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Disrupted Biogeochemical Cycles in the Great Lakes: Challenges and Opportunities for
Aquatic Science
R.E. Hecky
Large Lakes Observatory
Biology Department
University of Minnesota Duluth
Great Lakes of the World
Laurentian Great Lakes
African Great Lakes
Baikal
Canadian Great Lakes
>60% of surface fresh water
The Great Lakes are easily visible from space and are the globe’s most substantial freshwater resources.
All biogeochemical processes that occur in the oceans can occur in lakes
Special characteristics of Great Lakes relative to coastal oceans: 1) Unidirectional outflow 2) Mass budgeting simplified at large spatial scales allowing
geostrophic circulation and inertial currents of relevance to understanding coastal process
2) Weak mixing relative to coastal ocean; absence of significant tides (affects of biogeochemical processes accumulate)
3) Nutrient concentrations are low especially P and therefore sensitive to change amplifying biogeochemical signal
4) Logistically accessible; observations can be dense and at high resolution
5) Ionically dilute-simplifies geochemical modeling
Water Residence Time (WRT=volume/inflows)
CA:LA mean depth WRT NO3
(m) (y) µmol/L Tanganyika 7.1 557 6000 <0.5 Malawi 3 290 1225 <0.5 Baikal 12.1 730 327 7 Superior 1.6 148 191 20 Great Bear 5 72 131 10 Victoria 2.8 40 123 <0.5 Michigan 1.6 84 99 12 Huron 2.1 61 22 20 Ontario 3.4 86 6 16 Erie 2.3 18 2.6 18
Kalff. 2002. Limnology. Table 9-3 Bootsma and Hecky. 2003. JGLR. Evans. 2000. JAEHM. Weiss. 1991. Nature.
Influence of internal cycling/recycling on lakes generally a function of Water Residence Time both on scale of whole lakes as well as locally within lakes while influence of external loading is associated with the Terrestrial Catchment to Lake Area (CA:LA)
External
Loading:
rivers,
precipitation,
dryfall
Outflow
Burial
Internal Loading
Recycling
Nutrient Loadings are measured per unit time:
1) Easiest to measure –Outflow
2) Most difficult –Internal loading
3) On a daily basis, most algal demand is met by recycling
4) Concentrations are balance of internal and external loading
Nutrient Inputs and Outputs
N fixation Denitrification
TP mol.L-1
0 1 2 3
TN
m
ol.L
-1
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1 Victoria2 Arctic Ocean3 Scotian Shelf4 Malawi5 Slope6 Sargasso7 NW Ont Large Lakes8 NW Ont Small Lakes9 Superior
Victoria Oceans
Nyasa-Malawi
Superior
The GLOW cover the range of nutrient concentrations found in off shore
surface waters of the oceans; Nyasa/Malawi has lowest TN and Victoria
highest TP
Guildford and Hecky 2000
Common stresses among great lakes (and
many smaller lakes):
1) Eutrophication
2)Fisheries Exploitation
3) Exotic Species
4) Contaminants
5) Climate Change
Great Lakes -Great Experiments
1600s Exploration: Loss of innocence 1700s Exploitation: Forests, Fisheries, Mining and Transport 1800s Acculturation: Land Clearance, Agriculture, Settlement, Canalization, Urbanization 1900-1960 Degradation: Disease, Collapse of Fisheries, Eutrophication, Contamination 1960-1990 Restoration: GLFC, GLWQA, P and contaminant Management; habitat restored 1990 – Globalization: Faunal Marination and Climate Warming
Exploitation
>1830
1932
1936
1946
Demise of fisheries in deep lakes and
extinction of 3 deepwater species;
establishment of sea lamprey in the upper
lakes from Christie (1974) –first official
sighting of lamprey in the lakes
Acculturation
Great Lakes System Profile
Degradation
Chapra et al. 2012. JGLR.
Major ion concentrations were the first warning that biogeochemistry of the lakes was changing (Beeton, 1968)
P model of Chapra 1977 for Lake Michigan from Schelske et al. 1986
Modeling of P export from different landscapes and land uses as well as paleolimnological investigations could show that nutrient cycles of the water and air sheds of the lakes had been disrupted and the lakes were responding.
Schelske and Hodell. 1995. L&O
Chapra (1977) Science
Dolan and Chapra (2012) JGLR
Restoration
Ohio Task Force on P in Lake Erie. 2011.
GLWQA 1972 asked governments to focus on reducing P emissions from point sources and removal of P from detergents to reduce eutrophication especially in the lower lakes (Erie and Ontario)
Good News. Total Phosphorus (TP) Loading reductions have exceeded targets. Loading and its variability is now dominated by non-point sources (runoff) which respond to precipitation.
The standard errors vary from 1.5 to 19% of the total lake load depending on the lake and the year. Lake Superior typically has larger standard errors (9.1% of the load, on average), while Lake Erie has some of the smallest estimates (3.5% of the load, on average).
Chapra and Dolan. 2012. JGLR
Too much good news? Open-lake annual median values have been below targets especially since 1990 when a better fit to the observed data can be realized by using a higher settling coefficient e.g. 19 to 29 m/y in Ontario
Unexpected increase in settling velocity attributed to dreissenids and result is that Michigan, Huron and Superior are converging in their trophic status
Ohio Lake Erie Phosphorus Task Force Report (OLEPTFR) Ohio EPA 2010
Ohio Lake Erie Phosphorus Task Force Report (OLEPTFR) Ohio EPA 2010
2011 Lake Erie algal bloom largest on record
Settling correction is especially clear for Lake Ontario which initially agreed well with the model and is less subject to interannual variability as Niagara River dominates loading
Niagara River Loading has remained high despite declining TP concentrations in Lake Erie suggesting changing conditions at Erie outflow
Shoreline Fouling
Aesthetic complaints
Taste and Odour complaints
High bacterial counts (E.coli)
Tourism/Recreation
Property Values
Cost of Clean up
Tea Krulos
0
200
400
600
800
4-Apr 24-May 13-Jul 1-Sep 21-Oct 10-Dec 29-Jan
Bio
ma
ss (
g D
M m
-1)
Growth is rapid in late spring
and early summer as waters
warm
State of the Lakes 2009 Red is poor Green is good Diamond unchanging Rt. Arrow improving
State of Phosphorus in the Great Lakes: State of Confusion
Life was good except-
Globalization
Dreissenid mussels, Epibenthic filtering organism now occupies much of hard bottom substrata in all the lakes except Superior (where it is in the harbors)
Q: What are the effects of dreissenid mussels on nutrient cycling in the nearshore of the Laurentian Great Lakes?
To answer, turn to knowledge gained from estuarine and marine coastal environments.
Top-down effects of filter feeding bivalves • Clear phytoplankton, small zooplankton, some
detritus
• Clearance rate is a function of temperature, food flux to the bivalve bed (a function of physical processes and food density)
• Possible control of phytoplankton biomass when bivalve density is high, water residence time is longer than clearance time, phytoplankton not strongly nutrient limited
Bottom-up effects of filter feeding bivalves
• Concentrates nutrients into bivalve biomass, feces and pseudofeces (“biodeposits”), releases soluble nutrients at benthos
• Evidence of increased benthic fauna diversity and biomass
• Evidence of increase flora (seagrasses, red macroalgae)
…dreissenid mussels have been credited with re-engineering nutrient flux and distribution in the lower Great Lakes as well as improving nearshore water clarity
Post-mussels: Pre-mussels:
Nearshore Shunt Hypothesis: Hecky et al. Can. J. Fish. Aquat. Sci. 2004
Dreisenna influences on Cladophora growth
Increased Phosphorus
to benthos
1
Improved light climate
allows for growth at deeper
depths
2 0
2
4
6
8
10
0 100 200 300Biomass (g DM/m
2)
Depth
(m
)Post
Dreissena-1 ug/L SRP
SRP and
kPAR
-Increase in SRP + 1ug/L
-Increase water clarity 0.1
kPAR (m-1) (Markarewicz et al. 2000; Higgins et al. 2005)
Model predicts 2X increase
Post Dreissena in Lake Erie
Phosphorus excretion by
mussels was measured in
benthic chambers
Ozersky et al 2011 JGLR
16,200 kg SRP
11,000 kg TP
4,600 kg SRP
Dreissenid P excretion exceeds other P sources and P demand by Cladophora along rocky shorelines which host both organisms
Source of P presumed to be from lake based on C isotopes but P could also be regenerated from these same coastal sources—different management implications for managing P fluxes in the coastal zone
Ozersky et al. 2009. JGLR
Mussels have also reduced Total Phosphorus Export by 60 % from Inner Saginaw Bay to the Outer Bay. Saginaw Bay used to provide 20-40 % of total P loading to Lake Huron. To what extent has mussel interception of TP contributed to reduce concentrations in the lakes?
Cha et al. 2011. ES&T
Malkin et al. 2012. L&O
What limits dreissenid growth? Instrumented coastal zone (5, 10, 20 m moorings; fluorometer, thermisters and ADCP) to look at dynamics of chlorophyll (phytoplankton) and mussel growth in cages
2.0 m below above 1.0 m 0.15
Benthic boundary layer limits dreissenid access to water column food compared to suspended mussels Chlorophyll available near bottom (0.15 m) nearly 10x higher during first growth period Negative growth on bottom in SFDM in second experiment suggests strong food limitation for much of summer period due to BBL
5 m 10 m
Is the deep chlorophyll layer an important food resource during stratified season? How deep does the mussel interception of phytoplankton P remove TP from suspension? How effective is cross shelf transport from DCL to benthic boundary layer? Deep cage moorings (20 m) lost so no growth data available; but chlorophyll is higher at depth and cross shelf velocities higher than at shallower depths.
The African Great Lake Nearshore Shunt
Andre et al. 2003 JGLR
Water leaving radiance at 550 nm is correlated to suspended particle concentration and Secchi Disc depth. (Binding et al. 2007. JGLR). Satellite imagery from early 1980s can be compared with early 2000s (March to October for spatially resolved (1 km) evidence for changes in clarity
1979-1985
1999-2006
Great Lakes are becoming more transparent. Between early 1980s and early 2000s
Change from 1979-1985 to 1998-2005
Not only are lakes clearer on average but patterns of turbidity are altered with nearshore waters often clearer than offshore water; only in western and central basin of Erie has turbidity increased between these time periods
Ontario Erie
Barbiero et al. 2012. JGLR
NM
SM
NH
SH
SUP
Michigan
Huron
Superior
Transparency of Upper Lakes is converging on Superior; greatest change in chlorophyll and transparency in the spring
Lake Michigan Lake Superior
1% PAR
Barbiero et al. 2004. JGLR House et al. In prep.
Increased clarity has resulted in deepening of euphotic zone which allows better access to nutrient-rich deeper water in stratified season. True for both benthic and phytoplankton production in the Deep Chlorophyll Layers that form in the lakes. May be good news for cold water native stenothermal animals. Is energy being increasingly shunted to deep water food webs? Is there vertical Deep Water Shunt forming in the Great Lakes?
Chapra et al. 2012. JGLR.
Dreissenids as calcifying organisms continuously producing shell have caused decreases in Ca and alkalinity concentrations in the lower lakes . If decline in Ca is all in mussel shells would reduce SRP by approximately 3 µg/L (based on Arnott and Vanni (1996). Is the missing P in the lakes in mussel shells?
Late summer whiting event (precipitation of CaCO3 ) in Lake Michigan SeaWifs Image NOAA
http://eoimages.gsfc.nasa.gov/images/imagerecords/1000/1768/seawifs_lake_mich_lrg.jpg
In Lake Ontario, Ca and Alkalinity concentrations have fallen and summer “whitings” no longer occur (Barberio et al. 2006. JGLR ); pelagic sedimentation may be declining
Challenge to the Workshop: Open the black box that has guided Great Lakes management and. address spatial and temporal complexity of internal transport and biogeochemical reactivity within the lakes. Challenges: 1) Concurrent coastal eutrophication and pelagic oligotrophication 2) Clarification and redistribution of nutrients and productivity within the
lakes 3) Consequences of accelerated calcification on nutrient and trace element
cycling 4) Climate change on the biogeochemistry of the lakes
Leon et al. 2011 JGLR ELCOM-CAEDYM TP Chl SRP
June July Aug
Is the mussel shunt starving pelagic carbonate sedimentation and offshore sedimentation in general? Mass budgeting indicates “sedimentation/settling” coefficients are increasing while pelagic particulate concentrations are falling. Are we seeing a shift in the composition of offshore phytoplankton accelerating settling (e.g. more and larger diatoms) or is the increased “settling” a transfer of nutrient mass to the littoral? What are the consequences for nutrient and trace element cycling e.g. Total Hg concentrations in the lakes are falling. Eutrophication Oligotrophication Clarification Calcification
Lake Huron Open Lake Total Mercury Trend
20042005
20062007
To
tal M
erc
ury
(n
g/L
)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Lake Ontario Open Total Mercury Trend
20062007
20082009
20102011
Tota
l M
erc
ury
(ng
/L)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Lake Superior Open Lake Total Mercury Trend
20052006
20072008
20092010
2011
Tota
l M
erc
ury
(ng
/L)
0.0
0.2
0.4
0.6
0.8
Lake Erie East Basin Total Mercury Trend
20042005
20062007
20082009
Tota
l M
erc
ury
(ng
/L)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Open Lake Mercury Trends Lake Ontario
Slope -0.05 ng/L·yr
r2 = 0.55
Lake Erie East
Slope -0.03 ng/L·yr
r2 = 0.02
Lake Huron
Slope -0.06 ng/L·yr
r2 = 0.13
Lake Superior
Slope -0.05 ng/L·yr
r2 = 0.40
Mussels grew slower on the bottom in experiment 2 despite warmer temperatures; only other significant physical difference between experimental periods was higher near bottom stability measured as N2 which may have facilitated food depletion by mussels within the benthic boundary layer
5 m 10 m 20 m
No difference between experimental periods near bottom velocity but second period was warmer Also no difference in near bottom stability , n2, Ri, bottom shear velocity
Is it really resurgent? Yes.
0
500
1000
1500
2000
2500
1998 2000 2002 2004 2006
Volu
me c
olle
cte
d (
m3)
Debris removed from cooling water intake filters at Pickering Nuclear Plant—mostly Cladophora
