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Air-water carbon dioxide exchange in relation to chemical and
physical characteristics of the Churchill River and estuary,
southwestern Hudson Bay region
by
Emmelia C. Stainton
A Thesis submitted to the Faculty of Graduate Studies of
The University of Manitoba
in partial fulfilment of the requirements of the degree of
MASTER OF SCIENCE
Department of Environment and Geography
University of Manitoba
Winnipeg
Copyright © 2009 by Emmelia C. Stainton
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Air-Water Carbon Dioxide Exchange in Relation to Chemical
and Physical Characteristics of the Churchill River and Estuary,
Southwestern Hudson Bay Region
By
Emmelia C. Stainton
A Thesis/Practicum submitted to the Faculty of Graduate Studies of The University of
Manitoba in partial fulfillment of the requirement of the degree
Of
Master of Science
Emmelia C. Stainton©2009
Permission has been granted to the University of Manitoba Libraries to lend a copy of this thesis/practicum, to Library and Archives Canada (LAC) to lend a copy of this thesis/practicum, and to LAC's agent (UMI/ProQuest) to microfilm, sell copies and to publish an abstract of this
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i
Abstract
Work described in this thesis has increased our understanding of high
latitude river/estuarine air-water carbon dioxide (CO2) exchange through
examination of the Churchill River and estuary system of southwestern Hudson Bay,
which was studied throughout the 2007 open-water season. An automated
monitoring instrument was used to measure the concentrations of CO2 in air and
water of the Churchill River and estuary. River and estuarine waters were sampled
and analyzed to determine differences in CO2 flux with respect to season, space, and
source (river or marine). Riverine CO2 flux ranged from -51.4 to +30.8 mmol nr2
day1 (mean flux of-5.8 mmol nr2 day -1), while estuarine flux ranged from -11.04 to
+22.76 mmol nr2 day1 (mean fluxes of -1.07 at low tide, +0.06 at high tide, and -0.37
mmol nr2 day1 overall). These results indicate that the Churchill River and estuary
are net CO2 sinks throughout the 2007 open-water season. Analyses of water
samples collected from the Churchill River (surface waters) and estuary (surface
and bottom waters) indicate that the river is a source of chlorophyll a, dissolved
organic carbon, coloured dissolved organic matter, and particulate organic carbon
and nitrogen to the estuary basin, while marine waters are a source of dissolved
inorganic carbon, salinity, and alkalinity. A multi-sensor probe was used to
determine estuarine stratification and seasonal mixing patterns. The Churchill River
estuary was stratified by water source, with fresh river water flowing over brackish
marine water. The nature of the stratification varied seasonally with temperature
and changing river discharge and tidal amplitude.
11
Acknowledgments
I thank my supervisors, Drs. Tim Papakyriakou and David Barber, and my committee members, Drs. Ray Hesslein and Fei Wang for their guidance and support.
I also thank Drs. Gordon Robinson (Botany) and Brenda Hann (Zoology) for being excellent teachers throughout my undergraduate degree, and encouraging me to pursue aquatic research.
I would also like to thank several organizations for their support: the Natural Sciences and Engineering Research Council of Canada, the Northern Scientific Training Program, the ArcticNet program of the Network of Centers of Excellence, and the Churchill Northern Studies Centre.
I am very thankful for the continued support and encouragement from Simon and my family. To my father, Michael, I especially thank for his continued interest and helpful discussion with respect to this project.
Table of Contents
1 - Introduction 1
Motivation 1
Objectives 3
Thesis outline 4
2 - High latitude river-estuarine-shelf CO2 exchange 6
Introduction 6
Role of the surface waters in determining CO2 flux 8
Processes that affect [CCbJw 9
2.3.1 Chemical Processes 9
2.3.2 Physical processes 12
2.3.3 Biological processes 13
2.3.4 Summary 18
Gas exchange at water surfaces 19
2.4.1 Solubility 20
2.4.2 Stagnant Film Model 21
2.4.3 Determination of gas transfer velocity, k 24
2.4.4 Other considerations for gas transfer 26
Carbon exchange at the land-ocean interface 28
2.5.1 Rivers 29
2.5.2 Estuaries 32
2.5.3 Continental Shelves 35
2.6 CO2 transport from coastal systems
2.7 Climate implications 37
2.8 Summary 39
Chapter 3 - Summer air-water carbon dioxide exchange and water
chemistry of the Churchill River as it enters Hudson Bay 46
3.1 Introduction 47
3.2 Methods 51
3.2.1 Discrete water sampling 52
3.2.2 Continuous CO2, O2, and water sampling 54
3.2.3 Other sources of data 56
3.2.4 Laboratory procedures 57
3.2.5 Calculations 58
3.3 Results 61
3.3.1 Environmental conditions 61
3.3.2 Water chemistry 64
3.3.3 Continuous CO2,02, and water sampling 66
3.3.4 C02flux 68
3.3.5 Production and respiration 70
3.4 Discussion 72
Chapter 4 - Air-water carbon dioxide exchange and physiochemical
processes of the Churchill River estuary 82
4.1 Introduction 83
4.2 Methods 86
4.2.1 Study Area 86
4.2.2 Estuary survey sampling 87
4.2.3 Estuary basin CTD profiling 90
4.2.4 Other sources of data 91
4.2.5 Laboratory and analytical procedures 92
4.2.6 Calculations 93
Results 94
4.3.1 Hydrographic conditions 94
4.3.2 Physical conditions 94
4.3.3 Water chemistry 97
4.3.4 Air/water pC02 and C02 flux 104
4.3.5 pCC»2 concentration at depth 106
Discussion 109
4.4.1 Influence of season, location, depth, and tidal state on water chemistry 109
4.4.2 Influence of water source and physiochemical
conditions on pCCh and CO2 flux 110
4.4.3 Comparison to other high latitude estuaries 97
4.3.4 Air/water pC02 and C02 flux 114
Summary 115
5 - Conclusions 119
Summary and Conclusions 119
Future Directions 120
A - Latitude and longitude coordinates of all stations sampled within the Churchill River and estuary, June-October 2007. 122
B - Figure 1: Relationship between gas trasfer velocity, k, and wind speed from four different models: Wanninkhof (1992), Wanninkhof and McGillis (1999), Nightingale et al. (2000), and Liss and Merlivat (1986). The Wanninkhof (1992) model for calculating gas transfer was used for flux calculations in this study. 123
Table 1: Mean, standard deviation, and range of CO2 flux measurements based on four models for calculating gas transfer, k. The Wanninkhof (1992) model for calculating gas transfer was used for flux calculations in this study. 123
List of Tables
1 Commonly used gas transfer velocity parameterizations. 25
1 Mean and standard deviation of Churchill River annual flow and precipitation before and after the Churchill River Diversion (CRD, created in 1976) and before and after the Churchill River rock weir construction (10km south of the Town of Churchill, built in 1998). 61
2 Summary of water chemistry data from Churchill River site CH7 collected June-October 2007. 64
3 Mean, standard deviation and range of (i) all measured gas monitor data (n = 224), (ii) flux (n = 224), and (iii) net production, respiration, and gross production at 2.5m (Pnet/down-slope peak, n = 27; R/upslope peak, n = 26; Pgross/complete peak, n = 24) from Churchill River station CH7. 67
4 Summary of flux estimates made by Tremblay et al. (2006) for a Manitoba River and some northern Quebec hydroelectric reservoirs. Samples were collected from a 20L chamber connected to a non-dispersive infrared gas analyzer, and data was stored every 20s over a 5-10 minute sample period. 71
1 Water chemistry of Churchill River estuary waters collected from stations along a fresh (CH5) to marine (CHI) gradient, at both high tide (HT) and low tide (LT), from surface (0.5m) and bottom waters (3 and 10m), from June to October 2007. Surface water samples were also collected from the inflowing Churchill River (CH7) upstream of the weir (Stainton et al., in prep. 2009). 100
2 Pearson's Product Moment Correlation (r) between C02w (umol l1) and water chemistry variables. 113
3 Range of flux estimates and water pC02 from several mid- to high-latitude estuaries/coastal seas. 114
List of Figures
Carbon cycling and processes in aquatic systems (Ducklow and McCallister, 2004). pC02 in surface waters can increase/decrease through gas exchange, photosynthesis/respiration from aquatic organisms, precipitation/dissolution reactions with CaC03, and decomposition of organic material by heterotrophs. Reprinted by permission of the publisher from THE GLOBAL COASTAL OCEAN: THE SEA - IDEAS AND OBSERVATIONS ON PROGRESS IN THE STUDY OF THE SEAS, VOLUME 13, edited by Kenneth H. Brink and Allan R. Robinson, pp.287, Cambridge, Mass.: Harvard University Press, Copyright (c) 2006 by the President and Fellows of Harvard University. 15
Biogeochemical processes modifying Alk and DIC in aquatic systems (Zeebe and Wolf-Gladrow, 2001). This article was published in "CO2 in seawater, equilibrium, kinetics, isotopes", Zeebe and Wolf-Gladrow, Equilibrium, p. 7, Copyright Elsevier, 2001. 19
Relationship between solubility of CO2 and water temperature and salinity. Lines S=0, S=10, and S = 25 represent the relationship between water temperature (left y-axis) and solubility at salinities of 0,10, and 25 PSU. Lines T=273, T=281, and T=296 represent the relationship between salinity (right y-axis) and solubility at water temperatures of 273, 281, and 296 °K. 21
Stagnant film model (Sarmiento and Gruber, 2006). Gas transfer at the air-water interface is a relatively slow process, which results in a concentration gradient. CO2 is well mixed in the turbulent layer, uniform concentration of [CO2V and [C02]atm in the air. [C02]w and [C02]atm in the stagnant layers changes linearly to saturation levels [C02]w° and [C02]atm°. Gas transfer across the air-sea interface is governed by the rate of molecular diffusion of gas CO2 through the two stagnant films. 23
The lower Churchill River from Southern Indian Lake to Hudson Bay. Discharge for the Churchill River is gauged at Red Head Rapids. 31
6 Carbon and nutrient cycling between the ocean, atmosphere and land {Ducklow and McCallister, 2004). All three carbon
pumps - biological, solubility, and coastal shelf- are show. Arrows between organic carbon and DIC indicate biological production and respiration. Reprinted by permission of the publisher from THE GLOBAL COASTAL OCEAN: THE SEA -IDEAS AND OBSERVATIONS ON PROGRESS IN THE STUDY OF THE SEAS, VOLUME 13, edited by Kenneth H. Brink and Allan R. Robinson, pp.273, Cambridge, Mass.: Harvard University Press, Copyright © 2006 by the President and Fellows of Harvard University. 37
a) Map of the Churchill River as it enters Hudson Bay. b) CO2/O2 air/water gas monitor moored on float at Churchill River station CH7. Two 20W solar panels [right side) charge two 12V batteries that power the gas monitor system (inside white box). 52
Schematic of air/water gas monitor: wet box consisting of a peristaltic water pump, a thermocouple housing, a gas exchanger, and two solenoid valves (SV); dry box consisting of an air pump, drying compartment, O2 sensor, IR gas analyzer, and data logger. 55
a) Churchill daily mean air temperature and daily precipitation (Environment Canada), and river discharge (combination of Red Head Rapids and the Deer River, Water Survey of Canada) throughout the 2007 open-water season, b) precipitation and air temperature monthly averages from 2007 and historical climate norms (1971-2000) (Environment Canada). 62
Historical flow data from the Churchill River at Red Head Rapids (Water Survey of Canada) and monthly mean precipitation (Environment Canada) between 1971-2008. 63
Churchill River summer water chemistry data collected from 1995-2005 (data from Bezte, 2006) and from 2006-2007 (this study). 65
Diurnal fluctuations of CO2W, CO2A, O2W, O2A, and Tw at Churchill River site CH7 throughout the 2007 open-water season. This data was measured and recorded at 2-hour intervals using a continuous air/water CO2/O2 gas monitor. 68
a) and b) Turbidity, salinity, water temperature and pH collected at Churchill River station CH7 (data measured and recorded using an RBR XR-620) c) Churchill River discharge (combination of Red Head Rapids and the Deer. River, Water
X
Survey of Canada] and water level (recorded at the Churchill pump house station, approximately 3km upstream of station CH7). 69
3.8 Estimated CO2 flux from measured values at Churchill River site CH7 throughout the 2007 open-water season. A positive flux indicates a supersaturation of CO2 in water with respect to air (source), while a negative flux indicates an undersaturation of CO2 in water with respect to air (sink). The Churchill River is a CO2 sink throughout most of the monitored open-water season. 70
3.9 Estimated Pnet and R throughout the 2007 open water-season at Churchill River station CH7. Both Pnet and R were calculated from gas data measured using a continuous air/water gas monitor. 72
3.10 Delivery rate of C02w to the Churchill River estuary and 24-hour upstream flow distance upstream from station CH7. 73
4.1 a) Churchill River (CH7) and estuary (CH1-5) sampling stations at a) high tide and b) low tide (Landsat 7 satellite image, Natural Resources Canada). 87
4.2 Air/water gas monitoring instrument. Dry box: 1-air pump, 2-drying compartment, 3-O2 sensor, 4-gas analyzer, and 5-data logger; wet box: 6-peristaltic water pump, 7-thermocouple housing, 8- air/water gas exchanger, and 9- solenoid valves. 89
4.3 a) Churchill daily mean air temperature and precipitation (Environment Canada), b) Churchill River discharge (combination of Red Head Rapids and the Deer River, Water Survey of Canada) and water temperature (Manitoba Hydro pumphouse station), and c) Churchill harbour tide height (Fisheries and Oceans Canada), with sampling periods indicated by dashed vertical lines. 96
4.4 Depth, temperature, salinity cross-sections of Churchill River estuary CTD transects at high tide (left column) and low tide (right column) in June, July, August, and October 2007. Temperature is represented in colour (note all colour legends on the right side of figures are different), salinity is denoted by numbered contour intervals (ranging from 2-30), and individual CTD casts are shown as bold black vertical lines within the cross-sections. White circles on the high/low tide maps (bottom panels) are the CTD sampling locations. 99
XI
4.5 Temperature-salinity scatter plot of all CTD casts taken within the Churchill River estuary throughout the 2007 open-water season. Data is grouped by sampling month: red = June, blue = July, green = August (light green for high tide casts, dark green for low tide casts), black = September/October. 101
4.6 Water chemistry of the Churchill River estuary between June and October, 2007. Samples were taken at 0.5m from the surface, and 0.5m from the bottom at each of 5 stations throughout the length of the estuary (ranging from fresh to brackish water), at both high and low tide. The x-axis is salinity in all four panels and the right y-axis colour legend is reflected on the data points in each separate panel. Point distribution reflects the left y-axis parameters versus salinity. 103
4.7 Water-air difference in pCCh (top panels) and air/water CO2 flux from Churchill River estuary waters (both using average air pCC»2 = 374 uatm). Samples were collected using a continuous automated CO2 monitor that averaged air and water CO2 concentrations over a 30-minute period. Stations range from fresh (CH5) to brackish/marine (CHI) waters and were sampled at high and low tide, June to October, 2007. Overall mean CO2 flux is -0.62 mmol m2 day1. 105
4.8 pC02 concentration, salinity, and temperature variation with depth (surface, mid-depth, and bottom waters) at Churchill River estuary stations CH3 (top panels) and CH4 (bottom panels). Samples were collected between June and October at high tide using a continuous automated CO2 monitor. Water was pumped from each depth for 10 minutes, data logging every 10 seconds. Data points represent an average of pC02 over the 10-minute sampling period. 107
4.9 PCO2 concentration, salinity, and temperature variation with depth (surface, mid-depth, and bottom waters) at Churchill River estuary stations CH3 (top panels) and CH4 (bottom panels). Samples were collected between June and August at low tide using a continuous automated CO2 monitor. Water was pumped from each depth for 10 minutes, data logging every 10 seconds. Data points represent an average of pC02 over the 10-minute sampling period. 108
4.10 Churchill River CO2 flux and ApC02. Water was sampled using a continuous air/water CO2 gas monitor. The dashed line indicates August 1. I l l
Xll
5.1 Summary of processes within the Churchill River and estuary system. 120
xiii
List of Copyright Permissions
Figure 2.1 Reprinted by permission of the publisher from THE GLOBAL COASTAL OCEAN: THE SEA - IDEAS AND OBSERVATIONS ON PROGRESS IN THE STUDY OF THE SEAS, VOLUME 13, edited by Kenneth H. Brink and Allan R. Robinson, pp.287, Cambridge, Mass.: Harvard University Press, Copyright © 2006 by the President and Fellows of Harvard University. 15
Figure 2.2 This article was published in "CO2 in seawater, equilibrium, kinetics, isotopes", Zeebe and Wolf-Gladrow, Equilibrium, p. 7, Copyright Elsevier, 2001. 19
Figure 2.6 Reprinted by permission of the publisher from THE GLOBAL COASTAL OCEAN: THE SEA - IDEAS AND OBSERVATIONS ON PROGRESS IN THE STUDY OF THE SEAS, VOLUME 13, edited by Kenneth H. Brink and Allan R. Robinson, pp.273, Cambridge, Mass.: Harvard University Press, Copyright © 2006 by the President and Fellows of Harvard University. 37
1
Chapter 1: Introduction
1.1 - Motivation
Atmospheric carbon dioxide (CO2) concentrations have risen to 379 ppm [in
2005, IPCC, 2007] from the pre-industrial revolution level of 280 ± 10 ppm [IPCC,
2007]. Antarctic ice cores, which agree very well with atmospheric measurements of
CO2, have shown that the current atmospheric CO2 concentration has not been
exceeded in the past 420,000 years [IPCC, 2007]. This dramatic increase in
atmospheric CO2 is a result of anthropogenic CO2 production, primarily from fossil
fuel burning, although cement production and land use changes (e.g. biomass
clearing for agriculture, predominantly in the tropics) also release substantial
amounts of CO2 [IPCC, 2007]. Interestingly, the rates of atmospheric CO2 increase
and the estimated anthropogenic CO2 emission do not balance [IPCC, 2007]. A
"missing" 50-60% of anthropogenic CO2 emission is thought to be captured by the
global oceans and terrestrial ecosystems [Millero, 2000].
Increases in atmospheric CO2 and other greenhouse gases also linked to
industrialization (CH4, N2O, CFCs, HFCs, etc., not discussed in detail here) are driving
climate warming by their ability to absorb a portion of the IR radiation emitted from
Earth. Although greenhouse gasses are a critical regulator of the Earth's
temperature, increasing concentrations have the potential to regulate Earth's
temperature at levels above historic. The resulting linear global surface temperature
increase throughout the past 50 years is 0.1-0.16 °C per decade and global
temperatures are projected to increase by another 1.4 - 5.8°C by 2100 [IPCC, 2007].
2
Estimates of temperature change for high latitude regions are quite different than
global averages, with surface temperatures having increased by an average of 2-3°C
and up to 4°C in the winter since 1950 [ACIA, 2005].
Climate warming has lead to ice cover reduction (spatial and temporal),
water temperature increases, and altered chemical and biological cycles in northern
coastal marine environments [Chen, etal, 2003]. Human activities also affect the
amount - damming, irrigation, hydroelectric power generation, river diversions -
and composition (nutrients, carbon, suspended sediments) of freshwater runoff to
northern estuarine-shelf environments, which could impact their biogeochemical
cycling and overall metabolism [Borges, 2005; Chen, etal, 2003]. All of these
changes from normal are likely to alter the rates of processes governing the
production and sequestration of CO2 in estuarine-shelf systems.
An understanding of current carbon pools and fluxes, especially CO2, within
the biosphere and within/between smaller scale environments will enhance our
ability to predict changes under higher atmospheric CO2 conditions. CO2 is
intimately connected with all subsets of the biosphere, and thus knowledge of
carbon budgets and chemical species/phase transformations within specific
environments is important for understanding both current and future cycling and
possible feedback mechanisms.
Global warming is resulting in physical, chemical, and biological
transformations in many global environments [IPCC, 2007]. This is especially of
concern for high latitude polar regions, though a dearth of scientific observations
3
renders uncertain the extent to which these changes will affect high latitude
environments - and the influence this will have globally.
ArcticNet, a multi-year collaborative research project, was designed to fill
some of these knowledge gaps in the Canadian Arctic. This study is one of hundreds
undertaken as part of ArcticNet, as we attempt to understand the main pathways of
carbon and nutrients into and through Hudson Bay. Prior to this research, only a
handful of studies existed for the Churchill River - none pertaining to gas exchange.
1.2 - Objectives
The objective of this thesis is to better understand current carbon and
nutrient cycling of the high-latitude Churchill River and estuary system, to
determine whether these environments are sources or sinks for atmospheric CO2
during the open-water season, and to determine what physical, chemical, and
biological factors govern these conditions. The work and its interpretation are
presented in two parts. Part 1 determines what carbon stocks exist and how they
seasonally vary between the components of the carbon pool of the Churchill River
flowing into Hudson Bay. Part 2 determines whether the Churchill River estuary is a
source or sink for atmospheric CO2, and how seasonal physiochemical changes affect
the carbon pool within the Churchill River estuary. The methods and results of this
thesis are described separately in each of these parts.
4
1.3 - Thesis Outline
Chapter two of this thesis consists of a detailed overview of carbon cycling in
aquatic systems by discussing the importance of the oceanic carbon cycle, and
provides an explanation and synthesis of 1) processes that affect CO2 in water, 2)
carbon sources and transformations in aquatic systems, and 3) how the Churchill
river and estuarine system fits within the context of these processes and
transformations, and how it compares with similar systems globally. This chapter
will explain gas exchange at water surfaces and provide a review of the currently
accepted models, determination of a gas transfer velocity for estimation of gas flux,
and mention other factors that should be considered for gas transfer estimations.
This chapter also outlines air-water CO2 exchange in rivers, estuaries, and coastal
marine systems and identifies the processes by which CO2 is transported to the deep
ocean.
Chapter three will discuss the results from the paper: Stainton, E.C., R.H.
Hesslein, T.N. Papakyriakou, and D.G. Barber. Summer air-water carbon dioxide
exchange and water chemistry of the Churchill River as it enters Hudson Bay.
Chapter four will discuss the results from paper: Stainton, E.C., T.N.
Papakyriakou, and D.G. Barber. Air-water carbon dioxide exchange and
physiochemical processes of the Churchill River estuary.
The concluding chapter of this thesis will summarize the results and outline
suggestions for future research in the Churchill River estuary and other high-
latitude estuaries with the purpose of expanding our knowledge of CO2 transfer and
cycling, and influences of estuarine processes on the global oceans.
6
Chapter 2: High latitude river-estuarine-shelf CO2 exchange
2.1 - Introduction
Coastal marine environments are defined as the ocean extending from
shoreline to the continental shelf break, and are among the most productive
ecosystems in the biosphere. Representing only 7% of ocean surface area and less
than 0.5% of the ocean volume, the global marine shelf environments produce 20%
of the total oceanic organic matter (OM), 80-90% of new oceanic production, 80% of
oceanic OM burial, up to 50% of denitrification, 90% of oceanic sedimentary
mineralization, 30% of the oceanic production of particulate organic carbon (POC],
and 50% of the oceanic accumulation of POC [Borges, 2005; Chen, etal, 2003]. The
high rate of production on marine continental shelves is due to riverine inputs and
coastal upwelling, both of which supply nutrients and influence water retention
times [Chen, etal, 2003]. Continental shelves have considerably higher air-water
CO2 exchange and higher respiration and photosynthetic rates (2x) than the open
ocean and as a result, accumulate more organic carbon and calcium carbonate
(CaC03) [Chen, etal, 2003].
The Arctic Ocean is quite different compared to other global oceans because
of its large proportion of shelf relative to ocean area (30% shelf), cooler water
temperatures, its significant freshwater inputs from terrestrial runoff, and the
presence of sea ice. Although several similarities exist between arctic shelves and
others globally - higher primary production, terrestrial influence, shelf stratification
7
from freshwater runoff, and wind-induced upwelling - many processes are unique
to these northern environments [Chen, etal, 2003]. There is a strong seasonality in
northern marine environments due to ice cover, snow deposition, and riverine
input, which causes water column stratification in the summer from freshwater
input via snow- and ice-melt and freshet, and stratification breakdown in the winter
from salt rejection during ice formation [Chen, et al, 2003]. The difference in
temperature and salinity between the converging fresh river [source of heat) and
saline marine waters also contribute to seasonal stratification in many high latitude
estuaries [Macdonald and Yu, 2006].
In the Arctic, marine shelf-estuarine systems receive large amounts of
terrestrial freshwater input - the Arctic Ocean receives approximately 10% of the
global river discharge - that carry in high volumes of dissolved and particulate
matter and nutrients from predominantly permafrost regions rich in organic carbon
[Dittmar and Kattner, 2003]. As a result, Arctic and sub-arctic estuarine-shelf
systems are highly dynamic with increased production, respiration/degradation,
resuspension, and sedimentation [Borges, 2005]. These processes all affect the
transport and sequestration of carbon and the air-water flux of CO2 in estuarine-
shelf systems.
Hudson Bay receives a freshwater load of 713.6 km3 year1, [Dery and Wood,
2005] about 18% of the total discharge to the Arctic Ocean. The majority of this
freshwater input is controlled as a result of hydroelectric development. The large
amount of riverine water delivered results in freshening of Hudson Bay coastal
8
surface waters (salinities <28 PSU relative to ~34 for peripheral seas and bottom
waters) [Prinsenberg, 1986). A dynamic boundary exists where the inflowing rivers
meet Hudson Bay waters and mix under the influence of wind and tides. Here,
physical, chemical, and biological responses to the mixing waters influence gas
exchange. Little work has been done that explains how estuarine systems affect air-
water CO2 exchange relative to the respective river and marine environments.
Though the Churchill River comprises only a small proportion (2.8%) of the
freshwater delivered to Hudson Bay [Dery and Wood, 2005], its location (adjacent to
a town, with access to air, rail, and marine transportation), past hydroelectric
development (background data from impact assessments and current hydrological
monitoring), and future significance as a high latitude marine port make the
Churchill River and estuary system an ideal location to conduct a process-based
river-estuarine study, and a site for monitoring of climate induced changes to the
carbon cycle. Understanding the physiochemical transformations of the subarctic
Churchill River and estuary system will provide some insight into local carbon
budgets and may lead to a better understanding of how high latitude estuaries cycle
carbon.
2.2 - Role of the surface waters in determining CO2 flux
The exchange of CO2 between surface waters and the atmosphere governs
whether a water body will be either a source or sink for atmospheric CO2. This
exchange, or flux of CO2 (fco2, equation 2.2), is equal to the concentration gradient of
9
CO2 between the surface water and the atmosphere (A[C02], equation 2.1)
multiplied by the gas transfer velocity, k [Liss, 1983; Liss and Merlivat, 1986; Millero,
2000].
A[C02] = [ C 0 2 ] w - [ C 0 2 ] A [ 2 < 1 ]
^ * t f«> 2 ] w " ^ A ) [ 2 2 ]
The resulting flux can either be positive, indicating a source of CO2 (flux from
a water body to atmosphere) or can be negative, indicating a sink for CO2 (flux into a
water body from atmosphere). A concentration of [CC>2]w greater than [C02]A
indicates a supersaturation of water with respect to air, whereas if [C02]w is smaller
than [C02]A, water would be undersaturated.
2.3 - Processes that affect [C02]w
2.3.1 - Chemical Processes
The carbonate system in oceans, which contains the majority of oceanic
carbon, is extremely important because it controls water pH and the exchange of
CO2 between the atmosphere, lithosphere, and biosphere [Millero, 2000].
Dissolved carbonate equilibria can be described with a set of equations, as
follows:
10
C°2(g)
PC02(w)
* H2C03(aq)
HC03(aq)
< •
K0
K1
K2 < > •
PC02(w)
* H2C03(aq)
H+ + HC03(aq)
H + + co3;aq)
[2.3]
[2.4]
[2.5]
[2.6]
[H2C03] [2.7]
pco2
_ [H+][HCO;] [ 2 .8 ] 1 [H2C03]
K = [^HCof] [2.9] 2 [Hccg
H2CO3* represents the analytical sum of CC^wand H2C03faqj [DOE, 1994]
[Sarmiento and Gruber, 2006; Stumm and Morgan, 1996a]. From these equations, we
can see that CCbwcan be increased by 1) air/water exchange (equation 2.3) 2)
shifting the reactions in equations 2.4, 2.5, or 2.6 to the left or 3) changing the
equilibrium constants in equations 2.7, 2.8, or 2.9.
The carbonate system can be described for aquatic systems through
measurement of two or more of the following variables: total [C02]w [or dissolved
inorganic carbon, DIC) (equation 2.10), partial pressure of CO2 (pCChw) (as seen in
equation 2.4), alkalinity (Alk, equation 2.11), and pH (equation 2.12) [Millero, 2000;
Stumm and Morgan, 1996b]. Alk represents the "acid-neutralizing capacity of an
11
aqueous system" [Stumm and Morgan, 1996a] or "the concentration of all the bases
that accept H+ when a titration is made with H+ to the carbonic acid endpoint",
where the principal bases include HCO3-, CO32-, and B(0H)4- [Millero, 2000].
DIC = [H2C03*] + [HCO3- ] + [C032 • ]
[2.10]
Alk = [HC03"] + 2[C032"] + [B(OH)4"] + [OH"]-[H+]
+ [SiO(OH)3- ] + [MgOH+ ] + [HP042' ] + [P04
3" ] 1
pH = - l o g { H + } [2.12]
pH is the activity of H+ ions in an aqueous solution, and therefore is affected
by carbonate equilibria that can either produce or consume H+ with an
increase/decrease in [CCbJw [Stumm and Morgan, 1996a]. For example, increases in
[C02]w by way of equations 2.3 and 2.4 will increase both the acidity (lower pH) and
the total concentration of dissolved carbon species in water. Millero [2000] suggests
that DIC, pC02w, and pH are the three best variables to measure (in that order)
because they undergo the largest amount of change over time (diurnally and
seasonally).
Alk is unaffected by changes in [C02]w because carbonic acid is the reference
proton level used to determine proton deficiency [Stumm and Morgan, 1996a]. Alk is
however, affected by reactions consuming or producing H+ and OH", such as calcium
carbonate (CaCOs) precipitation and dissolution, and photosynthesis and
12
respiration/decomposition (e.g. NO3" assimilation from photosynthesis increases Alk
while decomposition decreases it) [Stumm and Morgan, 1996a].
H2CO3* only contributes slightly to the DIC pool, and therefore carbonate and
bicarbonate ions are ultimately controlling pCC^w [Sarmiento and Gruber, 2006]. By
rearranging and combining equations 2.5 to 2.9 and replacing carbonate and
bicarbonate with DIC and Alk, we arrive at the following equation expressing pC02w
[Sarmiento and Gruber, 2006]:
pC02
K, (2 • DIC - Alk) '2
*0*1 Mk-D,C [2.13]
From equation 2.13 we can see that there are three main parts to what
controls surface water pCCh: 1) the ratio of equilibrium constants (K2/K0K1) 2) DIC,
and 3) Alk [Sarmiento and Gruber, 2006; Stumm and Morgan, 1996a]. It is easiest to
separate these pCCh controls into two sections: 1) those that are affected by physical
processes and 2) those that are affected by biological processes.
2.3.2 - Physical processes
Gas exchange at the air-water interface, controlled by k and A[C02], will
result in DIC concentration changes (Figure 2.1). Looking at equation 2.3, we can see
that [CO2V will increase or decrease in response to CO2A in order to maintain
equilibrium. The transformation of CO2W to H2CO3* translates into only a small
13
increase in [C02]w [Sarmiento and Gruber, 2006], and is quite slow and less
important compared other physical and biological processes influencing [C02]w.
2.3.3 - Biological processes
Photosynthetic Production and Respiration
Plankton growth and death in the euphotic zone has a strong influence on
[CO2V and carbonate equilibria, as well as carbon transport to the sediments
[Figure 2.1) [Portielje and Lijklema, 1995; Wetzel, 2001]. Biological production and
respiration affects [CChJwin aquatic systems through the following reaction
[Redfield, etal, 1963]:
106CCL + 16NO / + HPO.2" + 78hLO + 18H+ <—• C.^H.^O.-N^P + 150CL 2 3 4 2 106 175 42 16 2 >
[2.14]
where the forward reaction is photosynthesis and the reverse reaction is
respiration. Surface waters become undersaturated in CO2 when photosynthesis is
high; CO2 uptake rate is higher than transfer from the atmosphere across the air-
water interface or that released through respiration; while waters may become
saturated in CO2 at night when photosynthetic metabolism stops, and where
bacterial respiration is large (e.g. DOC rich fresh waters) [Millero, 2000; Stumm and
Morgan, 1996a; Wetzel, 2001]. Should this occur, the DIC pool in surface waters will
try to return to equilibrium conditions by CO2 diffusion at the air-water interface.
14
Net primary production is the difference between photosynthetic production
(daytime] and respiration (both day and night), and is determined by nutrient
concentrations (primarily N and P, though Si may be limiting to diatoms) and light
conditions (and to a degree, water temperature which controls respiration rate)
[Sakshaug, 2004]. Schindler has shown in several whole-lake experiments that
atmospheric carbon is generally sufficient to fulfill phytoplankton carbon needs for
growth when nitrogen and phosphorus concentrations are ample [Schindler, 1974;
Schindler, 1977], though DIC may become rate limiting for portions of the day when
primary production is very high in surface waters [Portielje and Lijklema, 1995].
Increasing concentrations of the limiting nutrient will shift the species composition
and thus alter the uptake of other compounds, including dissolved CO2 [Schindler,
1974; Schindler, 1977]. Redfield et al. [1963] described aquatic systems as N limiting
if the N:P ratio is < 16, while P is limiting if N:P >16. Sakshaug [2004] reports an N:P
ratio ranging from 11-16 (mol mol-1) for Arctic marine waters, making them
predominantly N limited.
Another indicator of algal production is chlorophyll a, a pigment in all algal
cells, though its relationship with carbon fixation depends on acclimation and thus
may not be a useful indicator of carbon biomass. The chla:C ratio in arctic waters
has been reported by Sakshaug [2004] to range 0.003 to 0.8 (weight:weight), and is
highest for high nutrient/low light conditions (e.g. under ice) and lowest for
nutrient poor/high light conditions.
Figure 2.1: Carbon cycling and processes in aquatic systems [Ducklow and McCallister, 2004). pCC»2 in surface waters can increase/decrease through gas exchange, photosynthesis/respiration from aquatic organisms, precipitation/dissolution reactions with CaC03, and decomposition of organic material by heterotrophs. Reprinted by permission of the publisher from THE GLOBAL COASTAL OCEAN: THE SEA - IDEAS AND OBSERVATIONS ON PROGRESS IN THE STUDY OF THE SEAS, VOLUME 13, edited by Kenneth H. Brink and Allan R. Robinson, pp.287, Cambridge, Mass.: Harvard University Press, Copyright (c) 2006 by the President and Fellows of Harvard University.
Atmosphere pC02 - 370 fiatm
£s?
I f £ F-kVApCO,
1* Ocean pCO, 100-1000 patm
Ca , t + 2HCOi<=>CaCOJi+H,0+C02 <=> H.CO, <=> H*+HCO; ** 2H'+COj-
\t7 Zooplankton « Phytoptankton
DOM i
Net downward transport of particles and dissolved organic matter
(Biological pump) Y
u
I i
Bacterial respiration CO,
Production can be sustained by nutrients either released by organisms in the
photic zone [regenerative production) or supplied externally to the photic zone via
coastal upwelling of nutrient-rich deep water, riverine input, etc. (new production)
[Sakshaug, 2004]. The incidence of new production is highest in areas with almost
continual upwelling, such as along shelf breaks and where wind-driven vertical
16
mixing is sufficiently deep, and smallest where vertical mixing does not reach the
nutrient-rich deep layer even in winter, such as in the deep Arctic Ocean [Sakshaug,
2004]. The combination of seasonal ice cover and light intensity in arctic marine
systems influence the algal community structure and thus primary production.
Phytoplankton growth in arctic and sub-arctic marine systems generally begins
under ice as light intensity ad penetration through the ice increases, and continues
as surface blooms as ice retracts from the nutrient rich water. In stratified open
arctic waters, production will decrease throughout the summer with new
production occurring just above the pycnocline (30-35m), where nutrients from
deeper waters can be introduced [Sakshaug, 2004]. In estuarine-shelf environments,
however, primary production remains high throughout the open water season due
to nutrient input from rivers, tidal current mixing, and coastal upwelling, with
growth generally ending in the fall with the formation of sea ice [Sakshaug, 2004].
Borges [2005] proposes that primary production may increase [for some
species) in marine coastal systems as a result of CO2 accumulation in surface waters.
Primary production may also be enhanced by an extended growth season due to
shorter annual ice cover and earlier spring melt, which is predicted by most climate
warming models to occur throughout the next 100 years [IPCC, 2007]. Other factors
such as riverine freshwater supply, coastal upwelling, and wind- and tidal-induced
mixing also influence coastal primary production and will all likely contribute to
variability in primary production as they are individually affected by climatic
change.
Remineralization is the process by which organic matter, particulate or
dissolved, is returned to an inorganic state through decomposition and respiration.
These reactions cause an increase in [CO2V through the breakdown of particulate
organic matter (POM) and dissolved organic matter (DOM), which can either be
recycled by autotrophs in the photic zone, or accumulate in deep waters (Figure
2.1). Since many heterotrophic organisms can function in both anaerobic conditions
and below the photic zone, accumulations of POM and DOM from sedimentation and
sinking cells/organisms can result in high concentrations oiDIC in deep waters
[Wetzel, 2001]. This accumulation can contribute significantly to the surface water
DIC concentrations in upwelling regions, as we will see in subsequent sections.
Precipitation and Dissolution
Interaction with solid CaC03 also influences [C02]w and Alk:
Ca2+ + 2HC0 3 - ^ -> CaC03 + H20 + C02 [ 2 l 5 ]
The forward reaction (precipitation) produces CO2, while the reverse
reaction (dissolution) consumes gaseous CO2 (2C02W can be produced from 2HCO3"
for ICO2A consumed) [Stumm and Morgan, 1996a]. Carbonate precipitation is an
important process for transporting CO2 to deep oceanic waters (Figure 2.1, 2.2); a
vast amount of oceanic carbon is stored as carbonate in rocks and sediment, much
larger than the amount of cycled CO2 [Millero, 2000]. Some biota (e.g.
coccolithophores-algae, and foraminifera-amoeboid protists) produce CaC03 scales
18
and shells, which upon death, fall to the sediments carrying the carbon stored as
CaC03 with them.
2.3.4 - Summary
The main processes that modify pCCb in water can be summarized in Figure
2.2. Photosynthesis, CO2 efflux, and to some extent CaC03 formation all reduce DIC,
while respiration, CO2 influx, and CaC03 dissolution increase DIC. Only processes
that consume or release H+ ions lead to changes in Alk, with CaC03 dissolution
increasing Alk while CaCCh formation decreases it [Stumm and Morgan, 1996a;
Wetzel, 2001; Zeebe and Wolf-Gladrow, 2001]. Alk is also affected indirectly by
photosynthesis when nutrients (NO3", PO43) are consumed (increased Alk] or
released when cells die (decreased Alk). Precipitation, dissolution, photosynthesis,
and respiration reactions, as well as the hydration or dehydration of CO2 are all
relatively fast reactions producing/consuming CO2W when compared to gas transfer
at the air-water interface [Stumm and Morgan, 1996a].
19
Figure 2.2: Biogeochemical processes modifying Alk and DIC in aquatic systems [Zeebe and Wolf-Gladrow, 2001). This article was published in "CO2 in seawater, equilibrium, kinetics, isotopes", Zeebe and Wolf-Gladrow, Equilibrium, p. 7, Copyright Elsevier, 2001.
DIC (mmol kg 1)
2.4 - Gas Exchange at Water Surfaces
The flux of CO2 to or from surface waters is balanced by biogeochemical
processes [photosynthesis, respiration, variation in the carbonate system) and
varies diurnally, seasonally, and globally with latitude [Borges, 2005; Ducklow and
McCallister, 2004; Millero, 2000; Sarmiento and Gruber, 2006]. The gas transfer
velocity, k, strongly influences the flux of CO2 across the air-water interface, and this
transport is driven primarily by the concentration gradient of CO2 in the boundary
20
layer [Borges, et al, 2004; Portielje and Lijklema, 1995]. The gas transfer reaction at
the air-water interface is relatively slow and as a result, CO2 in many surface waters
is not at equilibrium with atmospheric concentrations [Stumm and Morgan, 1996a].
2.4.1 - Solubility
Air-water CO2 transfer is driven by the concentration gradient of CO2 at the
air-water interface:
ApC02 =PC02W - pC02A [216]
A[C02] = [ C 0 2 ] W - [ C 0 2 ] A [ 2 1 7 ]
where w and A refer to water and air. For a system at equilibrium, A[C02] and ApC02
will equal zero, while if ApC02 is positive, the flux will be from the water to the
atmosphere. The reverse is true for a negative ApCCh.
The concentration of CChw is determined by the solubility and partial
pressure of CO2 in water, following Henry's law [Stumm and Morgan, 1996a]:
[ C 0^ • ° • P C C \ [2.18]
where
a = EXP ( /A1 +A2 (100/ 7) + A3 LN (77 100) +A^ (7/ 100)2 + S• [8,+ S2 (T 1100) + S3 (7/ 100)2] ) t
[2.19]
a is the solubility coefficient (mol I/1 atnr1), Ai, A2, A3, A4, Bi, B2 and B3 are the
empirically-derived coefficients for molar solubility [Weiss, 197'A; Weiss and Price,
21
1980], and 7 is the water/air temperature (K). The solubility of C02, and thus [C02],
are also affected by both salinity and temperature (Figure 2.3).
Figure 2.3: Relationship between solubility of CO2 and water temperature and salinity. Lines S=0, S=10, and S = 25 represent the relationship between water temperature (lefty-axis) and solubility at salinities of 0,10, and 25 PSU. Lines T=273, T=281, and T=296 represent the relationship between salinity (right y-axis) and solubility at water temperatures of 273, 281, and 296 °K.
305
°_ 300
g 295 -•
t 290 +
£ 285 + £ 280 +
5 275 -•
270 -! 20
•S = 0
30 40 50 60 70
Solubility (mol m3 atm1)
T 30
•- 25
• • 2 0 w CL.
• - 1 5 ^
"S
• • 5
4- 0
•S = 10 •S = 25 •T = 273 •T = 281
80
•T = 296
2.4.2 - Stagnant Film Model
From the previous section, we established that the concentration gradient of
CO2 at the boundary layer occurs as a result of ApCCh from the air-water interface
equilibrium CO2 concentration difference. This gradient is steepest just above or
below the air-water interface (see Figure 2.4) [Portielje and Lijklema, 1995].
22
The stagnant film model assumes that two "stable stagnant films of finite
thickness", one in water and one in the atmosphere, are formed in the absence of
turbulence, and that gas diffusion through these films occurs by molecular
diffusivity, e [Whitman, 1923]. The flux of CO2 through a stagnant film can be
calculated from Fick's first law:
4> = -£ A z [2.20]
where § is flux (mmol m2 s-1), E is molecular diffusivity (air or water), [A] is the
concentration of gas A (air or water), and z is the thickness of the stagnant film layer
[Sarmiento and Gruber, 2006].
Using figure 2.4, we can rewrite equation 2.20 for both the atmosphere and
water stagnant film layers:
4>a = - "7T * (KOJa ~ [COJ?) a [2.21]
and
d>w = -V([co 2 ] w - [co 2 U 0
- rrn.i ^ [2.22]
where a is solubility (equation 2.19), ka = £a/Aza, kw = EW/AZW, and ka and /fw are
referred to as the gas transfer or piston velocity for air and water [Sarmiento and
Gruber, 2006]. Since actual measurements of CO2 concentration or partial pressures
are always made below or above the stagnant film layers, we can remove the terms
[C02]A° and [CO2V0 from equations 2.21 and 2.22 and calculate flux from:
23
* = -*-([co2]a-[co2]j [223]
where
J_ . k
= J- + i ^w ^a [2.24]
Liss and Slater [1974] have compared the gas transfer velocities of the air
and water film layers, and discovered that the water film layer, with a transfer 100
times slower than the air film layer, controls CO2 at the air-water interface. We can
therefore ignore the a/ka term, so k = kw [Sarmiento and Gruber, 2006].
Figure 2.4: Stagnant film model [found as Figure 3.3.1 in "Air-Sea Interface", Sarmiento and Gruber, 2006, p. 82]. Gas transfer at the air-water interface is a relatively slow process, which results in a concentration gradient. CO2 is well mixed in the turbulent layers, uniform concentrations of [CChJw and [C02]atm occur in the air. [CChJw and [C02]atm in the stagnant layers changes linearly to saturation levels [C02]w° and [CCbJatm0- Gas transfer across the air-sea interface is governed by the rate of molecular diffusion of gas CO2 through the two stagnant films.
2.4.3 - Determination of Gas Transfer Velocity, k
The gas transfer velocity is a function of turbulence in the mixed water layer
(due to primarily wind stress], and by the thickness of the molecular diffusive layer,
determined by the Schmidt number:
c £ [2.25]
where v is the kinematic viscosity of water.
The parameters to calculate the Schmidt number of CO2 between 0-30 °C have been
determined by Jahne et al. [1987b]:
S C =A-BT + CT2-DT3 ^
where A = 2073.1, B = 125.62, C = 3.6276, D = 0.043219, and T is water temperature
in °C \Jahne, etal, 1987b].
Measurements of k have been made by several techniques: isotope studies,
synthetic dye evasion methods, wind tunnel experiments, and through direct
observation using eddy-correlation (Table 1) [Liss and Merlivat, 1986; Wanninkhof,
1992; Wanninkhof and McGillis, 1999, Nightingale etal, 2000].
Since the gas transfer velocity is the primary source of error in calculating
CO2 flux, it is recommended that k be determined for specific locations until
empirical models are developed that can be applied to a range of aquatic systems.
Experimental estimation of k, however, requires extensive monitoring, and
therefore it is much more common for studies to use k parameterizations based on
wind speed. Some of the most common k parameterizations in use today are shown
in Table 1 [Liss and Merlivat, 1986; Wanninkhof, 1992; Wanninkhofand McGillis,
1999, Nightingale etal, 2000].
Table 1: Commonly used k parameterizations estimated for some freshwater and marine environments under different wind conditions.
Source Basis Gas transfer velocity parameterization (cm hour-1) Nightingale et al. [2000] dual deliberate tracer k = (0.333-U + 0.222'U2)'(Sc/600)"0-5
k = 0.17«O(Sc/600)"2/3, U < 3.6 ms'1
Liss & Merlivat [1986] 'S?^*^ * = ( U ' 3-4>-2.8.(Stf600)*s, 3-6 ms"1 < U * 13 ms1
k = (U - 8.4)'5.9-(Sc/600)"0-5, U > 13 ms"1
Wanninkhof & McGillis [1999] d i r e c t o b s e r v a t i o n s k = o.0283«U3-(Sc/600)-°'5
using eddy covanance v ;
Wanninkhof [1992] isotope studies k = 0,31-U2'(Sc/600) " 5
However, relationships using only wind speed to predict gas transfer
velocities can be flawed since some of the factors regulating gas transfer are not
related to wind speed [Wanninkhof, 1992]. Based on this premise, Raymond and
Cole [2001] have found that predetermined wind speed-parameterized /rvalues
may be significantly different in estuarine environments than for adjacent marine
waters under the same wind conditions. Borges et al. [2004] suggest that this is due
to the highly turbulent nature of estuarine environments compared to the open
ocean and that other factors such as tidal currents and fetch limitation may increase
k. Since depth, tidal velocity, and wind strength and direction is variable between
estuaries, Borges et al. [2004] suggest that k parameterized as a function of wind
speed is site specific for estuarine environments. This implies that flux calculations
using pre-determined equations may be subject to significant error.
26
Through various studies it has been shown that k is also dependent on other
variables: waves, bubble formation, surface films and surfactants, air-water
interface turbulence, and tidal currents, all of which can be highly site specific
[Borges, etal, 2004; Portielje and Lijklema, 1995; Wanninkhof, 1992].
2.4.4 - Other considerations for gas transfer
Air-water interface CO2 fluxes are best and most robustly measured by
collecting in situ pCCb data, by equilibrating a water sample with air or nitrogen and
later measuring the equilibrated gas with an infrared gas analyzer, or by making
continuous measurements of surface water by passing water through an
equilibrator and infrared gas analyzer [Borges, 2005; Millero, 2000]. Though pQOi
can be measured accurately, the calculation of fluxes are influenced by the gas
transfer velocity parameterizations which are site specific and may not be
accurately scaled to certain environments [Borges, 2005].
Bubble enhancement of gas transfer
Few studies have attempted to quantify the effect that bubbles have on air-
water gas transfer [Atkinson, 1973; Thorpe, 1982]. It seems near impossible to
estimate the number and size of bubbles as they are a function of water body
conditions and depth from the air-water interface [Liss, 1983]. Other factors making
the quantitative assessment of bubbles challenging include: differential gas
absorption/desorption, pressure changes (as a function of depth], and films
27
surrounding bubbles [Liss, 1983]. Atkinson [1973] and Thorpe [1982] have
estimated that bubble contribution to gas flux resulting form breaking waves is
likely only significant at wind speeds over 10 (12) m s1.
Chemical enhancement of gas transfer
The chemical enhancement of CO2 exchange will increase CO2 flux at the air-
water interface at low wind speeds, though this factor has been ignored in most
wind speed-parameterized gas transfer equations [Wanninkhof, 1992]. CO2
enhancement occurs at the air-water interface by the reaction between CO2A and
H2O or OH- ions, which increases the concentration difference of CO2 between the
air and water turbulent layers, and thus gas transfer [Wanninkhof, 1992]. An
enhancement factor E(t, pH, k) (enhancement as a function of temperature, pH, and
gas transfer velocity) can be used to account for the chemical enhancement of CO2
[Wanninkhof, 1992], which may yield more accurate flux calculations at wind speeds
below 5 ms1 [Wanninkhof, 1992].
Fugacity of CO? It is a common oversight in aquatic chemistry use the terms pC02 and/C02
interchangeably, though in fact they describe relationship between CO2 in two
different hydration states:
[C02] / c ° 2 = "F"
0 [2.27]
28
where Ko is the solubility coefficient of CO2 in water from the reaction CO2A = CO2W.
This is different from pC02, the partial pressure of CO2, which is calculated
using Henry's Law constant, Kh, in pCCh = [CO2] / Kh, where Kh is a function of
temperature, the gas in question (CO2), and the solvent (water, CO2W + H2OQ) =
H2CO3).
Fugacity is the tendency for a substance to "prefer" one state, the most
thermodynamically stable (i.e. lowest Gibbs free energy of solid, liquid, and gas
phases), to another. Fugacity may also be defined as the pressure change necessary
for a real gas to behave as an ideal gas, which for low pressure gases in accordance
with the ideal gas law, fugacity * pressure. The fugacity coefficient, §, represents the
ratio of fugacity to pressure, which for an ideal gas equals to 1. For CO2, a non-ideal
gas, fugacity is not equal to partial pressure but values are very close in magnitude
[McGillisand Wanninkhof, 2006].
2.5 - Carbon exchange at the land-ocean interface
Arctic and subarctic environments, including terrestrial, freshwater, and
marine systems, demonstrate seasonality due to a wide variation in solar radiation,
temperature, and hydrological influences between seasons. Coastal oceans and
estuaries are highly dynamic systems that undergo marked seasonal changes, and
are affected by the seasonal evolution of both terrestrial and marine environments.
29
2.5.1 - Rivers
The physical, chemical, and biological properties of estuarine-shelf
environments are governed by both riverine and oceanic processes; watershed size,
flow volume, and the physiochemical composition of the inflowing freshwater from
rivers into the ocean significantly influences the estuarine-shelf environment.
Many arctic and sub-arctic rivers have expansive watersheds covering rich
permafrost soils, that currently store over half the global organic carbon [Dittmar
and Kattner, 2003], and this soil composition contributes directly to the amount and
types of carbon collected in rivers. As water flows over the land towards streams
and rivers during the spring thaw and ice-free months, it collects particles rich in
organic C. Due to the seasonality of river water discharge in high latitude
environments, riverine organic carbon concentrations are highest in summer and
lowest in winter (highest when discharge is highest) while nutrient concentrations
are generally highest in the winter and lowest in the summer (highest at minimum
discharge) [Dittmar and Kattner, 2003].
Approximately 80% of the total organic carbon total organic carbon (TOC)
input to the Arctic Ocean via rivers is in the form of dissolved organic carbon (DOC)
(discharge of 18-26 x 1012 g DOC/year), originating primarily from terrestrial
sources (60-70% humic compounds) [Dittmar and Kattner, 2003]. This abundance
of labile carbon is available to decomposers making most large rivers net
heterotrophic systems [Cole and Caraco, 2001]. Initially high DIC and [C02]w
production from heterotrophy make most rivers net sources of CO2 [Raymond, et ah,
30
2000]. Several studies have also found high latitude lakes to be net sources of CO2
[Cole, etal, 1994].
High latitude rivers supply the Arctic Ocean with large quantities of
freshwater and nutrients. These vast systems have been used more extensively for
their power generating capabilities over the past few decades, but not without
significant impact to both upstream and downstream ecosystems. Hydroelectric
development relies on a relatively constant annual water flow in order to be
profitable. Since the hydrograph of high latitude rivers vary considerably with
season, a significant amount of water must be impounded and reserved for periods
of natural low flow (winter months). This often results in major flooding and
particle accumulation upstream, altered flow and changes to chemical, physical, and
biological water characteristics downstream [Rosenberg, etal, 1997], and can lead
to an altered water budget for the affected catchment area due to enhanced
evaporation [Vorosmarty and Sahagian, 2000]. Hydroelectric development also has
potential to alter ocean currents (through changes in freshwater load, and thus
stratification stability) although transformations of this intensity have not yet been
documented [Rosenberg, etal, 1997].
The work undertaken here relates to the Churchill River and estuary. The
Churchill River passes through the interior Great Plains, the Canadian Boreal Shield,
and the Subarctic Taiga before reaching Hudson Bay. In 1976, Manitoba Hydro
undertook the Churchill River Diversion project that diverted approximately two
thirds of the Churchill River's flow from Hudson Bay to the Nelson River through
31
Southern Indian Lake (Figure 2.5) [Baker et a/., 1994]. Since this diversion, the
majority of flow in the lower Churchill River is from inflowing rivers and streams
downstream of Southern Indian Lake, and this is reflected in the river's water
chemistry [Bezte, 2006].
Figure 2.5: The lower Churchill River from Southern Indian Lake to Hudson Bay (Landsat 7 satellite image, Natural Resources Canada). Discharge for the Churchill River is gauged at Red Head Rapids.
The flow of the Churchill River (gauged at Red Head Rapids, Figure 2.5) is
lowest in April at 105 m3s_1 and peaks in late May/early June to 589 m3s-1 (mean
32
monthly discharges, 1981-1993) [Bezte, 2006]. Water chemistry of the Churchill
River was determined from samples collected between 1999-2005 (just upstream of
the weir) and have the following ranges: pH - 7.8 to 8.6, TSS - <2 to 21 mgL-1,
nitrate/nitrite N - <0.005 to 0.01 mgL1, total phosphorus - 0.007 to 0.056 mgL-1,
DOC - 8 to 14 mgL1, chlorophyll a - 0.9 to 7.3 ugL1 [Bezte, 2006].
2.5.2 - Estuaries
Estuarine systems have been studied for decades although due to their
dynamic nature, only a small proportion of studies have examined gas transfer, and
an even smaller proportion have taken place at high latitudes. Estuarine
environments vary in carbon cycling and CO2 exchange depending on the size of
inflowing river and the width of the continental shelf. Processes in inner and outer
estuaries are quite different and terrestrial matter carried through estuaries
undergo significant changes before being transported to the coastal shelf and
adjacent sea [Borges, 2005].
Inner estuaries are diverse and dynamic systems described by Borges
[2005] as "characterized by strong gradients of biogeochemical compounds,
enhanced organic matter production and degradation processes, and intense
sedimentation and resuspension". Physical characteristics of inner estuaries
(watershed area, freshwater discharge, geomorphology, water chemistry, and tides)
influence both nutrient and carbon exchanges and biogeochemical cycling as well as
water column stratification in the estuary, freshwater residence time, and spatial
33
extent of the plume [Borges, 2005]. The geographic boundaries of the inner estuary
are the river mouth [lower limit) and the upstream limit of tidal influence (upper
limit). Inner estuaries are identified by Borges [2005] as heterotrophic systems
(respiration, R > gross primary production, GPP), though this may shift for some
estuaries in the future due to increased nutrient loading. CO2 exchange from inner
estuaries depends on DIC inputs, ecosystem metabolism and the physical
characteristics mentioned above, though inner estuaries are generally a source of
nutrients and CO2 to oceanic waters, a possible source of CO2 to the atmosphere, and
a sink for organic matter [Borges, 2005].
Outer estuaries are defined by fresh water plumes floating upon dense sea
water and extending beyond the mouth of the river. Water temperature, depth, and
tidal currents govern the presence and degree of stratification (thermal or haline) in
outer estuaries, and the extent of plumes can be determined by both temperature
and salinity gradients, as well as gradients in turbidity, nutrients, total alkalinity,
and chlorophyll a [Borges and Frankignoulle, 1999]. Unlike inner estuaries, outer
estuaries have frequently been reported to be undersaturated in CO2 [Borges, 2005]
and have smaller air-water CO2 fluxes and reduced carbon and nutrient exchange
with the ocean [Borges and Frankignoulle, 1999]. Though outer estuary air-water
CO2 fluxes are often much lower than for inner estuaries (one order of magnitude
lower) and can vary both spatially and temporally, the surface area of the outer
estuary is quite large comparatively and thus contributes significantly to the CO2
budget of estuaries [Borges, 2005].
34
The inner part of the Churchill River estuary is enclosed within a basin that
extends approximately 13.2 km in length (latitude 58.793°, longitude -94.2048°), at
low tide is up to 1.7 km wide, and has an average depth of 3m at low tide (except for
a dredged, narrow shipping channel of 21m) (Baker et al. 1994). The outer estuary
of the Churchill River consists of a brackish plume extending into the Hudson Bay at
varying distances depending on tide (plume greatest at low tide) and river
discharge (plume extends furthest in the spring at freshet).
A recent study by Kuzyk et al. (2008) examined biophysical processes within
the Churchill River estuary during the spring melt period. Their findings explain the
relationship between river load and sea ice within the Churchill River estuary, and
demonstrate the importance of river input to the local marine system through ice
melt and the supply of nutrients. Aside from a few technical reports prepared for
Manitoba Hydro (impact assessments following the Churchill River diversion, and
prior to and following construction of a rock weir) little other physiochemical and
biological data exist for the Churchill River and estuary. (Data presented in these
reports were from samples collected upstream of the weir, and were outlined above
in the "River" section).
Significant physical, chemical, and biological transformations occur within
estuaries throughout the year, and it is crucial that we understand how these
processes currently vary in estuaries with respect to season in order to forecast how
climate warming may affect these systems in the future. What these transformations
35
represent to the global carbon cycle is largely unknown, and therefore further work
is necessary in order to understand these processes.
2.5.3 - Continental Shelves
Shelf environments can behave very differently from the open ocean with
respect to carbon cycling and CO2 exchange depending on shelf width, depth and
shelf-edge depth, river input, water retention time on the shelf, and heat balance
[Chen, et ah, 2003]. An extensive literature review on air-sea CO2 exchange by
Borges [2005] found that both high and temperate latitude continental shelves tend
to be sinks for atmospheric CO2, while subtropical and tropical shelves tend to be
sources [Borges, etal, 2005]. A similar review of coastal margins arrived at
comparable results [Cai, etal, 2006].
2.6 - CO2 transport from coastal systems
There are three main methods by which the oceans can take up atmospheric
CO2: the solubility pump, the biological pump, and the coastal shelf pump [Millero,
2000]. The solubility pump is driven by the difference in CO2 concentration between
the atmosphere and surface water, where higher concentrations in the surface
water cause a flux of CO2 from the ocean and a higher concentration in the air will
cause a flux of CO2 to the ocean surface water [Millero, 2000; Ducklow and
McCallister, 2004]. The biological pump is driven by primary production in the
surface mixed layer of oceanic waters. During the open water months,
36
phytoplankton will grow and die or be consumed by zooplankton and other
secondary producers, all of which will eventually settle down to deep oceanic
waters taking the carbon (CO2) sequestered over their lifetime with them [Millero,
2000; Ducklow and McCallister, 2004]. The coastal shelf pump transports carbon and
nutrients from rivers and coastal oceans to the deep ocean through particle settling
[Bates, 2006; Tsunogai, etal, 1999]. In high latitudes, this process is based on
surface water cooling on coastal shelves that produces dense waters, which along
with primary production increase the influx of atmospheric CO2 [Tsunogai, etal,
1999]. This denser, C enriched water from the shelf is transported below the
pycnocline along the shelf slope by isopycnal mixing [Tsunogai, etal, 1999]. This
process, if confirmed for coastal shelf environments globally, may have a significant
effect on our understanding of oceanic uptake of atmospheric CO2 [Borges, 2005].
Figure 2.6 highlights the main carbon and nutrient transformations at the
land-ocean interface and the pathways by which carbon can be buried (solubility,
biological, and coastal pumps). Many reactions seen in Figure 2.6 were covered in
previous sections and will not be discussed in detail here.
37
Figure 2.6: Carbon and nutrient cycling between the ocean, atmosphere and land [Ducklow and McCallister, 2004). All three carbon pumps - biological, solubility, and coastal shelf - are show. Arrows between organic carbon and DIC indicate biological production and respiration. Reprinted by permission of the publisher from THE GLOBAL COASTAL OCEAN: THE SEA - IDEAS AND OBSERVATIONS ON PROGRESS IN THE STUDY OF THE SEAS, VOLUME 13, edited by Kenneth H. Brink and Allan R. Robinson, pp.273, Cambridge, Mass.: Harvard University Press, Copyright (c) 2006 by the President and Fellows of Harvard University.
Atmosphere C0 2
2.7 - Climate implications
The estuarine-shelf environment in arctic and sub-arctic regions is
biogeochemically very active and is influenced by both freshwater and marine
processes. Estuaries integrate climate change impacts from large watershed areas
and may undergo significant changes due to the combined climate change effects on
both marine and terrestrial systems. It is important to understand carbon cycling
38
and water/atmosphere interactions in northern coastal environments as they may
play a significant role in global CO2 flux estimates either as sources or sinks for CO2,
though their current significance in the global carbon cycle is unknown.
Increasing atmospheric CO2 concentrations and climate warming have the
potential to affect sea water temperatures, the extent and duration of sea ice cover,
photosynthetic production and respiration, and overall carbonate equilibria, which
will all influence the ability of high latitude coastal areas to sequester CO2. In arctic
and sub-arctic coastal systems, changes to ice conditions will likely have a profound
effect on the light regime and primary production, stratification stability, and
transport of particles (and some nutrients) to the open seas from the coasts and
estuaries. As sea ice forms, salt is rejected forming a saline, nutrient rich layer under
the ice. This process is reversed as ice melts forming a freshwater layer on top of
marine water and influences thermohaline circulation in high latitude seas [Chen, et
ah, 2003].
Shortening the duration of sea ice cover and extending the open water season
would enhance total primary production and lengthen the period of tidal and wind-
induced mixing (which could increase coastal erosion [Macdonald, etal, 1998] and
introduce a higher volume of sediment to the coastal oceans). Where sea ice does
not effectively transport nutrients and salt, sediments and organic carbon are
carried from coastal margins to the deep ocean via ice rafting [Eicken, et ah, 2000].
Sea ice changes may therefore affect the biogeochemistry of both the arctic/sub
arctic coastal and open ocean environments [Chen, etal, 2003].
39
The affect of climate change on land-based processes will also influence the
delivery of water [Deryand Wood, 2005; Rouse, etal, 1997], and thus nutrients and
sediment, to estuarine-shelf systems. Changes in the amount and timing of
precipitation will affect the rate at which rivers deliver nutrients and reduced
carbon to marine environments. Changes in the delivery of freshwater to marine
environments also has the potential to alter stratification: surface freshening from
increased precipitation could enhance ocean stratification, thus reducing the
downward transport of carbon via the continental shelf pump [Sarmiento, etal,
1998]. This, along with oceanic surface water warming and sea ice reduction, has
the potential to alter the function of the Arctic and sub-arctic coastal regions as a
source or sink for CO2.
It is unknown whether climate change will increase or decrease the extent to
which coastal environments can sequester carbon. This is especially of concern for
arctic and subarctic coastal shelves where little existing CO2 monitoring has been
done [Borges, 2005]. Currently coastal oceans are not included in global budgets for
CO2 and it is unknown what their inclusion will mean for future modeling/budgeting
and remediation strategies.
2.8 - Summary
Much is known about aquatic carbon chemistry, gas exchange at water surfaces,
and carbon exchange in rivers, lakes, and coastal oceans. However, little is known
about the relationships between gas exchange and biological, physical, and chemical
40
processes within river-estuarine systems, and even less is known for arctic and
subarctic systems. High latitude estuaries are challenging environments to study as
a result of tides, strong currents, and ice floes during the spring and early summer.
The lack of temporal-spatial data for arctic and subarctic river-estuarine systems
makes this research both important and unique.
References
ACIA (2005), Arctic Climate Impact Assessment, Cambridge University Press, Cambridge, U.K.
Atkinson, L. P. (1973), Effect of air bubble solution on air-sea gas exchange, Journal of Geophysical Research, 78,962-968.
Bates, N. R. (2006), Air-sea C02 fluxes and the continental shelf pump of carbon in the Chukchi Sea adjacent to the Arctic Ocean, Journal of Geophysical Research, 111, doi:10.1029/2005JC003083.
Borges, A. V. (2005), Do we have enough pieces of the jigsaw to integrate CO2 fluxes in the coastal ocean?, Estuaries, 28, 3-27.
Borges, A. V., et al. (2005), Budgeting sinks and sources of CO2 in the coastal ocean: Diversity of ecosystems count, Geophysical Research Letters, 32, doi:10.1029/2005GL023053.
Borges, A. V., and M. Frankignoulle (1999), Daily and seasonal variations of the partial pressure of C02 in surface seawater along Belgian and southern Dutch coastal seas. Journal of Marine Systems, Journal of Marine Systems, 19, 251-266.
Borges, A. V., et al. (2004), Variability of the gas transfer velocity of C02 in a macrotidal estuary (the Scheldt), Estuaries, 27, 593-603.
Cai, W.-J., et al. (2006), Air-sea exchange of carbon dioxide in ocean margins: a province-based synthesis, Geophysical Research Letters, 33, doi:10.1029/2006GL026219.
Chen, C, et al. (2003), Continental Margin Exchanges in Ocean Biogeochemistry: The Role of the Ocean Carbon Cycle in Global Change, edited by Fasham, pp. 53-94, Springer.
Cole, J. J., and N. F. Caraco (2001), Carbon in catchments: connecting terrestrial carbon losses with aquatic metabolism, Marine and Freshwater Research, 51,101-110.
Cole, J. J., et al. (1994), Carbon dioxide supersaturation in the surface waters of lakes, Science, 265,1568-1570.
42
Dery, S. J., and E. F. Wood (2005], Decreasing river discharge in northern Canada, Geophysical Research Letters, 32, doi:10.1029/2005GL022845. Dittmar, T., and G. Kattner (2003), The biogeochemistry of the river and shelf ecosystem of the Arctic Ocean: a review, Marine Chemistry, 83,103-120.
DOE (1994), Handbook of methods for the analysis of the various parameters of the carbon dioxide system in sea water. Version 2, in ORNL/CDIAC-74, edited by A. G. Dickson and C. Goyet.
Ducklow, H. W., and S. L. McCallister (2004), The biogeochemistry of carbon dioxide in the coastal oceans, in The Sea, Volume 13, edited by A. R. Robinson, Brink, K.H., Harvard University Press.
Eicken, H., et al. (2000), A key source area and constraints on entrainment for basin scale sediment transport by Arctic sea ice, Geophysical Research Letters, 26,1919-1922.
Guo, L., and R. W. Macdonald (2006), Source and transport of terrigenous organic matter in the upper Yukon River: evidence from isotope (613C, Al4C, and 6l5N) composition of dissolved, colloidal, and particulate phases, Global Biogeochemical Cycles, 20.
IPCC (2007), Summary for Policymakers, Cambridge University Press, Cambridge, U.K. New York, U.S.A.
Jahne, B., et al. (1987b), Measuremen of the diffusion coefficients of sparingly soluble gases in water, Journal of Geophysical Research, 92,10767-10776.
Liss, P. S. (1983), Gas Transfer: experiments and geochemical implications, in Air-Sea exchange of gase and particles, edited by P. S. Liss and W. G. N. Slinn, pp. 241-298, D. Reidel Publishing Company.
Liss, P. S., and L. Merlivat (1986), Air-sea gas exchange rates: introduction and synthesis, in The role of air-sea exchange in geochemical cycling, edited by P. Buat-Menard, pp. 113-127, D. Reidel
Macdonald, R. W., et al. (1998), A sediment and organic carbon budget for the Canadian Beaufort Shelf, Marine Geology, 144, 255-273.
43
Macdonald, R. W., and Y. Yu (2006), The Mackenzie Estuary of the Arctic Ocean in Handbook of Environmental Chemistry (Estuaries), edited by Springer-Verlag, pp. 91-120, Berlin Heidelberg.
McGillis, W. R., and R. Wanninkhof (2006), Aqueous C02 gradients for air-sea flux estimates, Marine Chemistry, 98,100-108.
Millero, F. J. (2000), The carbonate system in marine environments in Chemical Processes in Marine Environments, edited by A. Gianguzza, Pelizzetti, E., Sammartano, S., pp. 9-41, Springer.
Nightingale, P.D., et al. (2000), In situ evaluation air-sea gas exchange parameterizations using novel conservative and volatile tracers, Global Biogeochemical Cycles, 14,373-387.
Portielje, R., and L. Lijklema (1995), Carbon dioxide fluxes across the air-water interface and its impact on carbon availability in aquatic systems, Limnology and oceanography, 40, 690-699.
Prinsenberg, S.J. 1986. Salinity and temperature distribution of Hudson Bay and James Bay. Elsevier Oceanographic series, 44,163-186.
Raymond, P. A., et al. (2000), Atmospheric C02 evation, dissolved inorganic carbon production, and net heterotrophy in the York River estuary, Limnology and Oceanography, 45,1707-1717.
Redfield, A. C, et al. (1963), The influence of organisms on the composition of sea-water, in The sea, edited by M. N. Hill, pp. 26-77, Wiley.
Rosenberg, D. M., et al. (1997), Large-scale impacts of hydroelectirc development, Environmental Reviews, 27-54.
Rouse, W. R., et al. (1997), Effects of climate change on the freshwaters of arctic and subarctic North America, Hydrological Processes, 11.
Sakshaug, E. (2004), Primary and secondary production in the Arctic seas, in The oganic carbon cycle in the Arctic Ocean, edited by R. Stein, Macdonald, R., pp. 57-81, Springer.
Sarmiento, J. L., and N. Gruber (2006), Ocean Biogeochemical Dynamics, 503 pp., Princeton University Press, Princeton.
44
Sarmiento, J. L, et al. (1998), Simulated response of the ocean carbon cycle to anthropogenic climate warming, Nature, 393, 245-249.
Schindler, D. W. (1974), Eutrophication and Recovery in Experimental Lakes: Implications for Lake Management, Science, 184, 897-899. Schindler, D. W. (1977), Evolution of phosphorus limitation in lakes, Science, 195, 260-262.
Stumm, W., and J. J. Morgan (1996a), Dissolved carbon dioxide in Aquatic Chemistry, edited by W. Stumm, Morgan, J.J., pp. 148-205, Wiley-Interscience.
Stumm, W., and J. J. Morgan (1996b), Atmosphere-Water interactions in Aquatic Chemistry, edited by W. Stumm, Morgan, J.J., pp. 206-249, Wiley-Interscience.
Takahashi, T., et al. (1993), Seasonal variation of C02 and nutrients in the high-latitude surface oceans: a comparative study, Global Biogeochemical Cycles, 7,843-878.
Thorpe, S. A. (1982), On the clouds of bubbles formed by breaking wind-waves in deep water, and their role in air-sea gas transfer, 304,155-210.
Tsunogai, S., et al. (1999), Is there a "continental shelf pump" for the absorption of atmospheric C02?, Tellus, 51B, 701-712.
Vorosmarty, C, and D. Sahagian (2000), Anthropogenic disturbance of the terrestrial water cycle, BioScience, 50, 753-765.
Wanninkhof, R. (1992), Relationship between wind speed and gas exchange over the ocean, Journal of Geophysical Research, 97, 7373-7382.
Wanninkhof, R., et al. (1985), Gas exchange-wind speed relation measured with sulfur hexafluoride on a lake, Science, 227,1224-1226.
Wanninkhof, R., and W. R. McGillis (1999), A cubic relationship between air-sea C02
exchange and wind speed, Geophysical Research, 26,1889-1892.
Weiss, R. F. (1974), Carbon dioxide in water and seawater: the solubility of a non-ideal gas, Marine Chemistry, 2, 203-215.
Weiss, R. F., and B. A. Price (1980), Nitrous oxide solubility in water and seawater, Marine Chemistry, 8, 347-359.
45
Wetzel, R. G. (2001), Limnology: Lake and River Ecosystems, 3 ed., Academic Press.
Whitman, W. C. [1923), The two-film theory of gas absorption, Chemical metallic engineering 29,146-148.
Zeebe, R. E., and D. Wolf-Gladrow (2001), C02 in seawater: equilibrium, kinetics, isotopes, 346 pp., Elsevier Ltd., Amsterdam.
46
Chapter 3: Summer air-water carbon dioxide exchange and water chemistry of the Churchill River as it enters Hudson Bay
Emmelia C. Stainton1, Raymond H. Hesslein2, Tim N. Papakyriakou1, and David G. Barber1
1Centre for Earth Observation Science Department of Environment and Geography, University of Manitoba, Winnipeg, Manitoba R3T2N2, Canada. fisheries and Oceans Canada Freshwater Institute, Department of Fisheries and Oceans, 501 University Crescent, Winnipeg, Manitoba R3T2N6, Canada.
Abstract
A continuous CO2 monitor was moored in the central channel of the Churchill
River with the purpose of measuring gas concentrations in water and air allowing
the calculation of the CO2 exchange of the river just before it enters the estuarine
system of Hudson Bay. The CO2 flux ranged from -51.4 to 30.8 with a mean flux of
-5.8 mmol nr2 day-1. The delivery rate of C02wto Hudson Bay via the Churchill River
estuary was calculated based on morphometry and flow, and ranges from 39 to
1335 kmol day1, with a mean rate of 355 ± 328 kmol day1. Net production (Pnet)
and respiration (R) for the Churchill River were estimated directly from diurnal
peaks measured using the gas monitor, resulting in a mean Pnet of 82.6 and a mean R
of 77.0 mmol m2 day1 respectively. These results indicate that the Churchill River is
a net CO2 sink throughout much of the 2007 open-water season, which is striking
47
given that high latitude rivers and aquatic systems in general are considered strong
sources of atmospheric CO2.
3.1 - Introduction
Arctic and sub-arctic rivers, many with expansive watersheds covering
several biomes, pass over carbon rich permafrost soils that currently store over
one-third of the global terrestrial soil carbon (Gorham 1991). This process results in
high concentrations of organic carbon in rivers, which are available for
decomposition to dissolved CO2 (CChw). C02w is determined by photosynthetic
uptake, decomposition of organic carbon (respiration), air-water gas exchange, and
precipitation/dissolution reactions (Wetzel 2001). These factors are in turn
controlled independently by physical, chemical, and biological processes, resulting
in many variables influencing the aquatic carbon cycling.
An abundance of organic carbon in most high latitude rivers and lakes is the
reason that many are net heterotrophic systems, typically with CO2 levels in surface
waters well in excess of the overlying atmosphere (Kling et al. 1992; Cole et al.
1994). This excess of CO2 in surface waters evades into the atmosphere. However, a
recent study by Tank et al. (2008) found several shallow lakes of the Mackenzie
Delta (with varying connectedness to the Mackenzie River in western Canada) to be
sinks for CO2 throughout the summer. With dissolved organic carbon (DOC)
concentrations similar to those reported for other high latitude rivers (Guo and
Macdonald 2006; Wetzel 2001), the partial pressure of CO2 (PCO2) in surface waters
48
reported for most of the Mackenzie Delta lakes investigated was less than
atmospheric pC02, and interestingly was lowest when DOC was highest (Tank et al.
2008].
Biogeochemical processes and carbon dynamics in rivers are complicated by
the fact that they reflect physical, chemical, and biological conditions integrated
throughout the watershed. As a result only a few studies have investigated the
relationships among riverine biogeochemistry, flow dynamic and delivery
constituents, and air-riverine CO2 exchange (Telmer and Veizer 1999). Most
freshwater CO2 flux estimates have been made for mid-latitude lakes (Kelly et al.
2001; Sobek et al. 2005) and rivers (e.g. Jones et al. 2003), though some stream
studies have been published in recent years (Hope et al. 2001, Jones and Mulholland
1998). Few studies exist for high latitude lakes and river systems (Kling et al. 1992;
Hamilton et al. 1994; Tank et al. 2008), where baseline estimates are important,
especially since climate scale temperature changes are expected to influence DOC
mobilization (Guo and Macdonald 2006; Guo et al. 2007), seasonal hydrological
processes, and metabolic rates (Rouse et al, 1997). By comparison several studies
have reported on the nature of gaseous carbon cycling from northern wetlands and
bogs, with a strong regional representation within the Hudson Bay Lowland (HBL)
area of sub-arctic central Canada (Macrae et al. 2004).
Hudson Bay drains a vast area (~3.7xl06 km2, approximately one third the
area of Canada) and receives a huge amount of freshwater —710 km3 year1 (18%
of the annual discharge to the Arctic Ocean) - from several major river systems
49
(Dery et al. 2005). The natural flow and annual discharge cycle of many of these
rivers has been altered by hydroelectric development (Dery et al. 2005).
The lower reaches of inflowing rivers pass through the HBLs along the south
and south-western fringe of Hudson Bay over soils that are rich in organic carbon
(OC) and nutrients, resulting in high input of DOC in the coastal marine waters
(Granskog et al. 2007). The prevalence of high DOC associated with freshwater
plumes within the Bay differentiates this water body from other coastal seas within
the Canadian Arctic, and as a result Hudson Bay is far less effective in taking in
atmospheric CO2 relative to other peripheral seas (Else et al. 2008), which
characteristically are strong CO2 sinks (e.g. Miller et al. 2002).
To date, studies have reported on the OC delivery into coastal environments
by rivers, but none have examined the nature of the air-water CO2 exchange
upstream of the estuarine environment. This information would provide insight into
possible differences in carbon cycling that may exist between the riverine and
estuarine environments. Despite some recent advances in our understanding of the
physiochemical and gas exchange characteristics of the estuary/shelf system in
Hudson Bay (Else et al. 2008), little is known of the air-water CO2 exchange and
water chemistry of the inflowing rivers.
The primary objective of this study is to describe air-water CO2 exchange at
the lower reaches of the Churchill River waters as it enters the Churchill River
estuary and to relate observed fluxes to basic flow dynamics and characteristics. In
the following we present an overview of 1) the variability of dissolved/atmospheric
50
CO2/O2 and general water chemistry variables measured in the Churchill River
throughout the 2007 open-water season, 2) the flux of CO2 from collected field data
estimated using the thin boundary-layer (TBL) model (Wanninkhof 1992), 3)
production and respiration estimates for the lower reaches of the Churchill River,
and 4) the delivery rate of dissolved CO2 from the Churchill River to Hudson Bay.
This represents part 1 of a 2-part study examining the relationships among
biogeochemistry, flow dynamic and delivery constituents, and air-water CO2
exchange in both a river and estuarine system. It is important that we understand
the processes that define the transport and transformation of carbon from the
terrestrial landscape, through the aquatic network and into the marine system. A
better understanding of the inflowing Churchill River will allow us to gauge the
relative contributions of marine and riverine (terrestrial] influences in the mixing
estuarine waters, and ultimately identify the Churchill River watershed's input to
Hudson Bay. This research is part of a larger study that investigates the coupling
between freshwater quality and quantity and marine processes within Hudson Bay,
and how Hudson Bay may be impacted by the combined effects of climate change
and hydroelectric development (Freshwater-Marine Coupling in the Hudson Bay
IRIS, ArcticNet, http://www.arcticnet.ulaval.ca).
51
3.2 - Methods
All sampling associated with this work took place during the summer of 2007
at the lower reaches of the Churchill River (station CH7, Lat: 58.6693°, Long: -
94.1883°, Figure 3.1a]. The Churchill River enters into the coastal waters of
southwestern Hudson Bay (Lat: 58.7910°, Long: -94.2082°; Figure 3.1a) draining an
area of 281,300 km2 across the Canadian Shield, the HBLs, and the northern interior
prairies where the river's headwaters are situated. Approximately two-thirds of the
lower Churchill River's historical flow was diverted in 1976 for hydroelectric power
generation on the Nelson River, and hence the Churchill River's discharge is
controlled. The resulting reduction in water level negatively impacted use of the
river by residents of Churchill, Manitoba, and as a result a rock weir extending the
width of the river (~ 2km) was constructed approximately 10km upstream and
south of the town site in 1998. This raised the local water level of the Churchill River
by approximately 2m, flooding an area of approximately 6.5 km2 extending 10km
upstream (south) from the weir (Bezte, 2006). Our sampling location was situated
approximately 0.5 km upstream of the weir (Figure 3.1a). Sampling was done in two
ways: through discrete sample collection, and though continuous monitoring.
52
Figure 3.1. a) Map of the Churchill River as it enters Hudson Bay (Landsat 7 satellite image, Natural Resources Canada] b) CO2/O2 air/water gas monitor moored on float at Churchill River station CH7. Two 20W solar panels [right side) charge two 12V batteries that power the gas monitor system (inside white box).
3.2.1 - Discrete water sampling
A Van Dorn style water sampler was used to collect surface water samples
(0.25m) at station CH7 1-2 times per month. Dissolved inorganic carbon (DIC) was
collected from the water sampler first by piercing the outlet tubing and drawing 4ml
of water into a syringe triple-rinsed with sample. This was expelled through a
septum cap into Milli-Q triple-rinsed 12 ml borosilicate glass vials containing 20 u.1
of concentrated phosphoric acid. These vials had been evacuated and filled three
times (20 s cycles) with ultra high purity (UHP) helium containing 1% neon and 1%
acetylene. Neon and acetylene had been added as tracers to determine whether
sample contamination from atmosphere had occurred and to validate the
performance of the gas chromatograph used. Using this approach, neon, a very
53
insoluble gas, should be the same concentration in all samples unless the sample is
contaminated with air. Acetylene is similar to CO2 in solubility. The ratio of
acetylene/neon changes relative to sample volume making the addition of this gas a
useful check for sampling technique. Vials were pressurized to 6 PSI such that
leakage, if it occurred, would be out and not into the vials. In the field, vials were de-
pressurized to atmospheric pressure by piercing the septum with a 20-gauge needle
prior to sample collection. Addition of 4 mL of sample liquid to the sealed vial raised
the internal pressure to again insure that any leakage would be out of the vials.
Added acid both preserved the sample and converted the DIC contents to CO2.
A proportion of the water sample for other variables was transferred into 2L
amber polyethylene bottles that were acid-cleaned with 10% hydrochloric acid
(HCL) solution and triple rinsed with Milli-Q water. Upon sample collection, bottles
were triple rinsed with a small aliquot of sample water, filled to the shoulders,
tightly capped, and kept cool until processed. Laboratory processing took place
immediately upon return from the field. Sub-samples were collected for DOC,
particulate organic carbon and nitrogen (POC/N), chromophoric dissolved organic
matter (CDOM), alkalinity, chlorophyll a (Chla), and total suspended solids (TSS)
analyses.
Background water chemistry data (pH, DOC, POC/N, and nutrients) had been
collected in the vicinity of station CH7, and was made available to this study by the
provincial hydroelectric provider, Manitoba Hydro. These samples were collected
and analyzed during 1995-2005. Ancillary information on Chla and POC/N were
54
also available to this study from June and July 2006 as part of another study by
Centre for Earth Observation Science (Department of Environment and Geography,
University of Manitoba) personnel.
3.2.2 - Continuous CCh, 02, and water sampling
In situ measurement of dissolved gasses has been a successful technique for
determining air/water concentrations and flux estimates (Sellers et al. 1995). Here,
a continuous automated air-water CO2 monitoring instrument (following Carignan,
1998) was used to measure CO2 and O2 concentration in both air and water. This
instrument was moored on a 4 by 6 foot float and powered by two 20-watt solar
panels connected to two 12-volt deep cycle batteries. The water intake tube was
covered with 100-micron copper mesh and positioned at 25cm below the water
surface, while the air intake was affixed at approximately 1 meter above the water
surface (Figure 3.1b).
The gas monitoring instrument (GM) consists of two main parts: 1) a "dry"
box containing a gas pump (Spectrex Corp.® AS-200), an infrared CO2 gas analyzer
(LiCor® LI 820), a programmable data logger controller (Campbell Scientific®
CR10X), an oxygen sensor (Qubit® S101 diffusion based O2 sensor), and in-line gas
drying Nafion tubing (Perma Pure® LLC ME-060-24BB) surrounded by silica
desiccant 2) a "wet" box containing a peristaltic pump (Autoclude® M500), a gas
exchanger (Celgard Inc.®), and a thermistor (Omega Engineering®) (Figure 3.2).
Figure 3.2. Schematic of air/water gas monitor: wet box consisting of a peristaltic water pump, a thermocouple housing, a gas exchanger, and two solenoid valves (SV); dry box consisting of an air pump, drying compartment, O2 sensor, IR gas analyzer, and data logger.
DRY BOX
drying compartment
infrared gas
analyzer
Air IN -(gas loop)
WET BOX
pump *~1 I I
data logger
+ r\.u
3 Air OUT
' gas loop)J ^SV
4 air/water gas exchanger
4 = 3 1 r m-ling I X
thermocouple n n
H20 peristaltic
pump
Air OUT (gas loop)
H20 OUT • 4
H2OIN
Atmospheric CO2 and O2 was measured by drawing air into the system with
the gas pump, drying it with the in-line Nafion tubing, and passing it through the
oxygen sensor and CO2 analyzer before leaving the unit. Dissolved gas in water was
measured by drawing water into the wet box via the peristaltic pump, through the
in-line thermistor, and then through the gas permeable tubing in the gas exchanger
before leaving the system. An air pump ran concurrently with the peristaltic pump
56
in a closed loop that was connected to the outer chamber of the gas exchanger. Gas
in this closed loop equilibrates with the dissolved gasses in the water flowing
through the gas exchanger, and is then pumped through the air loop (drying
compartment, O2 sensor, CO2 analyzer) (Figure 3.2).
The data logger was programmed to run the instrument for 30-minute cycles
every 2 hours throughout the 2007 open-water season. The CO2 analyzer was
calibrated monthly with gas standard (span CO2 = 380 and 777 parts per million
(ppm)) and zeroed with UHP nitrogen.
For part of the 2007 open water season, a conductivity, temperature, depth,
pH, and turbidity probe (RBR XR-620) was moored in surface waters adjacent to the
GM's intake and programmed to log hourly.
3.2.3 - Other sources of data
River discharge at station CH7 was estimated by summing flows from the
Churchill River at Red Head Rapids (Lat: 58.1208°, Long: -94.625°) and the
inflowing Deer River (Lat: 58.015°, Long: -94.1956°) using daily discharge data
obtained from the Water Survey of Canada
(http://www.wsc.ec.gc.ca/hydat/H20/index_e.cfm).
Water and air temperature, wind speed, and water level data are routinely
monitored by Manitoba Hydro at the Churchill River pump-house station
(approximately 6km upstream of the weir), and the data were made available to this
study. Meteorological data (wind speed, air temperature, precipitation) were also
57
collected at the Churchill weather station by Environment Canada
[www.weatheroffice.gc.ca/city/pages/mb-42_metric_e.html) and hourly data were
downloaded and used here.
3.2.4 - Laboratory procedures
A pre-combusted 25 mm glass fiber (GF/F) filter was positioned on an acid-
soaked [24 hours) and Milli-Q water triple-rinsed polypropylene filter holder and
tightly sealed with a Milli-Q water rinsed o-ring. An acid-soaked and Milli-Q water
rinsed polypropylene syringe was triple rinsed with a small volume of sample, and
then filled with 60 mL of sample. This sample was then pushed through the filter [in
filter holder), discarding the first 30 mL of sample, and collected into 1) a pre-
combusted 5mL glass vial containing 25 uL of 50% high pressure liquid
chromatography grade phosphoric acid [DOC), and then 2) a pre-combusted 50 mL
amber borosilicate glass vial [CDOM). Samples were stored at 4°C until analysis.
Another 60 mL of sample was collected and pushed through the same filter, filter
holder and syringe [total volume of 120 mL). This filter was folded in half, carefully
wrapped in pre-combusted aluminum foil, and frozen for analysis of POC/N.
Nutrient samples [NO2VNO3", PO43") were prepared after Simpson et al.
(2008) and concentrations were measured on a continuous flow Bran + Luebbe
Auto-Analyzer 3 [AA3). DOC samples were analyzed on a Shimadzu TOC-5000A.
CDOM absorbance was measured using a Hewlett-Packard 845 2A diode array
spectrophotometer in an acid-cleaned 10-cm quartz cell and scanned at 2 nm
58
intervals between 250 and 820 nm against Milli-Q water. Absorption (X) was
calculated after Stedmon et al. (2000]. POC/N was measured on a Carlo Erba NA-
1500 Elemental Analyzer following acid fuming of sample filters to remove
carbonate (following Verardo et al, 1990).
Using a vacuum filtration system, particulate matter from a known volume of
sample (ranging from 250 to 500 mL) was collected onto 42.5mm GF/F filters for
the analysis of TSS (pre- combusted and weighed filters) and Chla. These were
stored in Petri dishes and kept frozen and dark (to reduce Chla degradation) until
analysis. TSS was determined after Stainton et al. (1977). Chlorophyll a
concentrations were measured on a Turner Designs fluorometer (10-AU) following
24-hour extraction in 90% acetone at 4°C (JGOFS 1994).
DIC samples were analyzed on a Varian CP4900 MicroGC. Gas was introduced
into the GC by piercing the septum cap of the vials with a needle and allowing gas to
flow (vials pressurized in the field at time of sampling) through Nafion drying tubing
to the GC sample loop. The sample line was flushed with UHP helium between
samples. Alkalinity samples were analyzed on a PC TitratlON Plus following the
Gran alkalinity titration method (Stainton et al. 1977).
3.2.5 - Calculations
C02w data measured by the GM was recorded as parts per million (ppm) and
was converted to concentration (umol L1) using:
[C02] = a • PC02
59
where a is the solubility coefficient calculated for CO2 (mol L_1 atnr1) (Weiss and
Price 1980].
O2 measurements were converted from ppm to concentration (umol L1)
using:
a 0.2095 x 106
[OJ = x x pO0water 1 22.4136 /?02air
where a is the solubility coefficient calculated for O2 (mol I/1 atnr1) (Weiss 1970].
The flux of C02 was calculated after Wanninkhof (1992]:
Flux = a x kC02 x ApC02
where
^°2 = °-31Ul20mXf^)"1/2
1600/ f ( 4 ]
ApCCh is the air-water difference in pCOi, Uiom is the wind speed at 10m from the
water surface, and Sc is the Schmidt number (a dimensionless coefficient] (Jahne et
al. 1987). Fluxes are sensitive to the parameterization of gas transfer with wind
(see Appendix B). The Wanninkhof (1992) model was chosen because it lies in
between the highest and lowest k values calculated at high wind speeds. At low wind
speeds, little difference exists between the various models to calculate k.
Net production (Pnet) and respiration (R) rates were estimated using the
diurnal C02w concentration data as:
ACO. P^tCmmolm^day"1) = — x Z x 1000 - Flux
At (5)
60
and
A CO. R (mmol m'2 day"1) = — x Z x 1000 + Flux
where ACChw is in mmol I/1, At is time in hours, Z is a measure of river depth (2.5m),
and Flux is in mmol nr2 hour1. ACChw/ At was determined by plotting CO2 w versus
time for each diurnal peak (upslope for R = increase in CChw, and down slope for
Pnet= decrease in CChw). Since CO2 air/water flux was calculated for each data point,
the average flux between the bottom and top (for R, or top and bottom for Pnet) of a
diurnal curve was either subtracted (equation 5) or added (equation 6) to arrive at
an estimate of Pnet and R. Gross production (Pgross) is the sum of Pnet and R from one
diurnal curve.
We estimated the approximate upstream distance from station CH7 that a
parcel of river water travels within a 24-hour period in order to identify the possible
reaches of the river where Pnet and R influence dissolved gas concentrations. This
was done by dividing the flow rate by the wetted perimeter (channel depth x
channel width), where flow is the sum of Churchill River flow at Red Head Rapids
and the inflowing Deer River, channel width was assumed to be 2000m (the
approximate width of the Churchill River at CH7), and channel depth was assumed
to be 2.5m (depth at station CH7). Flow data was also used to estimate the delivery
rate of CChw from the Churchill River to Hudson Bay. This was calculated by
multiplying CChw concentration (mmol L1 from the GM, averaged to match daily
flow data) by river discharge (L day1) to get kmol day1.
61
3.3 - Results
3.3.1 - Environmental conditions
Climate data collected by the Environment Canada meteorological station in
Churchill indicate daily average air temperature peaks in late July and August, while
precipitation amount and frequency peaks in August and September (Figure 3.3a).
Discharge rates are initially high following spring ice breakup (June 5), decrease
steadily throughout July, and peak once again in early August coinciding with
several substantial rainfall events. On average, Churchill was warmer and dryer
(until September) in 2007 compared to climate norms (1971-2000) (Figure 3.3b).
Table 3.1: Mean and standard deviation of Churchill River annual flow and precipitation before and after the Churchill River Diversion (CRD, created in 1976) and before and after the Churchill River rock weir construction (10km south of the Town of Churchill, built in 1998).
Period Pre-CRD Post-CRD Pre-weir Post-weir
Year 1971-1976 1976-2008 1976-1998 1998-2008
Flow (mV) Mean S.D. 1361 280 380 435 373 386 396 521
Precipitation (mm) Mean S.D. 30.3 25.9 38.0 29.8 36.5 28.4 41.4 32.4
Historical discharge indicates the mean annual flow of the Churchill River
(metered at Red Head Rapids) has decreased by approximately 70% between pre-
(1971 to 1976) and post-diversion (1976 to 2008) flow regimes (Table 3.1, Figure
3.4). Prior to the diversion, the mean annual flow of the Churchill River was 1361 ±
280 m3s1, though in subsequent years mean annual flow was substantially reduced
and was much more variable (380 ± 435 m3s1j (Table 3.1, Figure 3.4). Mean annual
precipitation was greater and more variable following the diversion (38 ± 29.8 mm,
compared to 30.3 ± 25.9 mm Table 3.1).
Figure 3.3. a) Churchill daily mean air temperature and daily precipitation (Environment Canada), and river discharge (combination of Red Head Rapids and the Deer River, Water Survey of Canada) throughout the 2007 open-water season, b) precipitation and air temperature monthly averages from 2007 and historical climate norms (1971-2000) (Environment Canada).
1000
5/1 5/15 5/29 6/12 6/26 7/10 7/24 8/7 8/21 9/4 9/18 10/2 10/16 10/30 Date
U l 2 O
I
31971-2000 climate norms precip
-1971-2000 climate norms temp
12007 precip -2007 temp
J A Month
63
Following weir construction, the mean annual flow in the lower reaches of
the Churchill River increased slightly from 373 (pre-weir) to 396 m3s-1 (post-weir)
and was more variable (standard deviation (S.D.) of 385.9 for pre-weir to 521 m3s'1
for post-weir, Table 3.1), which can be attributed partly to the large peak flow in
2005 (Figure 3.4). Mean annual precipitation was also greater throughout the post-
weir period (41.4 mm compared to 36.5 pre-weir) and more variable (S.D. of 28.4
for pre-weir to 32.4 for post weir, Table 3.1)
Figure 3.4. Historical flow data from the Churchill River at Red Head Rapids (Water Survey of Canada) and monthly mean precipitation (Environment Canada) between 1971-2008.
180 dischage monthly mean precip
86 91 Year
3000
64
3.3.2 - Water chemistry
Water chemistry collected from station CH7 between June and October 2007
is summarized in Table 3.2. TSS, chlorophyll a, and DIC values all peak in August
while alkalinity is highest from July to mid-August. DOC, CDOM, and NO2" + NO3" are
highest in late August and October. PON and POC follow a similar pattern where
concentrations are higher in June and August, and lower in early July and October.
PO4 is exceptionally high after ice-out, however concentrations decrease throughout
the open-water season to levels that are consistent with previous years data for the
Churchill River (Table 3.2, Figure 3.5). pH deviates most from its mean of 8.2 in July
(Table 3.2).
Table 3.2: Summary of water chemistry data from Churchill River site CH7, collected June-October 2007.
Parameter
pH
Alkalinity (ueq L"')
DIC (nmol L1)
DOC (mg I/1)
PON(mgL')
POC(mgL')
N0 2 +N0 3 (mgL ' ) PO^mgL-') Chlorophyll a (\ig L"1)
CDOM (abs 350 nm) TSS(mgL')
11-Jun
8.2
1306
1031
8.79
0.10
0.70
0.05 0.18 1.3
12.2 7.7
17-Jun
8.2
1361
1244
8.68
0.07
0.58
0.11 0.02 2.0
10.8 4.4
12-Jul
7.5
1423
1162
8.34
0.05
0.36
0.03 ~
2.7
8.7 2.3
Sampling Date 28-Jul
8.7
1531
1290
8.39
0.07
0.50
0.23 0.04 2.7
8.6 1.0
16-Aug
8.2
1448
1325
12.06
0.08
0.54
0.06 -
6.3
20.5 10.4
20-Aug
—
1375
1325
11.12
0.08
0.62
0.06 —
3.4
18.3 8.2
2-Oct
8.1
1312
1017
14.56
0.04
0.33
0.09 0.06 1.4
30.2 2.1
Figure 3.5. Churchill River summer water chemistry data collected from 1995-2005 (data from Bezte, 2006) and from 2006-2007 (this study).
700
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300
200
100
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o z
n • Conductivity ApH
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7.6
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j oPOC BTSS U XPON
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Apr-95 Apr-97 Apr-99 Apr-01 Apr-03 Apr-05 Apr-07
Date
66
Data indicate that little variation occurred in each of pH, conductivity,
chlorophyll a, DOC, POC, TSS or dissolved nutrients between 1994 and 2007, though
PON decreased throughout this sampling period (R2 of 0.68) [Figure 3.5). NO2/NO3
concentrations were more variable for samples collected in 2006 and 2007
compared to those between 1994 and 2005 [Figure 3.5).
3.3.3 - Continuous CO2,02, and water sampling
Air and water concentrations of CO2 and O2, water temperature, and wind
speed were measured at station CH7 for consecutive days in June [4), July (16), and
August (6 and 4). Mean and standard deviation of C02w, C02a, 02w, and 02a
concentrations were 11.7 ± 6.6,15.8 ± 1.7, 315.5 ± 16.3, and 309.5 ± 17.7 umol L1
respectively, while mean water temperature and wind speeds at CH7 were 16.8 ±
2.8 °C and 4.1 ± 2.4 m s-1 (Table 3.3). Interestingly, our C02w mean value is similar to
those reported by Hamilton et al. (1994) from several bogs in the HBLs during the
early summer. Tank et al. (2008) report similar C02w (published as PCO2) in four out
of six intensively studied shallow lakes of the Mackenzie Delta (western Canadian
arctic) from July to September 2006.
Dissolved and atmospheric gas concentrations and water temperature were
plotted over time and vary diurnally throughout most of the monitoring period.
From the three periods of near-continuous operation, C02Wvaries asynchronously to
02w and water temperature most obviously in July (Figure 3.6). C02w drops during
the daylight hours while 02w levels increase, both of these as a result of primary
67
production. Not surprisingly water temperature follows a diurnal pattern of daytime
warming and nighttime cooling, although the signal is not as strong as for the
dissolved gasses. Atmospheric gas concentrations also fluctuate diurnally, following
a similar pattern to their dissolved counterparts (i.e. atmospheric CO2 tends to
increase at the same time as a decrease in dissolved CO2, and vice versa).
Table 3.3: Mean, standard deviation and range of (i) all measured gas monitor data (n = 224), (ii) flux (n = 224), and (iii) net production, respiration, and gross production at 2.5m (Pnet/down-slope peak, n = 27; R/upslope peak, n = 26; Pgross/complete peak, n = 24) from Churchill River station CH7.
Parameter Mean S.D. Max Min
TW(°C)
Wind (m s"1)
(^ (umolL 1 )
02w (umol L"1)
C02a (umol L"1)
C02w (y,mol L"1)
Flux (mmol m"2 day"1)
Pnet (mmol m"2 day"1)
R (mmol m"2 day"1)
PBross (mmol m"2 day"1)
16.8
4.1
309.5
315.5
15.8
11.7
-5.8
82.6
77.0
155.3
2.8
2.4
17.7
16.3
1.7
6.6
11.9
58.9
44.1
95.5
21.3
13.2
353.9
357.4
20.1
28.5
30.8
200.7
168.2
355.6
10.3
0.1
283.2
285.9
13.3
3.4
-51.4
5.8
18.5
31.3
At the beginning of August the concentrations of C02w and 02w increase and
their diurnal signals no longer follow the trend seen throughout much of July
(Figure 3.6). This takes place simultaneously with increases in river discharge,
water level, and CTDP measured turbidity, and decreases in pH, salinity, and water
temperature (Figure 3.7).
68
Figure 3.6. Diurnal fluctuations of CO2W, CO2A, O2W, O2A, and Tw at Churchill River site CH7 throughout the 2007 open-water season. Data were measured and recorded at 2-hour intervals using a continuous air/water CO2/O2 gas monitor.
v V
340 •
+ 320 S
3 300 a.
8/9 8/11
3.3.4 - C02 flux
Air-river flux estimates range between -51.4 to +30.8 mmol nr2 day-1 (mean -
5.8 mmol nr2 day1, Table 3.3) and indicate that this system is a net sink for CO2
throughout most of the 2007 open-water season. Fluxes tended to be more negative
during the longest sampling period (July) when river discharge was declining and
most water chemistry variables and water temperature were increasing (Figure
3.8). Days are also longer during this period. Fluxes shift to being mostly positive at
the very end of July, concurrent with increasing river discharge, decreasing water
temperature, and increasing turbidity (Figure 3.7). The transformation from a CO2
sink to a source at the end of July is likely due in part to light limitation of primary
production within the river.
69
Figure 3.7. a) and b) Turbidity, salinity, water temperature and pH collected at Churchill River station CH7 (data measured and recorded using an RBR XR-620) c) Churchill River discharge (combination of Red Head Rapids and the Deer River, Water Survey of Canada) and water level (recorded at the Churchill pump house station, approximately 3km upstream of station CH7)
7/27 7/29 7/31 8/10 8/12 8/14 8/16 Date
Tremblay et al. (2006) report CO2 fluxes from the Lee River in southern
Manitoba and some northern Quebec rivers and reservoirs that are well within the
range we estimate for the Churchill River throughout the 2007 open water season
(Table 3.4). Our results are also consistent with those reported by Tank et al.
(2008), who found that several Mackenzie delta lakes were net CO2 sinks
throughout the summer growing season. However, the results from this study differ
from those mentioned above in three key ways: 1) our fluxes were determined from
70
continuous measurements, and therefore our mean reflects the net carbon
movement and is a true indicator of net flux, 2) we collected a higher frequency of
data (n = 224), and 3) our results are over a much tighter range [82 mmol nr2 day1).
Figure 3.8. Estimated CO2 flux from measured values at Churchill River site CH7 throughout the 2007 open-water season. A positive flux indicates a supersaturation of CO2 in water with respect to air (source), while a negative flux indicates an undersaturation of CO2 in water with respect to air (sink). The Churchill River is a CO2 sink throughout most of the monitored open-water season.
7/28 7/30 8/1
Date
3.3.5 - Production and respiration
Estimated Pnet, R, and Pgross for the Churchill River average 82.6 ± 58.9, 77.0 ±
44.1, and 155.3 ± 95.5 mmol nr2 day1 respectively for the water depth observed at
CH7 (2.5m) (Table 3.3 and Figure 3.9). Kling et al. (1992) report primary production
values (based on 24hr incubation of discrete samples with 14C) from Toolik Lake
(northern Alaska, United States of America) surface waters ranging from 0 to ~125
mgC nr3 day1, which is at the lower end of our estimated Pnet range (101 to 3530
mgC nr3 day1, converted from mmol nr2 day1).
71
Table 3.4: Summary of flux estimates made by Tremblay et al. (2006) for a Manitoba River and some northern Quebec hydroelectric reservoirs. Samples were collected from a 20L chamber connected to a non-dispersive infrared gas analyzer, and data was stored every 20s over a 5-10 minute sample period.
Site Caniapiscau River Laforge 2 La Grande 3 La Grande 4 Lee River
Site type reservoir reservoir reservoir reservoir
river
Location lat 54.95°, long 69.49° lat 54.57°, long 70.15° lat 53.72°, long 75.68° lat 54.36°, long 73.13° lat 50.33°, lone 95.84°
Year sampled 2003 2003 2003 2003 2002
N 28 11 35 27 5
co2
Mean 15.20 18.93 38.79 26.77 7.50
(mmol
S.D. 23.38 29.06 29.04 15.38 24.45
m"2 day1)
Min Max -0.27 95.71 -6.29 95.37 -5.70 114.63
-27.18 58.94 -32.65 247.51
The Churchill River varies in channel width, depth, and substrate
composition, which will influence physical and chemical water properties, gas
exchange, and Pnet and R throughout its length. Pnet is the rate of drawdown of CCbw,
per unit time, per unit area, at a given depth, though this drawdown occurs
simultaneous to air-water interface gas exchange. The surface flux of CO2 is thus
subtracted from the rate of CO2 drawdown when flux is positive, and added when
flux is negative so that we do not overestimate the rate of change attributed to
biological production. R is thus the increase of CChw (per unit time, per unit area, at a
given depth), and is corrected by subtracting the flux when negative, and adding the
flux when positive. Since both A[C02]w and CO2 flux vary throughout the day, the
relative correction of CO2 flux to Pnet and R estimates also varies (mean corrections
of 11% and 10.8% respectively).
72
Figure 3.9. Estimated Pnet and R throughout the 2007 open water-season at Churchill River station CH7. Both Pnet and R were calculated from gas data measured using a continuous air/water gas monitor.
6/17 6/19
The Churchill River delivers between 39 to 1335 kmol CO2W day-1 to the Churchill
River Estuary and Hudson Bay (Figure 3.10), and represents a slight dilution based
on surface water CO2W concentration measurements made in the estuary basin and
Hudson Bay from June to October, 2007 (range 14.5 to 24.2 umol L1, mean 17.7 ±
2.8 |imol I/1, n = 34; E.C. Stainton, unpublished data). Clearly a function of river
discharge, the delivery rate of C02w declines throughout July (as discharge and
water level decline) and sharply increases in late July/early August as water levels
and river flow rate rises.
3.4 - Discussion
Relating an observed air-surface gas exchange to river properties and
processes is complicated because, depending on flow rate, conditions being
measured do not necessarily reflect biogeochemical processes at the site, but rather
73
the integration of processes over a stretch of the river and period of time. For part of
the monitoring period, our GM-measured air/water gas data has a time lag [up to 12
hours) from the diurnal signal expected for the light/dark periods of Churchill, and
we believe this to be dependent on the Churchill River's discharge. A faster flow rate
brings water that has traveled a greater distance in 24-hours, and thus represents
gas exchange and the influence of Pnet and R that have occurred over a greater
distance and at an earlier time upstream. Conversely, with slower flow the water
being sampled is presumably influenced to a greater extent by local primary
production and air-water gas exchange.
Figure 3.10. Delivery rate of CC>2w to the Churchill River estuary and 24-hour upstream flow distance upstream from station CH7.
16001 r 14
1400 +
O 400-U
200 f — 24-hour flow distance upstream
o Delivery rate 0 -I 1 ^
8 ° ^ o o >
12 £ cr o
3
o
C </> >-t a> P3
+ 6
+ 4
6/17 6/24 7/1 7/8 7/15 7/22 7/29 8/5 8/12
Date
74
For this reason, we estimated the upstream distance that a parcel of river
water travels within a 24-hour period. Based on flow and a 24-hour travel time, the
furthest upstream source is ~14 km (maximum flow] while the closest is ~3.5 km
(minimum flow) (Figure 3.10). The reaches of the river influencing the dissolved gas
measurements, CO2 flux, and Pnet and R are local at either end of the recorded flow
range and within the area flooded by the Churchill River weir construction
(approximately 10 km upstream of the weir). The relatively uniform basin
characteristics of the stretch of the river influencing the water we monitored are
consistent with the assumption to use the wetted perimeter area when estimating
Pnet and R.
Our Pnet and R estimates are unique in that they represent: 1) the integral of
riverine Pnet and R. This is a significant improvement over approaches that focus on
estimating pelagic or benthic Pnet and R, 2) have used methods that usually don't
measure R, and 3) have historically used discrete samples collected in very narrow
windows of space and time, and as such have huge variance.
Estimation of the C02w delivery rate is important because it provides a
reference point for estimating estuarine CO2 exchange. Implications of an altered
hydrological cycle and temperature regime resulting from climate warming are
largely unknown for high latitude watersheds, and therefore further changes in flow
of the Churchill River and riverine biogeochemical cycling with respect to CCtew will
affect the load delivered to Hudson Bay. It is thus necessary to establish baseline
conditions as climate change has the potential to influence the delivery of nutrients
75
and carbon from this and other large high latitude rivers. Regional variation in both
the quality and quantity of riverine inputs is expected, which will affect coastal
regions as the fresh water delivered integrates climate change impacts over entire
watersheds.
The interrelationship between nutrients, respiration, and temperature will
determine the fate of high latitude aquatic systems as sources or sinks for CO2 (Kling
et al. 1992). It is generally accepted that nutrient concentrations and light, rather
than temperature, limit primary production in high latitude aquatic and terrestrial
environments (Billings et al. 1984). However decomposition and respiration rates
are temperature dependent (Billings et al. 1982) making these high latitude
environments potential sources of CO2 with increasing mean annual temperature. In
spite of this, sufficient nutrient concentrations and increasing water temperatures
could enhance aquatic primary production and thus provide a greater sink for CO2.
This complex interplay of variables, most uncertain for a changing climate, makes
projections for the future role of high latitude aquatic systems as CO2 sources or
sinks speculative.
Using a continuous GM was a successful method for tracking changes in
air/water CO2 flux, Pnet, and R in the Churchill River, and could provide a means to
monitor climate feedbacks (water flow and temperature changes) in the future, as
climate change has the potential to change Pnet, R, and runoff water quality in
response to changes in hydrology and temperature. Little is known about the impact
of changes in hydrology and temperature in high latitude lakes and rivers, however
76
this data establishes a background for further studies/monitoring on a river that is
well positioned (location, large area integrated, ease of access, local infrastructure,
etc.) to be a sentinel watershed in tracking climate change.
Production and respiration estimates here indicate that the Churchill River is
a net productive system as it enters Hudson Bay, which is contrary to much of the
existing CO2 exchange literature for lakes and rivers (Kling et al. 1991; Cole et al.
1994; Sobek et al. 2005). The view that, globally, aquatic ecosystems are sources of
CO2 to the atmosphere has been perpetuated in the literature over the past >10
years, though recent findings reported here and by Tank et al. (2008) suggest
otherwise.
This paper represents one of few accounts of carbon cycling in a high latitude
river throughout the peak growing season, and as a result further studies examining
high latitude air-water CO2 exchange are required to better understand this issue.
Identifying current conditions may lead to more confident predictions of future
nutrient and carbon cycling in high latitude rivers, and may provide some insight
into how rivers will affect upstream estuarine and marine coastal environments.
77
Acknowledgements
We gratefully acknowledge the field support and assistance from K. Swystun,
R. Whitten, P. Fitzpatrick, J. Batstone, and the Churchill Northern Studies Centre. We
thank Manitoba Hydro, Water Survey of Canada, and Environment Canada for
providing river discharge, water chemistry, and meteorological data. We also thank
M. Stainton for helpful discussion and comments. E. Stainton is funded by awards
from the Natural Sciences and Engineering Research Council of Canada (NSERC,
PGS-M) and the Northern Scientific Training Program (NSTP, Indian and Northern
Affairs). This research was conducted within the umbrella of Arctic-Net, a Network
of Centers of Excellence (http:/www.arcticnet-ulaval.ca). Support for this research
was provided by NSERC grants (Barber & Papakyriakou) and the Canada Research
Chairs program (Barber).
78
References
Bezte, C.L. 2006. Lower Churchill River water level enhancement weir project post-project monitoring: assessment of water chemistry and phytoplankton responses to operation of the Project - Year VII, 2005, A report prepared for Manitoba Hydro by North/South Consultants Inc.
Billings, W.D., J.O. Luken, DA Mortensen, and K.M. Peterson. 1982. Arctic tundra: A source or sink for atmospheric carbon dioxide in a changing environment? Oecologia 53: 7-11, doi:10.1007/BF00377129.
Billings, W. D., K.M. Peterson, J.O. Luken, and D.A. Mortensen. 1984. Interaction of increasing atmospheric carbon dioxide and soil nitrogen on the carbon balance of tundra microcosms. Oecologia 65: 26-29, doi:10.1007/BF00384458.
Carignan, R. 1998. Automated determination of carbon dioxide, oxygen, and nitrogen partial pressures in surface waters. Limnol. and Oceanogr. 43: 969-975.
Cole, J. J., N.F. Caraco, G.W. Kling, and T.K. Kratz. 1994. Carbon dioxide supersaturation in the surface waters of lakes. Science 265:1568-1570.
Dery, S.J., M. Stieglitz, E.C. McKenna, and E.F. Wood. 2005. Characteristics and trends of river discharge into Hudson, James, and Ungava Bays, 1964-2000. J. of Climate 18: 2540-2557.
Else, B.G.T, T. Papakyriakou, M.A. Granskog, and J.J. Yackel. 2008. Observations of sea surface/CO2 distributions and estimated air-sea CO2 fluxes in the Hudson Bay region [Canada) during the open-water season. J. Geophys. Res. 113: C08026, doi: 10.1029/2007JC004389.
Gorham E. 1991. Northern peatlands: role in the carbon cycle and probable responses to climatic warning. Ecological Applications 1: 182-195.
Granskog, M.A., R.W. Macdonald, C.-J. Mundy, D.G. Barber. 2007. Distribution, characteristics and potential impacts of chromophoric dissolved organic matter (CDOM) in Hudson Strait and Hudson Bay, Canada. Cont. Shelf Res. 27: 2032-2050.
Guo, L. and R. W. Macdonald. 2006. Source and transport of terrigenous organic matter in the upper Yukon River: evidence from isotope (S13C, A14C, and 615N) composition of dissolved, colloidal, and particulate phases. Global Biogeochem. Cycles 20: GB2011, doi:10.1029/2005GB002593.
79
Guo, L., C.-L. Ping, and R. W. Macdonald. 2007. Mobilization pathways of organic carbon from permafrost to arctic rivers in a changing climate. Geophys. Res. Lett. 34: L13603, doi:10.1029/2007GL030689.
Hamilton, J.D., C.A. Kelly, J.W.M Rudd, R.H. Hesslein, and N.T. Roulet. 1994. Flux to the atmosphere of CH4 and CO2 from wetland ponds on the Hudson Bay lowlands (HBLs). J. Geophys. Res. 99: 1495-1510.
Hope, D., S.M. Palmer, M.F. Billet, and J.J.C. Dawson. 2001. Carbon dioxide and methane evasion from a temperate peatland stream. Limnology and Oceanography 46: 847-857.
Jahne, B., G. Heinz, and W. Dietrich. 1987. Measurement of the diffusion coefficients of sparingly soluble gases in water. Journal of Geophysical Research 92:10767-10776, doi:10.1029/JC092iC10pl0767.
JGOFS. 1994. JGOFS Report No. 19-Protocols for the Joint Global Ocean Flux Study fJGOFS) core measurements.
Jones, J.B., JR and P.J. Mulholland. 1998. Carbon dioxide variation in a hardwood forest stream: an integrative measure of whole catchment soil respiration. Ecosystems 1:183-196.
Jones, J.B., JR, E.H. Stanley, and P.J. Mulholland. 2003. Long-term decline in carbon dioxide supersaturation in rivers across the contiguous United States. Geophysical Research Letters 30:1495, doi: 10.1029/2003GL017056.
Kelly, C.A., E. Fee, P.S. Ramlal, J.W.M. Rudd, R.H. Hesslein, C. Anema, and E.U. Schindler. 2001. Natural variability of carbon dioxide and net epilimnetic production in the surface waters of boreal lakes of different sizes. Limnology and Oceanography 46:1054-1064.
Kling, G.W., G.W. Kipphut, and M.C. Miller. 1991. Arctic lakes and streams as gas conduits to the atmosphere: implications for tundra carbon budgets. Science 251: 298-301.
Kling, G.W., G.W. Kipphut, and M.C. Miller. 1992. The flux of C02 and CH4 from lakes and rivers in arctic Alaska. Hydrobiologia 240: 23-36.
Macrae, M. L., R. L. Bello, and L.A. Molot. 2004. Long-term carbon storage and hydrological control of CO2 exchange in tundra ponds in the Hudson Bay Lowland. Hydrological processes 18: 2051-2069.
80
Miller, LA, P.L. Yager, K.A. Erickson, D. Amiel, J. Bade, J. Kirk Cochran, M.-E. Garneau, M. Gosselin, D.J. Hirschberg, B. Klein, B. LeBlanc, and W.L. Miller. 2002. Carbon distributions and fluxes in the North Water, 1998 and 1999. Deep-Sea Research Part II49: 5151-5170.
Rouse, W. R., M.V. Douglas, R.E. Hecky, A.E. Hershey, G.W. Kling, L. Lesack, P. Marsh, M. McDonald, B.J. Nicholson, N.T. Roulet, and J.P. Smol. 1997. Effects of climate change on the freshwaters of arctic and subarctic North America. Hydrological Processes 11:873-902.
Sellers, P., R.H. Hesslein, and C.A. Kelly. 1995. Continuous measurement of CO2 for estimation of air-water fluxes in lakes: an in situ technique. Limnology and Oceanography 40: 575-581.
Simpson, K. G., J.-E. Tremblay, Y. Gratton, and N. M. Price. 2008. An annual study of inorganic and organic nitrogen and phosphorus and silicic acid in the southeastern Beaufort Sea. Journal of Geophysical Research. 113: C07016, doi: 10.1029/2007 JC004462.
Sobek, S., L.J. Tranvik, and J.J. Cole. 2005. Temperature independence of carbon dioxide supersaturation in global lakes. Global Biogeochemical Cycles 19: GB2003, doi:10.1029/2004GB002264.
Stainton, M.P., M.J. Capel, and F.A.J. Armstrong. 1977. The chemical analysis of fresh water. Canadian Fisheries Marine Service, Miscellaneous Special Publication 25,180 pp.
Stedmon, C.A., S. Markager, and H. Kaas. 2000. Optical properties and signatures of chromophoric dissolved organic matter (CDOM) in Danish coastal waters. Estuarine, Coastal, and Shelf Science 52: 67-278.
Tank, S.E., F.W. Lesack, and R.H. Hesslein. In press. Northern delta lakes as summertime CO2 absorbers within the arctic landscape. Ecosystems, DOI: 10.1007/S10021-008-9213-5.
Telmer, K. and J. Veizer. 1999. Carbon fluxes, pC02 and substrate weathering in a large northern river basin, Canada: carbon isotope perspectives. Chemical Geology 159: 61-86.
Tremblay, A., J. Therrien, B. Hamlin, E. Wichmann, and L.J. LeDrew. 2005. GHG emissions from boreal reservoirs and natural aquatic ecosystems, p. 209-232. In A. Tremblay, L. Varfalvy, C. Roehm, and M. Garneau [eds.], Greenhouse gas emissions -fluxes and processes. Springer.
81
Wanninkhof, R. 1992. Relationship between wind speed and gas exchange over the ocean. Journal of Geophysical Research 97: 7373-7382.
Weiss, R.F. 1970. The solubility of nitrogen, oxygen, and argon in water and seawater. Deep Sea Research 17: 721-735.
Weiss, R.F. and B.A. Price. 1980. Nitrous oxide solubility in water and seawater. Marine Chemistry 8: 347-359.
Wetzel, R.W. 2001. Limnology: lake and river ecosystems. Academic Press.
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Chapter 4: Air-water carbon dioxide exchange and physiochemical processes of the Churchill River estuary
Emmelia C. Stainton, Tim N. Papakyriakou, and David G. Barber
Centre for Earth Observation Science Department of Environment and Geography, University of Manitoba, Winnipeg, Manitoba R3T2N2, Canada.
Abstract
A continuous CO2 monitor was used to measure pCQi in water and air along a
fresh to marine transect in the Churchill River estuary allowing the calculation of
air-water CO2 exchange. A multiparameter probe was used to profile conductivity,
temperature, depth, and pH at stations along the transect, and water was sampled
for chemical analysis of various parameters. Estuarine stratification and seasonal
mixing patterns were determined using the integrated data set. The Churchill River
estuary was stratified by water source, with fresh river water flowing over brackish
marine water. The nature of the stratification varied seasonally with temperature
and changing river discharge and tidal amplitude. Conservative mixing of several
water chemistry parameters was observed. The river was a source of chlorophyll a,
dissolved organic carbon, coloured dissolved organic matter, and particulate organic
carbon and nitrogen to the estuary basin, while the marine water was a source of
salinity, alkalinity, and dissolved inorganic carbon to the estuary basin. Air-water
flux estimates ranged between -11.04 to +22.76 mmol nr2 day-1, with mean fluxes
being -1.07 ± 1.90 mmol nr2 day1 at low tide, 0.06 ± 5.93 mmol nr2 day1 at high
83
tide, and -0.37 ± 4.79 mmol nr2 day-1 overall. Both between and within month
variation in CO2 flux exists. These results indicate that the Churchill River estuary is
a net CO2 sink throughout much of the 2007 open-water season, which is striking
given that high latitude estuaries are generally considered sources of atmospheric
CO2. These results are consistent with other data collected from the Churchill River
[upstream of the estuary) and with flux estimates made for Hudson Bay.
4.1 - Introduction
Growing evidence linking increased levels of atmospheric anthropogenic
carbon dioxide (CO2) to climate change has heightened interest in regional
budgeting and modeling of atmospheric CO2 concentrations to identify current
trends and understand carbon pathways among various interconnected carbon
stocks at or near to the Earth's surface. Model and budget accuracy are still quite
limited by major gaps in understanding of important components of the global
carbon cycle, in particular within the productive coastal marine environments of the
high latitudes in both hemispheres (Borges, 2005). Coastal shelves and estuaries,
though only making up 7% of oceanic surface area, are not included in oceanic
carbon budgets (Borges, 2005). The role of these regions in the global CO2 cycle is
largely unknown, with uncertainties of up to 75% (Takahashi et al., 2002), and it is
not known whether the inclusion of polar shelves and estuaries will increase the
efficiency of high latitude oceans as a carbon sink, or alter the region's role from a
strong sink to source of atmospheric CO2 (Borges, 2005). An understanding of the
84
underlying dynamics of carbon cycling in these environments is particularly
important as there is a general lack of good observational data on the time/space
characteristics of CO2 cycling (Borges, 2005). The estuarine environment in the
northern high latitudes is generally unknown, and their complexity is hard to study
because it involves processes at a spatially and temporally dynamic boundary.
The surface waters of Hudson Bay are slightly fresher relative to other
peripheral seas (Prinsenberg 1986) resulting from a large freshwater input from
several large rivers (713.6 km3 year1, Dery et al. 2005) that drain the Northern
Great Plains and the boreal belt of eastern Canada. In 1976 Churchill River flow was
reduced by approximately two-thirds when water was diverted to the Nelson River
to enhance hydroelectric power generation (Bezte 2006). Although the Churchill
River comprises only 2.8% of the freshwater input to Hudson Bay (20.57 km3 year1,
Dery et al. 2005), it's watershed is similar in characteristic to other larger rivers
entering Hudson Bay, like the Nelson and La Grande Rivers of northern Manitoba
and Quebec, Canada. The lower Churchill River passes through the Hudson Bay
Lowlands, a vast area with organic carbon-rich peatlands (Gorham 1991) dotted
with thousands of small ponds, bogs, and fens.
To date, research conducted within the Churchill River and estuary system
has focused largely on water quality/chemistry in relation hydroelectric
development (Technical Reports by North-South consultants prepared for Manitoba
Hydro), however a recent study by Kuzyk et al. (2008) investigated the
85
physical/chemical/biological coupling within the Churchill estuary system through
the spring/melt period.
The objective of this research is to describe 1) the current physical and
chemical conditions of the Churchill River estuary, 2] the air-water exchange of CO2,
and how physiochemical conditions influence the CO2 exchange, and 3) how these
conditions and processes vary with sampling location, season, depth, and tidal
amplitude. We relate CO2 flux to water column properties and processes along the
estuarine salinity gradient in order to determine the extent to which this area is a
source or sink for CO2, and the conditions (chemical, biological, and physical) that
govern CO2 flux.
An understanding of the CO2 flux between estuarine waters and the
atmosphere in polar regions is important for assessing potential climatic feedbacks,
especially with the increasing influence of climate change in these areas. This work
also ties in with other CO2 budgeting studies in the Churchill area, and will lead to a
better understanding of carbon transformations at the land/freshwater/marine/air
interface. The accessibility of the Churchill region compared to other high latitude
estuaries makes it a useful study area for modeling the impact of climate change in
an arctic coastal area over a range of potential changes. This work at this locale
suggests that there may be significant value in developing a long-term monitoring
program in the Churchill estuary that would serve to track the influence of climate
change on northern estuaries over time.
86
4.2 - Methods
4.2.1 - Study area
The Churchill River enters into the coastal waters of southwestern Hudson
Bay (Lat: 58.791°, Long: -94.2082°; Figure 4.1] draining an area of 281,300 km2
across the Canadian Shield, the Hudson Bay Lowlands, and the northern interior
prairies where the river's headwaters are situated. During the summer months, the
Churchill River flows over a rock weir and into an enclosed estuary basin (~13 km
long by 3 km wide) before entering Hudson Bay (Figure 4.1). The aerial extent of
estuarine waters varies significantly with tidal cycle (Figure 4.1), with exposed
rocks and mudflats visible at low tide. At high tide seawater flows into the estuary
basin raising the water level by 2-4 meters. This causes the inflowing river water to
pile up downstream of the weir and flow over the dense salt water, forming a "salt
wedge" (Kuzyk et al., 2008). From late November to May the Churchill River and
Hudson Bay are ice-covered (Kuzyk et al., 2008).
All sampling associated with this work took place between June-October
2007, and was conducted in two ways: 1) estuary survey sampling (CTD casts,
discrete water sampling, and discrete dissolved/atmospheric CO2 sampling), and 2)
estuary basin quick CTD casting. Information on the inflowing Churchill River water
was also collected throughout the 2007 open-water season (Stainton et al., in prep.
2009).
87
Figure 4.1. Churchill River (CH7) and estuary (CH1-5) sampling stations at a] high tide and b) low tide (Landsat 7 satellite image, Natural Resources Canada).
4.2.2 - Estuary survey sampling
Sample locations were situated within the estuary basin and extend 13 km
along a transect from fresh to marine water (Figure 4.1). Discrete water samples
were collected from each of 5 stations at high tide (CH5 to CHI), and each of four
stations at low tide (CH4 to CHI) once each month throughout the sampling period.
Water samples
A Van Dorn style water sampler was used to collect surface water (0.5m from
top) and bottom water samples (0.5m from bottom, stations CH4-CH1 only) once
per month, at both high and low tide. Sample for dissolved inorganic carbon (DIC)
analysis was collected from the water sampler first by piercing the outlet tubing and
88
drawing 4mL of water into a syringe triple-rinsed with sample. Water was expelled
through a septum cap into 12 mL borosilicate glass vials containing 20 iL of
concentrated phosphoric acid following Stainton et al. (in prep. 2009).
Sample for other variables was transferred into acid-cleaned (10% HCL
solution) and Milli-Q and sample water rinsed 2L amber polyethylene bottles, filled
to the shoulders, tightly capped, and kept cool until processed. Laboratory
processing took place in Churchill immediately upon return from the field. Sub-
samples were collected for alkalinity, chlorophyll a, total suspended solids (TSS),
dissolved organic carbon (DOC), chromophoric dissolved organic matter (CDOM),
and particulate organic carbon and nitrogen (POC/N) analyses.
Air-water CO2 sampling
Concurrent with water sample collection, dissolved CO2 concentrations were
measured in the surface waters of each station (CH5 to CHI) using a continuous
flow automated air-water CO2 monitoring instrument (Figure 4.2). The system is
described in detail by Stainton et al. (in prep. 2009), and is only briefly reviewed
here. The gas monitor (GM) consists of two main parts: 1) a "dry" box containing an
infrared CO2 gas analyzer (LiCor® LI 820), a programmable data logger controller
(Campbell Scientific® CR10X), a gas pump (Spectrex Corp.® AS-200), and in-line gas
drying Nafion tubing (Perma Pure® LLC ME-060-24BB) surrounded by silica
desiccant 2) a "wet" box containing a gas exchanger (Celgard Inc.®), a peristaltic
pump (Autoclude® M500), and a thermistor (Omega Engineering®) (Figure 4.2).
89
Figure 4.2. Air/water gas monitoring instrument. Dry box: 1-air pump, 2-drying compartment, 3-O2 sensor, 4-gas analyzer, and 5-data logger; wet box: 6-peristaltic water pump, 7-thermocouple housing, 8- air/water gas exchanger, and 9- solenoid valves.
Atmospheric CO2 was measured by drawing air into the system with the gas
pump, drying it with the in-line Nafion tubing, and passing it through the CO2
analyzer before leaving the unit. Dissolved gas in water was measured by drawing
water into the wet box via the peristaltic pump, through the in-line thermistor, and
then through the gas permeable tubing in the gas exchanger before leaving the
system. An air pump ran concurrently with the peristaltic pump in a closed loop that
was connected to the outer chamber of the gas exchanger. The water intake tube
was covered with 100-micron copper mesh and positioned at 0.25 m below the
90
water surface, while the air intake was affixed at approximately 0.5 m above the
water surface. The GM was run for 30 minutes at each station resulting in one air
and one water pCOi measurement.
Following surface air/water pCCh measurement, water was also sampled
from mid-column and bottom waters (0.5m from bottom) at stations CH3 and CH4
using the GM to determine whether variation existed with depth. Water was
pumped through the gas exchanger for 10 minutes at each depth (with data
recorded every 10 seconds), resulting in one average water pCOi measurement.
The CO2 analyzer was calibrated bi-weekly with a certified gas standard
(span CO2 = 380 parts per million (ppm)) and zeroed with ultra high purity
nitrogen. Drift was less than 2 ppm between calibrations.
CTD profiling
At each sampling station, a conductivity, temperature, depth, and pH probe
(model RBR XR-620) was used for water-column profiling. Data were collected and
recorded 6 times per second.
4.2.3 - Estuary basin CTD profiling
Water column profiling (using RBR XR-620) was also done by visiting
stations within the estuary basin during a two-hour window around high or low
tide. Here, more stations were sampled along a transect between stations CH5 and
91
CH2 each month at both high and low tide in order to get a "snapshot" of the
estuary's stratification and mixing patterns.
4.2.4 - Other sources of data
Discharge of the inflowing Churchill River was estimated by summing flows
from the Churchill River at Red Head Rapids (Lat: 58.1208°, Long: -94.625°) and the
inflowing Deer River (Lat: 58.015°, Long: -94.1956°) using daily discharge data
obtained from the Water Survey of Canada
(http://www.wsc.ec.gc.ca/hydat/H20/index_e.cfm).
Meteorological data (precipitation and air temperature) were collected at the
Churchill weather station by Environment Canada
(www.weatheroffice.gc.ca/city/pages/mb-42_metric_e.html) and hourly data were
downloaded and used here. River water temperature data are routinely monitored
at the Churchill River pump-house station by Manitoba Hydro (~6km upstream of
the weir), and the data were made available to this study.
Water chemistry and air/water CO2 concentrations were collected from
Churchill River station CH7 throughout much of the 2007 open-water season. Water
chemistry samples were collected and processed as outlined above. Air/water CO2
concentrations were measured on a second GM (identical to that used for estuary
surveying) that collected continuous data (sampling every 2 hours). Details on this
system appear in Stainton et al. (in prep. 2009).
92
4.2.5 - Laboratory and analytical procedures
DIC samples were analyzed on a Varian CP4900 MicroGC. Gas was introduced
into the GC by piercing the septum cap of the vials with a needle and allowing gas to
flow [vials pressurized in the field at time of sampling) through Nafion drying tubing
to the GC sample loop following Stainton et al. (in prep. 2009). The sample line was
flushed with UHP helium between samples. Alkalinity samples were analyzed on a
PC TitratlON Plus following the Gran alkalinity titration method (Stainton et al.
1977).
Using a vacuum filtration system, particulate matter from a known volume of
sample (ranging from 250 to 500 mL) was collected onto 42.5mm GF/F filters for
the analysis of chlorophyll a and TSS (pre- combusted and weighed filters). These
were kept frozen and dark (to reduce chlorophyll a degradation) until analysis.
Chlorophyll a concentrations were measured on a Turner Designs fluorometer (10-
AU) following 24-hour extraction in 90% acetone at 4°C (JGOFS 1994). TSS was
determined after Stainton et al. (1977).
Samples for DOC and CDOM were filtered through a 25mm glass fiber (GF/F)
filter contained in a polypropylene filter holder previously rinsed with Milli-Q and
sample, and collected into 1) a pre-combusted 5mL glass vial containing 25 uL of
50% HPLC grade phosphoric acid (DOC), and 2) a pre-combusted 50 mL amber
borosilicate glass vial (CDOM). Samples were stored at 4°C until analysis. A total
sample volume of 120mL was filtered through this GF/F filter, which was wrapped
in pre-combusted aluminum foil and frozen for analysis of POC/N.
93
DOC samples were analyzed on a Shimadzu TOC-5000A. CDOM absorbance
was measured using a Hewlett-Packard 8452A diode array spectrophotometer in an
acid-cleaned, Milli-Q water rinsed, 10-cm quartz cell and scanned at 2 nm intervals
between 250 and 820 nm against Milli-Q water, and absorption [X) was calculated
after Stedmon et al. (2000). POC/N was measured on a Carlo Erba NA-1500
Elemental Analyzer following acid fuming to remove carbonate (following Verardo
et al., 1990). Nutrient samples (NO2/NO3", PO43) were analyzed following Simpson
et al. (2008) and concentrations were measured on a continuous flow Bran +
Luebbe Auto-Analyzer 3 (AA3).
4.2.6 - Calculations
Dissolved CO2 data measured by the GM was converted to concentration
(umol Lr1) from molar fraction using:
[C02] = a • PC02 w
where a is the solubility coefficient calculated for CO2 (mol L1 atnr1) (Weiss and
Price 1980).
CO2 flux was calculated after Wanninkhof (1992):
Flux = a x kCO x A/CO, 2 2, (2)
where
*CO2 = 0.31U12
0m*[_ScV''2
94
Uiom is the wind speed at 10m from the water surface, Sc is the Schmidt number (a
dimensionless coefficient) (Jaime et al. 1987), and ApCCh is the water-air difference
inpC02.
4.3 - Results
4.3.1 - Hydrographic conditions
Both the amount and frequency of precipitation peaked in late August and
throughout September, while air temperature peaked in mid July (Figure 4.3a).
Water temperature also peaked in mid July (Figure 4.3b) and variation is in line
with the air temperature record throughout the entire open-water season. River
discharge was initially high following ice breakup, decreased to a minimum in late
July, and peaked once again in early August coinciding with several substantial
rainfall events (Figure 4.3b).
Tidal amplitude was greatest around September 1 and at the beginning/end
of October, and lowest around the 20th day of each month. Much of our sampling
took place when tidal amplitude was high, except for in August where amplitude
decreased from 4 meters to 2 meters during the sampling trip (Figure 4.3c).
4.3.2 - Physical conditions
From the estuary basin water column profiles, we produced depth-distance
plots of salinity and temperature conditions within the Churchill River estuary at
95
both high and low tide, from June to October (Figure 4.4). Inflowing Churchill River
water was warmer than Hudson Bay marine waters during all sampling trips except
October. This caused thermohaline stratification within the estuary basin with
warm, river water flowing into and over the cooler, denser marine waters. In the
fall, the slightly cooler inflowing river water led to inverse temperature
stratification within the estuary basin.
In June, high river discharge volume and melting ice floes from the break-up
of Hudson Bay resulted in both the river plume extending further towards the
estuary basin mouth, and the presence of a thicker fresh surface-water layer at both
high and low tide compared to the other sampling months. The effect of river
discharge volume on estuary basin thermohaline stratification can also be seen in
the high tide panels of July and August. Reduced discharge volume in July led to a
sharp thermocline (and halocline) when marine waters intrude up the estuary
basin. In August, high river discharge (Figure 4.3b) caused river water to flow over
the marine water at high tide, resulting in more gradual stratification and river
water extending further north in the estuary basin compared to that measured in
July. At low tide (June, July, August) river water reached the estuary mouth, creating
a brackish mixture with the underlying marine water in the estuary basin. Cold
temperature and windy conditions in October are a possible explanation for the
pocket of cold water sampled in the estuary at low tide. Strong south winds during
sampling may have enhanced mixing in this uniformly shallow (<5m) area of the
estuary basin.
Figure 4.3. a) Churchill daily mean air temperature and precipitation (Environment Canada), b) Churchill River discharge (combination of Red Head Rapids and the Deer River, Water Survey of Canada) and water temperature (Manitoba Hydro pumphouse station), and c) Churchill harbour tide height (Fisheries and Oceans Canada), with sampling periods indicated by dashed vertical lines.
97
Temperature-salinity data (collected during estuary survey sampling and
basin CTD casting) is shown in Figure 4.5. All June casts at either tidal amplitude
were stratified in both temperature and salinity. This was also true for the July CTD
casts, though river and marine waters were warmer. The slight separation in the
July point cluster between 20-30 psu and ~7-12 °C was a result of separate CTD
casting days, where the cooler data were collected on July 12, and the warmer
portion was collected between July 15-19. In August there was a clear separation
between the low tide and high tide CTD casts, and for this reason the data was
divided into two groups (all low tide casts in dark green, and all high tide casts in
light green) (Figure 4.5). Thermohaline stratification persisted in August, however
at low tide the basin was largely isothermal with observed vertical variation only in
salinity. In the fall (September and early October) the estuary was typically
isothermal, as both marine and river water undergoes seasonal cooling. Haline
stratification was present in the fall, both at high and low tide.
4.3.3 - Water chemistry
Results from the water chemistry analyses at estuary stations CHI to CH5
and river station CH7 from June to October are summarized in Table 4.1. Mean
dissolved oxygen, chlorophyll a, PON, POC, DOC, CDOM, Si, and PO4 concentrations
and pH were generally highest for river station CH7 and estuary station CH5, and
showed a decreasing trend from the river/upstream estuary stations towards
Hudson Bay (Table 4.1). Conversely, mean alkalinity concentration and salinity were
98
greatest for estuary station CHI, and values decreased moving upstream towards
river station CH7. DIC and TSS, and NO3+NO2 however, did not follow a decreasing
pattern from the river to marine station. TSS and NO3+NO2 were greatest for
stations CH4 and CHI, though variability in TSS was also greatest at these stations.
Mean DIC concentration is highest at estuary stations CH2 and CH4 (Table 4.1).
Stations CH2 and CH4 also appeared (based on visual observation in the field) to
experience significant turbulence due to fresh-marine water confluence (CH4), and
tidal movement (CH2). When sampling at high tide, the surface waters near CH4
would show a delineation between fresh and marine water in the form of ripples
(calm days) and wave fronts (windy days). Station CH2 was located in the mouth of
the estuary basin where tidal currents were quite strong, especially during the
receding tide.
Within station variability also existed and is evident in Table 4.1. Comparing
surface and bottom water means for each station, chlorophyll a, DOC, CDOM, and to
some extent PON, POC, and Si were all greater for surface waters. Conversely, DIC,
TSS (except station CH2), salinity and alkalinity were all larger at the bottom
compared to surface waters. Consistent vertical variation in DO, NO3+NO2 and PO4
was not observed. NO3+NO2 concentration was observed to be higher in surface
waters for stations CHI and CH2, though higher in bottom waters for stations CH3
and CH4, while PO4 concentration is greater for CHI and CH3 bottom waters and for
CH2 and CH4 surface waters (Table 4.1).
99
Figure 4.4. Depth, temperature, salinity cross-sections of Churchill River estuary CTD transects at high tide [left column) and low tide [right column) in June, July, August, and October 2007. Temperature is represented in colour (note all colour legends on the right side of figures are different), salinity is denoted by numbered contour intervals (ranging from 2-30), and individual CTD casts are shown as bold black vertical lines within the cross-sections. White circles on the high/low tide maps (bottom panels) are the CTD sampling locations. Temperature-salinity plots were created using Ocean Data View (Schlitzer, 1998).
High Tide
12.5
10
7.5
5
, 5
0 June
- rn^^v •-
' • « . - .
. _
, ^ _
^^^^^^^^^^^^^^i j j j
'
X 9 1
^ 7
56
18
16
14
12
10
8
L ^ * j | ^ ^ _ | ' .
\ «• July
W\ 17 ?
•I 13
12
August
^^^^MPT^^ 1 -7
VSs 7
| -----f V>
iff
n
13.5
13
II" f ^ 10.5
100
Table 4.1: Water chemistry of Churchill River estuary waters collected from stations along a fresh (CH5) to marine (CHI) gradient, at both high tide (HT) and low tide (LT), from surface (0.5m) and bottom waters (3 and 10m), from June to October 2007. Surface water samples were also collected from the inflowing Churchill River (CH7) upstream of the weir (Stainton et al., in prep. 2009).
Station
CH3 CH3 CH4 CH4 CH4
g CH4 5 CH5
CH5 CH7 CH7
CHI CHI CHI CH2 CH2 CH3 CH3
>. CH3 •3 CH3
CH4 CH4 CH5 CH7 CH7
CHI CHI CHI CHI CH2 CH2 CH2 CH2 CH3
1 CH3 | CH3
CH3 CH4 CH4 CH4 CH5 CH7 CH7
CH2 CH3 CH3 CH3
_ CH3
1 CH4 3 CH4
CH4 CH5 CH7
Depth Tide height m 0.5 10 0.5 0.5 3 3
0.5 0.5 0.5 0.5
0.5 0.5 10 0.5 10 0.5 0.5 10 10 0.5 0.5 0.5 0.5 0.5
0.5 0.5 10 10 0.5 0.5 10 10 0.5 0.5 10 10 0.5 0.5 3
0.5 0.5 0.5
0.5 0.5 0.5 10 10 0.5 0.5 3
0.5 0.5
LT LT HT HT HT HT HT HT
--
Mean SD. HT HT HT HT HT LT LT LT LT HT LT HT
--
Mean S.D. LT LT LT LT HT LT HT LT HT LT HT LT HT LT HT HT
--
Mean S.D. LT LT HT LT HT LT HT HT HT
-Mean S.D.
DO umol L'1
488 631 474 455 504 497 492 435 488 521 499 53 350 306 394 389 400 500 499 531 335
-330 480 439 399 412 73 326 415 357 412 374 420 365 417 436 399 417 324 429 441 437 424 416 430 402 37 357 361 385 346 373 379 428 369 436 395 383 30
DIC Umol L"'
1114 1012 883 1087 1292 1595 1098 943 1031 1244 1130 205 385 360 948 890 1213 596 552 887 562
-136 1354 1162 1290 795 393 1227 1292 1795 1904 1408 1278 1780 1914 1466 1268 2001 1335 1359 1355 2011 1381 1325 1325 1524 285 1140 899 1460 1073 1743 1241 1148 1521 1106 1017 1235 260
TSS mgL-1
6.5 13.0 4.9 6.8 4.6 10.2 7.4 5.5 7.7 4.4 7.1 2.7 3.0
0.6 7.2 0.4 2.8 1.2 4.4 3.4 1.3 4.0 2.5 2.3 1.0 2.6 1.9 4.6 4.4 6.2 18.6 2.6 5.8 2.4 3.8 4.2 3.8 2.7 6.6 5.6 16.3 2.8 7.2 10.4 8.2 6.5 4.5 5.7 4.0 2.7 4.2 8.0 5.6 4.4 45.2 5.2 2.11 8.7 12.9
Chla ftpL-1
1.03 0.80 1.54 1.59 0.42 0.44 1.84 1.96 1.33 1.97 1.29 0.59 0.53 0.06 0.38 0.25 0.28 1.10 1.42 0.40 0.52 2.29 2.46 2.66 2.71 2.67 1.27 1.06 1.66 2.77 1.17 0.58 1.85 2.86 0.26 0.55 2.13 2.51 0.20 1.60 3.00 4.68 0.55 3.93 6.27 3.43 2.22 1.65 1.10 0.94 0.70 1.03 0.67 1.05 0.94 0.70 1.33 1.41 0.99 0.25
PON ugN mL"1
-0.085
-0.034
-0.047
--
0.097 0.069 0.066 0.026
-0.045 0.031 0.082
-0.050 0.050 0.032 0.049
---
0.049 0.070 0.051 0.016
----------
0.052 0.069 0.023 0.076 0.065 0.122 0.036 0.088 0.081 0.075 0.069 0.028
-0.044 0.052 0.051 0.056 0.048 0.034 0.037 0.046 0.045 0.046 0.007
POC HgC mL''
-0.621
-0.551
-0.267
---
0.698 0.578 0.543 0.164
-0.128 0.120 0.160 0.491 0.297 0.348 0.221 0.281
---
0.355 0.499 0.290 0.137
--------
0.391 0.531 0.100 0.426 0.506 0.991 0.220 0.608 0.538 0.623 0.493 0.242
-0.230 0.353 0.192 0.415 0.315 0.280 0.218 0.292 0.329 0.291 0.071
DOC HM 691 579
-725
--
737
-732 723 698 61
-197 171 222 215 454 538 174 279
--
708 695 699 396 228
---------
846 958 114 523 903 825 248 921 1005 927 727 318
-1148 583 1010 206 881 1175 416 1194 1214 870 378
Salinity PSU
--10.43 22.14
---
0.08
---
10.88 11.04
-25.58 27.69 24.18 25.34 14.41 9.80 27.82 22.01
-9.17 0.08
--
18.61 9.61 12.44 1.97
28.73 27.34 8.58 1.37
30.75 27.70 4.24 1.31
30.51 15.72 4.10 1.68 1.52 0.08
--
12.38 12.35 5.56 7.91 18.77 5.48
27.72 8.97 3.11 22.77
--
12.54 9.22
CDOM abs 350
12.6 10.3 11.7 11.4 6.8 3.6 12.1 11.1 12.2 10.8 10.3 2.9 3.7 2.1 1.5 2.5 2.3
-6.9 1.7 3.6 8.2 7.5 8.8 8.7 8.6 5.1 3.0 10.7 16.2 1.4 2.5 14.5 16.9 0.6 1.9 16.7 18.9 0.7 8.8 17.7 15.4 3.6 17.6 20.5 18.3 11.3 7.4 25.2 34.8 15.0 27.4 3.6 22.8 32.4 10.5 30.6 30.2 23.3 10.3
Alk ^ q L"1
1445 1631 1320 1357 1858 1997 1330 1342 1306 1361 1495 249 1740 1891 2043 1938 1924 1741 1554 2061 1705 1513 1940 1435 1423 1531 1746 224 1664 1492 2114 2082 1634 1460 2227 2083 1504 1525 2222 1850 1413 1352 2099 1376 1448 1375 1718 329 1250 1517 1815 1577 2118 1550 1369 1958 1320 1312 1578 295
SiuM HM 3.88 1.34
-9.10 13.92 6.62
-6.09 16.58 20.89 9.80 6.73 2.53 2.24 1.74 3.60 2.95 3.76 2.11 2.08 5.27 2.08 0.21 0.33
--
2.41 1.40 5.89 4.98 1.93 5.10 6.94 7.02 2.44 4.22 9.94 10.66 1.90 6.71 6.04 7.53 5.19 7.40
--
5.87 2.52 11.90 8.08 6.16 7.57 4.32 10.51 5.30 6.40 21.84 20.65 10.27 6.22
N03
mgL-' 0.008 0.040 0.019 0.009 0.109 0.031 0.047 0.020 0.048 0.114 0.045 0.038 0.106 0.049 0.202 0.035 0.046 0.052 0.041 0.072 0.085 0.074 0.039 0.060 0.033 0.232 0.080 0.062 0.447 0.255 0.076 0.065 0.167 0.011 0.038 0.037 0.108 0.058 0.190 0.457 0.017 0.162 0.069 0.321 0.061 0.062 0.144 0.140 0.465 0.056 0.182 0.079 0.250 0.224 0.719 0.446 0.268 0.089 0.278 0.210
P04 pH mgL1
0.020 8.04 0.002 7.89
- 8.51 0.010 8.43 0.051 7.81 0.047 7.85 0.076 --0.109 8.13 0.177 8.17 0.018 8.19 0.057 8.11 0.057 0.25 0.024 8.20
-- 8.29 0.044 8.23 0.044 8.30 0.039 8.27 0.002 8.21
- 8.49 0.035 8.13 0.043 8.24 0.011 8.86
-- 8.51 0.105 8.69
-- 7.50 0.036 8.73 0.038 8.33 0.028 0.33 0.017 8.60 0.016 8.71 0.034 8.44 0.059 7.96 0.012 8.58 0.013 8.83 0.053 8.44 0.054 8.42 0.025 8.76 0.020 8.74 0.051 8.42 0.016 8.35 0.020 8.86 0.032 8.78 0.045 8.37 0.063 8.59
- 8.25
.. 0.033 8.54 0.018 0.24 0.149 8.64 0.147 8.63 0.031 8.61 0.215 8.58 0.116 8.56 0.220 8.64 0.070 8.88 0.028 8.52 0.003 -0.058 8.14 0.104 8.58 0.078 0.20
101
Figure 4.5. Temperature-salinity scatterplot of all CTD casts taken within the Churchill River estuary throughout the 2007 open-water season. Data is grouped by sampling month: red = June, blue = July, green = August (light green for high tide casts, dark green for low tide casts), black = September/October.
20 • June • July
August (HT) • August (LT) • Sept/Oct
0 + V-v
X 10 20
Salinity (psu) 30
When partitioned by sampling month, DO decreased from June to October,
while NO3+NO2 concentration increased from June to a maximum in the fall (Table
4.1). All of chlorophyll a, PON, and POC concentrations were greatest in June and
August, while TSS, DOC, and CDOM were all highest in October, and lowest in July.
DIC concentration was greatest in August, while alkalinity was higher in the summer
102
months relative to the spring and fall. Both PO4 and Si concentrations were highest
in October, while pH was slightly higher in July (Table 4.1).
Figure 4.6 shows the relationship between the water variables (DOC, CDOM,
POC, PON, Alk, DIC, chlorophyll a, and pH) and salinity. Here, point location is
determined by the left y-axis and x-axis variables, while the point colour reflects the
right y-axis variable. DOC and CDOM concentrations were both greatest for the
lower salinity samples. This trend is also evident for POC and PON, however there is
more variability in concentration for samples with salinity ranging 0 to 10 psu.
Chlorophyll a and pH follow a similar trend, decreasing with increasing salinity,
though there are a few data points with a pH of less than 8.2 with salinities between
0-10 psu. In general, alkalinity and DIC concentrations increase with increasing
salinity (Figure 4.6).
Some water chemistry variables were also sampled for the Churchill River
estuary region between March and May 2005 by Kuzyk et al. (2008), DOC, NO3+NO2,
and chlorophyll a, which are comparable to values presented here.
Figure 4.6. Water chemistry of the Churchill River estuary between June and October, 2007. Samples were taken at 0.5m from the surface, and 0.5m from the bottom at each of 5 stations throughout the length of the estuary [ranging from fresh to brackish water), at both high and low tide. The x-axis is salinity in all four panels and the right y-axis colour legend is reflected on the data points in each separate panel. Point distribution reflects the left y-axis parameters versus salinity. Plots were created using Ocean Data View (Schlitzer, 2008).
1200
1000
^ 800
o O 600 Q
400
200
2200
^.2000 u
•f 1800
I < 1600
1400
1200
o
o
° o o °
o
o
0 o
G
0
. P-
35
bo
25
o
©
o
" .' O ^
° r.o c o"
. o o
o
o
S3 9,
°o. ®
-C)
0
10
5
0
12000
1.0
0.8
o » r > , — i
20 | ^0 .6
1 5 o y
§ 20-4
0.2
-7 ao 1500^ ^
It 'I n g, O
IOOO^ 8
U
50
o o o
Sb
Q D
O o
O O
o ° 0 O o o
0.12
0.1
0.08 j ?
o 0.06 **
0.04
0.02
8.8
8.6
8.4 ^
8.2
10 20 30 Salinity [psu]
10 20 30 Salinity [psu]
II
104
4.3.4 - Air/water pC02 and C02 flux
Air/water pC02
Estuary surface water was in general undersaturated in CO2 with respect to
the atmosphere, with a mean difference in partial pressure (ApCCh) of-4.30 ± 29.33
uatm and a ApCC>2 range of-64.26 to +86.65 uatm. At high tide (i.e. marine water)
ApC02 averaged -0.86 ± 29.55 uatm, while at low tide (i.e. river water) the average
was -9.86 ± 29.26 uatm. Estuarine surface waters were generally undersaturated
with respect to the atmosphere at low tide, with the exception of station CH3 in June
and CH4 and CHI in July. At high tide, ApC02 was much more variable, though the
following trends exist: 1) surface waters are undersaturated in June, 2) marine
dominated stations CHI and CH2 are undersaturated, while river-dominated
stations CH5, CH4, and CH3 are supersaturated in July, 3) there is no obvious trend
in the August samples, however waters are supersaturated at station CH5 (86.65
uatm) and undersaturated at station CHI, 4) in October, ApCC»2 decreases from
station CH5 to CH3 (Figure 4.7).
C02 flux
The air-water CO2 flux ranged between -11.04 to +22.76 mmol nr2 day1, with
mean values of -1.07 ± 1.90 mmol nr2 day1 at low tide and +0.06 ± 5.93 mmol nr2
day_1 at high tide. At low tide, CO2 flux estimates are predominantly negative with
the exception of station CH4 and CHI in July, and station CH3 in June (Figure 4.7). At
high tide, flux magnitude is greatest at stations CH5 and CH4, with waters being a
105
sink for CO2 in June and a source in July and October [Figure 4.7). Overall, June flux
estimates were predominantly negative while July fluxes were mostly positive, with
the exception of station CH3 at low tide (June) and stations CH2 and CHI at high tide
(July) (Figure 4.7). Flux estimates for August were all negative at low tide, however
much variability exists among the high tide samples, where a positive flux of 22.76
mmol nr2 day _1 was observed at station CH5. Total mean CO2 flux for the estuarine
surface waters was -0.37 ± 4.79 mmol nr2 day-1 and indicate that this system is a net
sink for CO2 throughout the 2007 open-water season.
Figure 4.7. Water-air difference in pC02 (top panels) and air/water CO2 flux from Churchill River estuary waters. Samples were collected using a continuous automated CO2 monitor that averaged air and water CO2 concentrations over a 30-minute period. Stations range from fresh (CH5) to brackish/marine (CHI) waters and were sampled at high and low tide, June to October 2007. Overall mean CO2 flux and ApC02 was -0.62 mmol nr2 day1 and -4.30 uatm respectively.
High Tide Low Tide
-a
(N o u
4 3 2 1 0
- 1 - 2 - 3 - 4 --5
•
•
+22.76 BJune °July a August • October
1 - 1 1 . 0 4
J 1 n 1 wm
I
CH5 CH4 CH3 CH2 CHI CH5 CH4 CH3 CH2 CHI
106
4.3.5 - pC02 concentration at depth
At high tide, stations CH3 and CH4 were stratified with warmer, fresher
water overlying cooler, marine water (Figure 4.8]. Higher pCCh was generally
associated with the marine bottom water, which is consistent with information
presented in Figure 4.7, and indicates that fresher surface water was typically more
undersaturated in dissolved CO2 compared to marine water. In July however, little
variation in pCCb with depth was observed at CH3.
At low tide, station CH4 was well mixed with respect to temperature and
salinity, resulting in little variation in pCCh with depth (Figure 4.9). Station CH3 was
stratified, although the pattern differed depending on date. In June, water column
pCCh at CH3 was greater than for the other station (Figure 4.9), which is different
than what we measured at high tide where water pCCh was lowest in June (CH4)
(Figure 4.8). Little variation existed in pCOz with depth on July 14, indicative of a
well-mixed water column (Figure 4.9). CTD and pCCh profiles for station CH3 on
August 26 varied the most in pCOi and salinity, though temperature decreased by
less than 2 °C with depth.
107
Figure 4.8. pCCh concentration, salinity, and temperature variation with depth (surface, mid-depth, and bottom waters) at Churchill River estuary stations CH3 (top panels) and CH4 (bottom panels). Samples were collected between June and October at high tide using a continuous automated CO2 monitor. Water was pumped from each depth for 10 minutes, data logging every 10 seconds. Data points represent an average of pCOi over the 10-minute sampling period. Temperature-salinity plots were created using Ocean Data View (Schlitzer, 2008).
0
2
?4
a6
8f
10 0
- B - 17-Jul •-*•- 21-Aug -a— 1-Oct
CH3
2 +
o. Q
4 +
CH4
o 16-Jun -•^-21-Aug —e— 1-Oct
6 300 320 340 360 380 400 420 440 0
pC02 (|iatm) 10 20 30
Salinity (psu)
108
Figure 4.9. pCCh concentration, salinity, and temperature variation with depth [surface, mid-depth, and bottom waters) at Churchill River estuary stations CH3 [top panels) and CH4 (bottom panels). Samples were collected between June and August at low tide using a continuous automated CO2 monitor. Water was pumped from each depth for 10 minutes, data logging every 10 seconds. Data points represent an average of pCC>2 over the 10-minute sampling period. Temperature-salinity plots were created using Ocean Data View (Schlitzer, 2008).
0
2
1 4 Si o, a> Q u 6
8 1
10 0
2 +
Q 4 +
•13-Jun -14-Jul -26-Aug
P CH3
*
CH4
26-Aug
300 320 340 360 380 400 420 440 0 pC02 (uatm)
10 20 30 Salinity (psu)
Information from Figures 4.8 and 4.9 indicate that freshwater and marine
water sources (and thus water temperature, salinity, and PCO2) are better separated
at high tide compared to low tide. Although tidal amplitude varied with season, this
did not appear to affect the extent of upstream marine water intrusion at high tide
109
(see Figure 4.8 bottom right panel). River discharge, which was greatest at the
beginning of June and middle of August, is the cause for estuarine stratification and
this is reflected in the CTD measurements at CH3 on June 13 and August 26 where
both surface and bottom waters were fresher, compared to the July 14 profile.
Thermoclines were observed for the June 13 and August 26 CTD profiles, where
only one thermocline was present on July 14 (Figure 4.9 top right panel).
4.4. Discussion
4.4.1 - Influence of season, location, depth, and tidal state on water chemistry
Water samples corresponding to high tide had greater DIC concentrations
relative to those collected at low tide. It appears that DIC is supplied to the estuary
basin by marine waters (which make up the majority of bottom waters); DIC
concentrations were elevated at station CH4, where the incoming tide causes
turbulent mixing at the confluence of the river and marine waters, and at station
CH2 where incoming/receding tide flows through a narrow channel creating
turbulent waters. TSS and NO3+NO2 followed a similar pattern, with highest
concentrations at stations CH4 and CHI. NO3+NO2 concentration were also higher at
high tide compared to low tide, although there was no obvious relationship between
TSS and tide height.
Chlorophyll a, PON, POC, DOC, CDOM, and P04 all had higher concentrations
at the river station and in surface waters, and follow a decreasing trend towards
110
Hudson Bay. This trend is also evident if the data are discriminated by tide height,
that is, all parameters had higher concentrations at low tide (fresher) compared to
high tide (more saline). It is clear that 1) the concentration of these variables in the
Churchill River estuary was determined by river load, and 2) in general the
concentration of these variables was negatively correlated with salinity. Recall that
chlorophyll a, PON, and POC concentrations were all greatest in June and August,
which correspond to periods of maximum river discharge.
Alkalinity and salinity decreased in concentration from station CHI to river
station CH7. When samples are divided by tide height, the same pattern emerges:
alkalinity and salinity were higher at high tide compared to samples collected at low
tide. DO concentration was highest for the freshwater influenced stations and
decreased towards Hudson Bay, however there was little difference in DO between
surface and bottom water samples, or between samples at the far range of tidal
heights.
4.4.2 - Influence of water source and physiochemical conditions on pCQz and C02 flux
ApC02 was continuously monitored for the river water entering the estuary
in June, July, and August 2007 (Stainton et al., in prep. 2009), and the associated CO2
flux averaged -5.8 mmol nr2 day1 (Figure 4.10). ApC02 and CO2 flux measured at
station CH7 were all negative in July, however river waters were supersaturated at
times in mid-June and throughout most of August (Stainton et al., in prep. 2009).
I l l
This is contrary to what was measured in the estuary, where ApCCb and the CO2 flux
were in fact negative at most stations sampled in June, and positive at most stations
sampled in July. The river monitoring dates in August [1-12) unfortunately do not
match up to the estuary survey sampling dates (21-26), so it is difficult to interpret
the difference in ApCCh and CO2 flux between river (mostly positive) and estuary
(mostly negative, with the exception of station CH5 at high tide) during this time
period without the collection of further data.
Figure 4.10. Churchill River CO2 flux and ApCCb. Water was sampled using a continuous air/water CO2 gas monitor. The dashed line indicates August 1.
E
d"
O
2 4 6 8 10 12 14 August
The average ApCCh observed here for the estuary is -4.39 uatm, which is in
between results associated with an open-water Churchill River mean A/9CO2
estimate of-103.9 uatm (Stainton et al., in prep. 2009), and measurements made by
112
Else et al. (2008), where summer Hudson Bay ApCC>2 was +3.3 uatm (sampled 25 km
north-east of the Churchill River estuary mouth). Else et al. (2008) estimate the
mean open-water flux for all of Hudson Bay to be -0.73 mmol nr2 day-1, which is also
in agreement with our estuary flux estimate of-0.37 mmol nr2 day-1.
It appears that estuarine mixing changes the fluxes measured in the river
waters such that fluxes were more negative at stations where river and marine
water converge (station CH4 at high tide, and stations CH3 and CH2 at low tide, see
Figure 4.4). For example, of the surface waters measured at high tide in June,
convergence between the fresh river water and the intruding marine waters may
account for the more negative flux at station CH4 compared to station CH5, where
waters are fresh and isothermal (Figures 4.4 and 4.7). Similarly, fresh and marine
waters converge at stations CH3 and CH2 in August which may be attributed to the
lower fluxes of-4.85 and -4.05 mmol nr2 day1 compared to station CH4 (1.23 mmol
nr2 day-1) that was mostly isothermal (Figures 4.4 and 4.7). In August, flux at high
tide is positive for station CH5, but negative at station CH4. There does not appear to
be an explanation for this based on the profile cross-sections or from the water
chemistry collected at these sites.
Pearson's product moment correlation (r2) between CO2 flux and other
measured variables is very small. However, since CO2A varies by only a small
amount, CC»2w is ultimately dictating ACO2 and CO2 flux; therefore it is appropriate to
relate C02w to other water chemistry variables in order to decipher what may
influence the distribution of C02w in the estuary. CChw correlates negatively with
water temperature, pH, chlorophyll a, and alkalinity, and correlates positively with
CDOM, DOC, P04, NO3+NO2, and Si (Table 4.2). This indicates that processes other
than mixing account for the distribution of pCChw in the Churchill River estuary. The
negative correlation between temperature and CO2W indicates that the distribution
of CO2W within the estuary is determined primarily by the relationship between CO2
solubility and temperature (higher solubility at lower temperatures, see Figure 2.3).
Table 4.2: Pearson's Product Moment Correlation (r) between CChw (umol 1_1) and several water chemistry variables.
Variables R^_
CCL vsCDOM 0.42 2w
CO., vsALK -0.11 2w
CO. vsDOC 0.21 2w
CO. vsPO. 0.16 2w 4
CO, vsNCL+NCL 0.26 2w 3 2
CO. vsSi 0.25 2w
C02 wvspH -0.50 C0 2 %sH 2 0 temp -0.66 CO„WvsCHLA -0.12 Why is there a difference in the mean ApCCh and CO2 flux between the river
station and the estuary? One possibility is that gas exchange is enhanced once the
river waters flow over the weir into the shallow estuary basin. Stainton et al. (2009,
in prep.) found that the diurnal signal for CO2 in the river was in fact a result of net
production and respiration upstream from the monitoring station. It is possible that
the riverine phytoplankton community ceases to photosynthesize (and thus be a
drawdown of CO2) once they mix with saline water in the estuary basin. This could
114
cause an increase in estuarine CCW Another possibility is that waters undergo
enhanced gas exchange as they flow turbulently over the weir and into the estuary
basin. Where the river is fairly uniform in depth (mean ApCCb of 103.9 uatm), water
flows over the rock weir into a shallow basin where air-water exchange would take
place on a greater surface area (mean ApCC»2 of -4.39 uatm].
4.4.3 - Comparison to other high latitude estuaries
Relatively few studies have examined the relationship between air/water
CO2 exchange and physiochemical processes in estuaries, with fewer still
investigating high latitude systems. Table 4.3 lists flux and pCC»2 ranges for several
mid- and high-latitude estuaries where data does exist. Churchill River estuary flux
estimates are most similar in range to those from Hudson Bay (Else et al. 2008) and
the Beaufort Sea (Murata et al. 2008), though no similar high latitude estuarine
studies exist for comparison.
Table 4.3: Range of flux estimates and water pCC»2 from several mid- to high-latitude estuaries/coastal seas.
Location Churchill River estuary (Canada) Churchill River (Canada) Hudson Bay (Canada) Beaufort Sea (Canada) Lena delta/Laptevs Sea (Russia) Randers Fjord (Denmark) Rhine (Netherlands) Sheldt (Belgium/Netherlands) Thames (UK) Gironde (France) Hudson (US) York River (US)
Flux (mmol m'2 day"1) -11.04 to+22.76
-51 to+ 30.8 -19.6 to+16.5 -17.3 to-15
-36 to +290 70 to 160
-21 to+2028 0 to 1728 50 to 110 16 to 36
6.1 to 36.1
pC02 (uatm) 308 to 441 87 to 626
259 to 425 (/C02) 252.6 to 490 522 to 1171 355 to 400
570 to 1870 349 to 460 400 to 460 440 to 2860 515 to 1795
113to3467(ppmv)
Reference present study Stainton et al. 2009 (unpublished) Else et al. 2008 Murata et al. 2008 Semiletov 1999 Borges et al. 2004 Frankignoulle et al. 1998 Borges et al. 2004 Borges et al. 2004 Frankignoulle et al. 1998 Raymond et al. 1997 Raymond et al. 2000
115
Water pCC>2 estimates are also similar to those from Hudson Bay (Else et al.,
2008) and the Beaufort (Murata et al., 2008), though are also within the same range
as data collected by Borges et al. (2004) for the Thames, Sheldt, and Randers Fjord
estuaries. However, the upper limit of flux estimates for these European estuaries is
much greater than that of the Churchill (Table 4.3). The watersheds of the lower
Churchill River and that of the Lena River are similar (both are composed of carbon-
rich soils and peatlands), however the pCC>2 range for the Lena River delta and
adjacent Laptevs Sea is much larger than that of the Churchill and Hudson Bay.
4.5 - Summary
We found the Churchill River estuary to be a net sink for CO2 throughout the
2007 open-water season. Comparing this data to that collected from the Churchill
River indicates that transformations within the estuary basin supply dissolved CO2
to the estuarine waters. Future research should include monitoring of the diurnal
variability in air-water CO2 exchange in both the estuary basin and the outer
estuarine waters through the summer growing season, as data presented here
represents only a snapshot of air-water CO2 exchange in the estuary basin.
Continuous monitoring of the inflowing river, estuary basin (at station CH5 or CH4),
and the outer plume waters would build on this work to create a complete picture of
air-water CO2 exchange and cycling in the Churchill River and estuary system.
116
Acknowledgements
We gratefully acknowledge the field support and assistance from K. Swystun,
E. Chmelnitsky, T. Kelly, P. Fitzpatrick, J. Batstone, the Port of Churchill, and the
Churchill Northern Studies Centre. We thank Manitoba Hydro, Water Survey of
Canada, Environment Canada, and Natural Resources Canada for providing water
chemistry, river discharge, meteorological data, and satellite images. We also thank
M. Stainton for helpful discussion and comments. E. Stainton is funded by awards
from the Natural Sciences and Engineering Research Council of Canada (NSERC,
PGS-M) and the Northern Scientific Training Program (NSTP, Indian and Northern
Affairs). This research was conducted within the umbrella of Arctic-Net, a Network
of Centers of Excellence (http:/www.arcticnet-ulaval.ca). Support for this research
was provided by NSERC grants [Barber & Papakyriakou) and the Canada Research
Chairs program (Barber).
References
Bezte, C.L. 2006. Lower Churchill River water level enhancement weir project post-project monitoring: assessment of water chemistry and phytoplankton responses to operation of the Project - Year VII, 2005, A report prepared for Manitoba Hydro by North/South Consultants Inc
Borges, A. V. (2005), Do we have enough pieces of the jigsaw to integrate C02 fluxes in the coastal ocean?, Estuaries, 28, 3-27.
Borges, A.V, J.P Vanderborght, et al. 2004. Variability of the gas transfer velocity of CO2 in a macrotidal estuary (the Scheldt). Estuaries. 27:593-603.
Carignan, R. 1998. Automated determination of carbon dioxide, oxygen, and nitrogen partial pressures in surface waters. Limnol. and Oceanogr. 43: 969-975.
Else, B.G.T, T. Papakyriakou, M.A. Granskog, and J.J. Yackel. 2008. Observations of sea surface/CO2 distributions and estimated air-sea CO2 fluxes in the Hudson Bay region (Canada) during the open-water season. J. Geophys. Res. 113: C08026, doi: 10.1029/2007JC004389.
Gorham E. 1991. Northern peatlands: role in the carbon cycle and probable responses to climatic warning. Ecological Applications 1: 182-195.
Jahne, B., G. Heinz, and W. Dietrich. 1987. Measurement of the diffusion coefficients of sparingly soluble gases in water. Journal of Geophysical Research 92:10767-10776.
JGOFS. 1994. JGOFS Report No. 19-Protocols for the Joint Global Ocean Flux Study (JGOFS) core measurements.
Kuzyk Z.A., R. Macdonald, et al. 2008. Sea ice, hydrological, and biological processes in the Churchill River estuary region, Hudson Bay. Estuarine, Coastal and Shelf Science.
Murata A., K. Shimada, S. Nishino, and M. Itoh. 2008. Distributions of surface water CO2 and air-sea flux of C02 in coastal regions of the Canadian Beaufort Sea in late summer. Biogeosciences Discussions. 5: 5093-5132.
Schlitzer, R. 2008. Ocean Data View, http://odv.awi.de.
Simpson, K. G., J.-E. Tremblay, Y. Gratton, andN. M. Price. 2008. An annual study of inorganic and organic nitrogen and phosphorus and silicic acid in the southeastern
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Beaufort Sea. Journal of Geophysical Research. 113: C07016, doi: 10.1029/2007 JC004462.
Stainton, E.C., R. Hesslein, T.N. Papakyriakou, and D.G. Barber. 2009. Summer air-water carbon dioxide exchange and water chemistry of the Churchill River as it enters Hudson Bay. In prep.
Stainton, M.P., M.J. Capel, and F.A.J. Armstrong. 1977. The chemical analysis of fresh water. Canadian Fisheries Marine Service, Miscellaneous Special Publication 25,180 pp.
Stedmon, C.A., S. Markager, and H. Kaas. 2000. Optical properties and signatures of chromophoric dissolved organic matter [CDOM] in Danish coastal waters. Estuarine, Coastal, and Shelf Science 52: 67-278.
Verardo, David J., P.N. Froelich and A. Mclntyre [1990). Determination of organic carbon and nitrogen in marine sediments using the Carlo Erba NA-1500 Analyzer. Deep Sea Research, vol.37, no.l pp 157-165.
Takahashi, T., S.C. Sutherland, C. Sweeney, et al. 2002. Global sea-air C02 flux based on climatological surface ocean pC02, and seasonal biological and temperature effects. Deep Sea Research Part II: Topical Studies in Oceanography. 49:1601-1622.
Wanninkhof, R. 1992. Relationship between wind speed and gas exchange over the ocean. Journal of Geophysical Research 97: 7373-7382.
Weiss, R.F. and B.A. Price. 1980. Nitrous oxide solubility in water and seawater. Marine Chemistry 8: 347-359.
Wetzel, R.W. 2001. Limnology: lake and river ecosystems. Academic Press.
Chapter 5: Conclusions
5.1 - Summary and Conclusions
This research provides baseline water chemistry and air-water CO2 flux
estimates of the Churchill River and estuary.
Chapter 1 introduces the rationale behind this work, and outlines the main
research objectives.
Chapter 2 provides a literature review of: processes that affect aqueous CO2,
gas exchange at water surfaces, carbon exchange at the land-ocean interface, CO2
transport from coastal systems, and of climate implications with respect to carbon
cycling.
Chapter 3 presents the findings of data collected from the Churchill River.
Through air-water gas monitoring, the Churchill River was found to be a sink for CO2
throughout the 2007 monitoring period (Figure 5.1). Changes to the diurnal signal
of dissolved gases, water temperature, and turbidity were measured at times of
increased river flow. Production and respiration were calculated from the
uptake/release of CO2, and indicate that production outweighs respiration
throughout most of the monitoring period.
Chapter 4 presents the findings of data collected from the Churchill River
estuary throughout the 2007 open-water season. Unlike many high latitude rivers,
the Churchill was found to be a net productive system throughout the monitoring
period, however the riverine pCChw trends were not observed in estuarine waters,
120
likely due to microbial respiration of river waters, and enhanced gas exchange as a
result of shallower water depths and turbulence within the estuary (Figure 5.1).
Most parameters measured for Churchill estuary waters mix conservatively within
the estuary basin (based on water source, river or marine). Estimated CO2 flux was
more negative at low tide compared to high tide. Variation in flux magnitude was
greatest at sampling locations with turbulence where fresh and marine water
masses meet and where the tide recedes through a narrow channel.
ApC02-103.9 uatm flux C02 - 5.4 mmol m 2 day1
HB ApC02 + 3.3 uatm flux C03 - 0.73 mmol nrr2 day1
Figure 5.1: Summary of processes within the Churchill River and estuary system.
5.2 - Future Directions
The Churchill River and estuary are an ideal location to study climate change
impacts on fresh-marine water physiochemistry and air-water carbon exchange.
The proximity of the Town of Churchill to the estuary, and the boat launching and
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docking facilities accessing the river make water-based sampling relatively easy
compared to other northern estuaries (e.g. the Nelson River).
In the beginning, this thesis was meant to include air-water pCC»2 and water
chemistry monitoring of Hudson Bay waters using a marine buoy (Environment
Canada) as a platform. Due to logistical issues, the buoy was not deployed. Future
research should include sampling of Hudson Bay waters throughout the open-water
season using this infrastructure in order to obtain a marine reference for ApCC»2. On
a larger scale, further measurements of air-water CO2 exchange should be taken
from other river-estuarine systems in Hudson Bay with the purpose of
understanding the collective importance of arctic and subarctic estuaries in
sequestering carbon.
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Appendix A
Summary of all Churchill River Estuary 2007 stations with latitude and longitude positions.
Station CH9 CH10 CH11 CH12 CH13 CH14 CH15 CH16 CH17 CH18 CH19 CH20 CH21 CH22 CH26 CH27 CH28 CH29 CH30 CH31 CH32 CH33 CH34 CH35 CH36 CH37 CH38 CH39 CH1 CH2
CH3A CH3B CH4 CH5 CH7
Long [deg. east] -94.1978 -94.2012 -94.2079 -94.2107 -94.2203 -94.2152 -94.2098 -94.2049 -94.1997 -94.1940 -94.1951 -94.2089 -94.2252 -94.1845 -94.2159 -94.2067 -94.2132 -94.2183 -94.2094 -94.1991 -94.1899 -94.2016 -94.1878 -94.1951 -94.1913 -94.2170 -94.2051 -94.2056 -94.1865 -94.2082 -94.2098 -94.2039 -94.2054 -94.1911 -94.1875
Lat [deg. north] 58.7104 58.7253 58.7595 58.7864 58.7717 58.7715 58.7720 58.7722 58.7719 58.7717 58.7044 58.7927 58.7719 58.7732 58.7880 58.7866 58.7874 58.7819 58.7803 58.7595 58.7604 58.7402 58.7002 58.6978 58.7267 58.7262 58.7489 58.7352 58.8098 58.7910 58.7756 58.7798 58.7406 58.6983 58.7904
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Appendix B
Figure 1: Relationship between gas trasfer velocity, k, and wind speed from four different models: Wanninkhof [1992), Wanninkhof and McGillis (1999], Nightingale et al. (2000), and Liss and Merlivat (1986). The Wanninkhof (1992) model for calculating gas transfer was used for flux calculations in this study.
/—"\
hour
S u o
100 -
90 -
80 -
70 -
60 •
50 •
40 -
• W92
• WM9 9
°N00
30
20 --
10 •-
0
6 8 10
Wind speed (m s 1 )
12 14 16
Table 1: mean, standard deviation, and range of C02 flux measurements based on four models for calculating gas transfer, k. The Wanninkhof (1992) model for calculating gas transfer was used for flux calculations in this study.
Flux C02 (mmol m"2 day"1)
AVG STDEV MAX MIN
W92 -5.8 11.9 30.8
-51.4
WM99 -3.2 9.0
20.7 -57.8
N00 -5.4 10.4 26.5
-43.5
LM86 -3.0 7.5
20.3 -33.1
