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