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Stream Nutrient Processing: Spiraling, Removal and Lotic Eutrophication
EcohydrologyFall 2013
Nutrient Cycles• Global recycling of elemental requirements
– Major elements (C, H, N, O, P, S)– Micro nutrients (Ca, Fe, Co, B, Mg, Mn, Cu, K, Z, Na,…)
• These planetary element cycles are:– Exert massive control on ecological organization– In turn are controlled in their rate, mode, timing and
location by ecological process– Are highly coupled to the planets water cycle– In many cases, are being dramatically altered by
human enterprise– Ergo…ecohydrology
Global Ratios of Supply and Demand – Aquatic Ecosystems
Inducing Eutrophication
Leibig’s Law of the Minimum– Some element (or light or
water) limits primary production (GPP)
– Adding that thing will increase yields to a point; effects saturate when something else limits
– What limits productivity in forests? Crops? Lakes? Pelagic ocean?
Justus von Liebig
(GPP)
Phosphorus Cycle
• Global phosphorus cycle does not include the atmosphere (no gaseous phase).– Largest quantities found in mineral deposits and
marine sediments.• Much in forms not directly available to plants.
– Slowly released in terrestrial and aquatic ecosystems via weathering (and, not slowly, by mining).
• Numerous abiotic interactions– Sorption, co-precipitation in many minerals (apatite),
solubility that is redox sensitive
Phosphorus Cycle
http://arnica.csustan.edu/carosella/Biol4050W03/figures/phosphorus_cycle.htm
Nitrogen Cycle• Includes major atmospheric pool - N2.
– N fixers use atmospheric supply directly (prokaryotes).• Energy-demanding process; reduces to N2 to ammonia (NH3).
– Industrial N2- fixation for fertilizers exceeds biological N fixation
annually. (We do it with Haber-Bosch)– Denitrifying bacteria release N2 in anaerobic respiration (they
“breathe” nitrate).– Decomposer and consumers release waste N in form of urea or
ammonia.– Ammonia is nitrified by bacteria to nitrate.– Basically no abiotic interactions (though recent evidence of rock
sources in Rocky Mountain forests)
Global Nitrogen Enrichment
• Humans have massively amplified global N cycle– Terrestrial Inputs
• 1890: ~ 150 Tg N yr-1
• 2005: ~ 290+ Tg N yr-1
– River Outputs• 1890: ~ 30 Tg N yr-1
• 2005: ~ 60+ Tg N yr-1
• N frequently limits terrestrial and aquatic primary production– Eutrophication
Gruber and Galloway 2008
Watershed N Losses
• Applied N loads >> River Exports– Slope = 0.25
• Losses to assimilation (storage) and denitrification– Variable in time and
space – Variable with river
order and geometry– Can be saturated
Boyer et al. 2006
Van Breeman et al. 2002
Rivers are not chutes(Rivers are the chutes down which slide the ruin of continents. L. Leopold)
• Internal processes dramatically attenuate load– Assimilation to create particulate N – Denitrification – a permanent sink
• Understanding the internal processing is important– Local effects of enrichment (i.e., eutrophication)– Downstream protection (i.e., autopurification)
• Understanding nutrient processing (across scales) is a major priority
Nutrient Cycling in Streams• Advection it commanding organization process in
streams and rivers – FLOW MATTERS• Nutrients in streams are subject to downstream
transport.– Nutrient cycling does not happen in one place.– Flow turns nutrient cycles in SPIRALS– Spiraling Length is the length of a stream required for a
nutrient atom to complete a cycle (mineral – organic – mineral).
• Uptake (assimilation + other removal processes)• Remineralization
Nutrient Spiraling in Streams
2) Cycling in open ecosystems
[creates spirals]
Longitudinal Distance
Inorganicforms
Organicforms
1) Cycling in closed
systems
Advective flow
Nutrient Cycling vs. Spiraling
Distance
Tim
e
Uptake length
(Sw)
Turnover length
(So)+Spiral
length (S) =
Inorganicforms
Organicforms
Components of a Spiral
From : Newbold (1992)
Nutrient Spiraling
Uptake Length
• The mean distance traveled by a nutrient atom (mineral form) before removal
• Flux– F = C * u * D– F = Flux [M L-1 T -1], C = Conc. [M L-3], u = velocity [L
T-1], D = depth [L]• Uptake rates
– Usually assumed 1st order (exponential decline)– Constant mass loss FRACTION per unit distance
xkLoxeFF
Constant Fractional Loss
• Basis for exponential decline– dF/dx = -kL * F– k = the longitudinal uptake rate (L-1)
• Integrating yields F at location x as a function of uptake rate, distance (x) and initial upstream concentration F0:
1/k = Sw
Longitudinal distance
Trac
er a
bu
nd
ance
Field data
Best-fit regression line using:Fx = F0e-kx
where: Fx = tracer flux at distance x
F0 = tracer flux at x=0
x = distance from tracer addition
k = longitudinal loss rate (fraction m-1)
1/k = Sw
Uptake Length (Sw)
Turnover Lenth (SB)
• Distance that a nutrient atom travels in organic (biotic form) before being remineralized to the water column
• Hard to measure directly• Regeneration flux (M L-2 T-1] is:
– R = kB * XB where kB is regeneration rate [T-1] and XB is the organic nutrient standing stock (M L-2]
– XB includes components in the sediments – XS which stay put - and the water column - XB which move.
– The turnover length is the velocity of organic nutrient transport (vB) divided by the regeneration rate.
– Transport velocity depends on the allocation to sediment and water column pools (vB = u * XS/XB)
Time
Longitudinal distance
Advective flow
Uptake length (Sw)
Turnover length (So)
Spiral Length in Headwater Streams(dominated by uptake length)
Open Controversy• Headwater systems have short uptake lengths
– Direct (1st) contact with mineral nutrients– Shallow depths
• Alexander et al. (2000), Peterson et al. (2001)– Large rivers have much longer uptake lengths (therefore no net N
removal)• Wollheim et al. (2006)
– Uptake length doesn’t measure removal, it measures spiral length– Uptake rates per unit area may be more informative when the
question is “where does nutrient removal occur within river networks”
– Most of the benthic area and most of the residence time in river networks is in LARGE rivers
Linking Uptake Length to Associated Metrics
• Uptake velocity (vf; rate at which solutes move towards the benthos; measure of uptake efficiency relative to supply) [L T-1]– vf = u * d / Sw = u * d * kL
• Uptake rate (U; measure of flux per unit area from water column to the benthos) [M L-2 T-1]– U = vf * C
solutetriad
U
vf SWvf = (u * d)/SW
Solute SpiralingMetricTriad
Spiraling Metric Triad
Uptake Kinetics – Michaelis-Menton• Uptake of nutrients (among MANY other
processes) in ecosystems is widely modeled using saturation kinetics– At low availability, high rates of change– Saturation at high availability
M
MAX
KC
CUU
Nutrient availability
U
Umax C
C + Km
U =
Linear Transitional Saturated
Umax
C + Km
vf =
vf
Sw
vd
Umax
Sw = Cvd Km
Umax
+
M-M Kinetics for U provides predictions for Sw and Vf profiles
How Do We Measure Uptake Length?
• Add nutrients – Since nutrients are spiraling (i.e., no longitudinal
change in concentration), we need to disequilibrate the system to see the spiraling curve
• Adding nutrients changes availability• Changes in availability affects uptake kinetics• Ergo – adding nutrients (changing the
concentration) changes the thing we’re trying to measure
Mulholland et al. (2002)
Enrichment Affects Kinetics
Alternative Approach
• Add isotope tracer (15N)– Isotope are forms of the same atom (same atomic
number) with different atomic mass (different number of neutrons)
– Two isotopes of N, 14N (99.63%) and 15N (0.37%)– We can change the isotope ratio (15N : 14N) a LOT
without changing the N concentration• Trace the downstream progression of the 15N
enrichment to discern processes and rates
1000)(
std
stdsmpl
R
RR ‰
10001)(
std
smpl
R
R ‰
‰ Notation• The “per mil” or “‰” or “δ” notation
• R is the isotope ratio (15N:14N)• Reference standard (Rstd) for N is the
atmosphere (by definition, 0‰)• More 15N (i.e., heavier) is a higher δ value
light heavy
0
+-
-10 +30
Natural Abundances of Isotopes
Accounting for Isotope Fractionation• Many processes select for the lighter isotope
– Fractionation (ε) measures the degree of selectivity against the heavier isotope
– N fixation creates N that is lighter than the standard (εFix = δN2 – δNO3 = 1 to 3‰)
– N uptake by plants is variable, but generally weak (εA = δNO3 – δON = 1 to 3‰)
– Nitrification is strongly fractionating (εNitr = δNH4 – δNO3 = 12 to 29‰)
– Denitrification is also strongly fractionating (εDen = δNO3 – δN2
= 5 to 40‰)• Note that where denitrification happens, it yields nitrate that
“looks” like its from organic waste and septic tanks
So – How to Uptake Length (Addition vs. Isotope) Compare?
Not So Good
• Our two methods give dissimilar information• Isotopes are impractical for large rivers• Large rivers are important to network removal
• But…if we’re interested in the entire kinetic curve, then this may be a GOOD thing
• Enter TASCC and N-saturation methods
What Happens to Uptake Length as we Add Nutrients
• Sequential steady state additions (Earl et al. 2006)
Back-Extrapolating From Nutrient Additions
• Multiple additions (Payn et al. 2005) result in a curve from which ambient (background) uptake rate can be inferred
Laborious but Fruitful(back extrapolation to negative ambient)
Lazy People Make Science Better
• Use a single pulse co-injection to get at multiple concentrations in one experiment (Covino et al. 2010)
Method Outline• Add tracers in known ratio• Measure the change in
ratio with concentration; the ratio at each time yields an uptake length (Sw) which can be indexed to concentration
• U can be obtained from Sw from the triad diagram (U = u*d*C/Sw = Q*C/w*Sw)
• Fit to Michaelis-Menten kinetics and back extrapolate to ambient
Data
Stream Biota and Spiraling Length
• Several studies have shown that aquatic invertebrates can significantly increase N cycling.– Suggested rapid recycling of N by macroinvertebrates may increase
primary production.• Excreted and recycled 15-70% of nitrogen pool as ammonia.
• Stream ecosystem organization creates short spirals for scarce elements– In a “pure” limitation, uptake length goes to zero and all downstream
transport occurs via organic particles• CONCENTRATION GOES TO ZERO @ LIMITATION
– Any biota that accelerate remineralization (e.g., shorten turnover length) amplify productivity
– Invertebrates accelerate remineralization
19_16.jpg
Invertebrates and Spiraling Length
Eutrophication
• Def: Excess C fixation– Primary production is
stimulated. Can be a good thing (e.g., more fish)
– Can induce changes in dominant primary producers (e.g., algae vs. rooted plants)
– Can alter dissolved oxygen dynamics (nighttime lows)
• Fish and invertebrate impacts• Changes in color, clarity, aroma
Typical Symptoms: Alleviation of Nutrient Limitation
(GPP)
• Phosphorus limitation in shallow temperate lakes
• Nitrogen limitation in estuarine systems
V. Smith, L&O 2006V. Smith, L&O 1982
Local Nitrogen Enrichment• The Floridan Aquifer (our
primary water source) is:– Vulnerable to nitrate
contamination– Locally enriched as much as
30,000% over background (~ 50-100 ppb as N)
• Springs are sentinels of aquifer pollution– Florida has world’s highest
density of 1st magnitude springs (> 100 cfs)
Arthur et al. 2006
Weeki Wachee20011950’s
Mission Springs Chassowitzka (T. Frazer)
Weeki Wachee
Mill Pond Spring
In Lab Studies:Nitrate Stimulates Algal Growth
In laboratory studies, nitrate increased biomass and growth rate of the cyanobacterium Lyngbya wollei.
Cowell and Dawes 2004
Stevenson et al. 2007
• Hnull: N loading alleviated GPP limitation, algae exploded (conventional wisdom)
• Evidence generally runs counter to this hypothesis– Springs were light limited even at low concentrations (Odum 1957)
– Algal cover/AFDM is uncorrelated with [NO3] (Stevenson et al. 2004)
– Flowing water mesocosms show algal growth saturation at ~ 110 ppb (Albertin et al. 2007)
– Nuisance algae exists principally near the spring vents, high nitrate persists downstream (Stevenson et al. 2004)
Field Measurements:Nitrate vs. Algae in Springs
From Stevenson et al. 2004 Ecological condition of algae and nutrients in Florida Springs DEP Contract #WM858
Fall 2002 (closed circles) Spring 2003 (open triangles)
No useful correlation between algae and nitrate concentration
Alexander Springs (50 ppb N-NO3)
Visualizing the Problem
Silver Springs (1,400 ppb N-NO3)
Synthesis of Ecosystem Productivity:Nitrate vs. Metabolism in Springs
Data Sources:- WSI (2010)- WSI (2007)- WSI (2004)- Cohen et al. (2013)
Slight Digression - Nutrient Contamination Broadly in Florida
Source: USEPA (http://iaspub.epa.gov/waters10/state_rept.control?p_state=FL&p_cycle=2002)
Recent Developments – Numeric Nutrient Criteria
• Nov 14th 2010 – EPA signed into law new rules about nutrient pollution in Florida– Nutrients will be regulated using fixed numeric
thresholds rather than narrative criteria– Became effective September 2013
• Result of lawsuit against EPA by Earthjustice arguing that existing rules were under-protective– Why?
Stressor – Response for Streams• No association found between
indices of ecological condition and nutrient levels
• Elected to use a reference standard where the 90th percentile of unimpacted streams is the criteria
Eutrophication in Flowing Waters?
• Why no clear biological effect of enrichment in lotic systems?– What is ecosystem N demand? – How does this compare with supply (flux)?– What does this say about limitation?
• Is concentration a good metric of response in lotic systems?– In lakes/estuaries, diffusion matters.– In streams, advection continually resupplies nutrients.
Qualitative Insight: Comparing Assimilatory Demand vs. Load
• Primary Production is very high– 8-20 g O2/m2/d (ca. 1,500 g C/m2/yr)
• N demand is basically proportional– 0.05 – 0.15 g N/m2/day
• N flux (over 5,000 m reach) is large– Now: ca. 30 g N/m2/d (240 x Ua)
– Before: ca. 2.5 g N/m2/d (20 x Ua)
– This assumes no remineralization (!)
• In rivers, the salient measure of availability may be flux (not concentration)
• Because of light limitation, this is best indexed to demand
• When does flux:demand become critical?
Metrics of Nutrient Limitation• Concentration
– Ignores the fact that flux/turbulence reduces local depletion, and that light conditions affect demand
• Flux-to-demand (Q*C/Ua) (unitless)– Requires arbitrary reach length to estimate demand
• Autotrophic uptake length (Sw,a) (length units)– Consistent with nutrient spiraling theory (Newbold et al. 1982)
– Ratio of flux to width-adjusted benthic uptake
Autotrophic Uptake Length• Mean length (downstream) a molecule of
mineral nutrient travels before a plant uses it– Not dissimilatory use, which typically dominates
• Shorter lengths imply greater limitation• For N: Sw,a,N
• For P: Sw,a,P
Predicting GPP Response • Nutrient Limitation Assay (NLA)
– Relative response (RR) of N enrichment:control– Regressed vs. Concentration and Sw,a,N
NLA Response Data from Tank and Dodds (2003); Analysis by Sean King
Estimating Ua from Diel Nitrate Variation(Ichetucknee River, 5 km downstream of headspring)
Submersible UV Nitrate Analyzer (SUNA)
YSI Multiprobe
0:00 5:00 10:0015:0020:00 1:00 6:00 11:0016:0021:00 2:00 7:00 12:0017:0022:00 3:00
Autotrophic Assimilation[N
O3
- ]
[NO3-]min
[NO3-]max
Assumptions: No autotrophic assimilation at [NO3-]max
Other processes constant (unknown)Other N species constant (validated)
Diel Method for Estimating Autotrophic N Demand
Heffernan and Cohen 2010
Ua Estimates Yield Reasonable C:N Stoichiometry at the Ecosystem Scale
NPP = Ua * 25.4R2 = 0.67, p < 0.001
C:N RatiosVascular Plants ~ 25:1Benthic Algae ~ 12:1N
et P
rimar
y Pr
oduc
tion
(NPP
) (m
ol C
/m2 /
d)
N Assimilation (Ua) (mol N/m2/d)
Inducing N Limitation in Spring Runs[some were, many springs were not N limited at 0.05 mg/l]
Autotrophic Uptake Length Globally
Summary
• Spiraling the dominant paradigm for nutrient dynamics in flowing water– Stream ecological self-organization creates short
spirals for scarce elements• Measuring spiraling (esp. in larger rivers) can
leverage new methods (diel, TASCC)• Lotic eutrophication is different than other
aquatic ecosystems, and requires a spiraling basis
So – Why All the Algae?
Back to First Principles:Controls on Algal Biomass
bottom up effects
top down effects
Algae Biomass
Grazers Flow RatesDissolved Oxygen
Nutrients Light
mediating factors
What else has changed? – Water Chemistry.
• Despite relative constancy, variability in springs flow and water quality can be large and ecologically relevant
• The changes are poorly understood because of a) uncertain flowpaths, and b) uncertain residence times
• The changes are understudied because of the plausibility of the N loading story
Data from Scott et al. 2004
What else has changed? Flow.
• Changes in flow occur in response to climate drivers and human appropriation
• Kissingen Springs
Weber and Perry 2006
Munch et al. 2007
Field Measurements:Algal Cover Responds to Flow
• Flow has widely declined– Silver Springs– White Springs– Kissingen Spring
• Reduced flow is correlated with higher algal cover (King 2012)
Flow and DO Affect Grazers
0 1 2 3 4 5 6 7 8 9 100
50
100
150
200
250
300
DO (mg/L)
Gast
ropo
d Bi
omas
s (w
et w
eigh
t g/m
2)
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.40
50
100
150
200
250
300
Velocity (m/s)
Gast
ropo
d Bi
omas
s (w
et w
eigh
t g/m
2)
Observational Support: Grazer Control Algal Biomass Accrual
Liebowitz et al. (in review)
A) B)
C)
Gastropod biomass (g m-2)
Alg
ae
bio
ma
ss (
g m
-2)
y = 2350x-1.592
R² = 0.38p < 0.001
Note: Multivariate Model of Algal Cover explained 53% of variation, with gastropod density as a dominant predictor along with shading and flow velocity. Nutrients were pooled (no significant effect).
Evidence of Alternative States?• Below 20 g m-2 – always high algae• Above 20 g m-2 - both high a low algae• Mechanism?
Residual algae biomass
Pro
po
rtio na
l F
req
ue
nc y
Gastropod biomass < 20 g m-2Gastropod biomass > 20 g m-2
0.00
0.05
0.10
0.15
0.20
0.25
0.00
0
.05
0.10
0.15
0.2
0
0.
25
-6 -4 -2 0 2 4 6-6 -4 -2 0 2 4 6
Residual algae biomass
A) B)
Qualitative Confirmation: Gastropods Control Algal Biomass
Quantitative Confirmation
0 50 100 150 200 250 300 3500
5
10
15
20
25
30
35
40
45
f(x) = 2.46086606987551 exp( − 0.00313473225145049 x )R² = 0.640491892532922
f(x) = 12.8376995134515 exp( − 0.00459539280306784 x )R² = 0.55239167058656
f(x) = 14.9535593057998 exp( − 0.00782615105298781 x )R² = 0.847684879293451
f(x) = 38.1271888766727 exp( − 0.00923800694915261 x )R² = 0.934313949655305
HS
Exponential (HS)
GF
Exponential (GF)
MP
Exponential (MP)
ST
Exponential (ST)
Gastropod wet weight (g m-2)
Alg
ae
AF
DM
(g
m-2
)
Further Evidence of Alternative States
Experiment 1 – Low Initial Algae: Intermediate density of snails able to control algal accumulation.
Experiment 2 – High Initial Algae: No density of snails capable of controlling accumulation.
Shape of hysteresis is site dependent.
Alternative Mechanisms?Declines in animal populations that
control algae [top-down effects]– Mullet excluded (90+% loss) from
Silver Springs with construction of Rodman dam
– ~2 orders of magnitude increase in snail density with distance downstream in Ichetucknee
• Changes in flow (direct and indirect effects)– Significant declines regionally
(Kissingen Springs)• Changes in human disturbance
– Recreational burden is 25,000 visitors/mo at Wekiva Springs
0
5
10
15
20
0-1 1-2 2-3 3-4 4-5 5 +
Num
ber o
f Spr
ings
Dissolved Oxygen (mg/L)
2002
0
5
10
15
20
Num
ber o
f Spr
ings
1972
Heffernan et al. (2010)
Controls on Grazers• Dissolved oxygen is an important control• Multivariate model explained 60% of grazer
variation with DO, pH, shading, SAV and salinity
Dissolved Oxygen (mg L-1)
A)
C)
B)
Ga
stro
po
d
bio
ma
ss
(g
m-2)
DO Management Thresholds?
Experimental Manipulation of DO
Short Term DO Effects (2-day pulses of hypoxia)
• DO dramatically controls snail grazing rates
Behavioral and Mortality Responses
Complex Ecological Controls?
Heffernan et al. (2010)
Why is Grazing SO Important in Springs
• General theory on what controls primary producer community structure (Grimes 1977)– Nutrient stress (S)– Disturbance (R)– Competition (C)
• In springs, nutrients are abundant, disturbances are absent, so competion controls dynamics
• Grazing is a dominant control on competition