Stream Nutrient Processing: Spiraling, Removal and Lotic Eutrophication Ecohydrology Fall 2015

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? (GPP) Justus von Liebig

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

Nutrient Cycling vs. Spiraling 1) Cycling in closed systems 2) Cycling in open ecosystems [creates spirals] Inorganic forms Advective flow Organic forms Longitudinal Distance

Components of a Spiral + Distance Time Inorganic forms Organic forms length (S) Uptake length (Sw) Turnover length (So) + =

Nutrient Spiraling From : Newbold (1992)

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

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:

Uptake Length (Sw) 1/k = Sw 1/k = Sw Tracer abundance Best-fit regression line using: Fx = F0e-kx 1/k = Sw 1/k = Sw 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) Tracer abundance Field data Longitudinal distance

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)

Spiral Length in Headwater Streams (dominated by uptake length) Time Longitudinal distance Advective flow Uptake length (Sw) Turnover length (So)

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

Spiraling Metric Triad Solute 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-M Kinetics for U provides predictions for Sw and Vf profiles Linear Transitional Saturated Umax C C + Km U = U vd Umax Sw = C vd Km + Sw vf Umax C + Km vf = Nutrient availability

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

Enrichment Affects Kinetics Mulholland et al. (2002)

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

‰ 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 ‰

Natural Abundances of Isotopes - + light heavy -10 +30

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

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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 Phosphorus limitation in shallow temperate lakes Nitrogen limitation in estuarine systems (GPP) V. Smith, L&O 2006 V. 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

Mission Springs Chassowitzka (T. Frazer) Mill Pond Spring Weeki Wachee Weeki Wachee 1950’s 2001

In Lab Studies: Nitrate Stimulates Algal Growth Stevenson et al. 2007 In laboratory studies, nitrate increased biomass and growth rate of the cyanobacterium Lyngbya wollei. Cowell and Dawes 2004

Evidence generally runs counter to this hypothesis 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 Fall 2002 (closed circles) Spring 2003 (open triangles) From Stevenson et al. 2004 Ecological condition of algae and nutrients in Florida Springs DEP Contract #WM858 No useful correlation between algae and nitrate concentration

Visualizing the Problem Silver Springs (1,400 ppb N-NO3) Alexander Springs (50 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) YSI Multiprobe Submersible UV Nitrate Analyzer (SUNA)

Diel Method for Estimating Autotrophic N Demand [NO3-]max [NO3-] Autotrophic Assimilation [NO3-]min Assumptions: No autotrophic assimilation at [NO3-]max Other processes constant (unknown) Other N species constant (validated) Heffernan and Cohen 2010

Ua Estimates Yield Reasonable C:N Stoichiometry at the Ecosystem Scale NPP = Ua * 25.4 R2 = 0.67, p < 0.001 Net Primary Production (NPP) (mol C/m2/d) C:N Ratios Vascular Plants ~ 25:1 Benthic Algae ~ 12:1 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 Rates Dissolved 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. Weber and Perry 2006 Changes in flow occur in response to climate drivers and human appropriation Kissingen Springs 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

Observational Support: Grazer Control Algal Biomass Accrual Gastropod biomass (g m-2) Algae biomass (g m-2) y = 2350x-1.592 R² = 0.38 p < 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). Liebowitz et al. (in review)

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 Proportional Frequency Gastropod biomass < 20 g m-2 Gastropod 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 0.20 0.25 -6 -4 -2 0 2 4 6 -6 -4 -2 0 2 4 6 A) B)

Qualitative Confirmation: Gastropods Control Algal Biomass

Quantitative Confirmation

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 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) Gastropod biomass (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