Stream Nutrient Processing: Spiraling, Removal and Lotic Eutrophication

1. Stream Nutrient Processing: Spiraling, Removal and Lotic Eutrophication Ecohydrology Fall 2013

2. 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

3. Global Ratios of Supply and Demand – Aquatic Ecosystems

4. Inducing Eutrophication Leibig’sLaw 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

5. 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

6. Phosphorus Cycle http://arnica.csustan.edu/carosella/Biol4050W03/figures/phosphorus_cycle.htm

7. 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)

9. 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

10. 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

11. 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

12. 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

13. Nutrient Spiraling in Streams

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

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

16. Nutrient Spiraling From : Newbold (1992)

17. 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 1storder (exponential decline) • Constant mass loss FRACTION per unit distance

18. 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:

19. 1/k = Sw 1/k = Sw Uptake Length (Sw) 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) Tracer abundance Field data Longitudinal distance

20. 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)

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

22. 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

23. 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

24. Spiraling Metric Triad Solute Spiraling Metric Triad

25. 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

26. UmaxC U = C + Km vd vd Km Sw = C + Umax Umax Umax vf = C + Km M-M Kinetics for U provides predictions for Sw and Vfprofiles Linear Transitional Saturated U Sw vf Nutrient availability

27. 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

28. Enrichment Affects Kinetics Mulholland et al. (2002)

29. 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

30. ‰ ‰ ‰ 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

31. Natural Abundances of Isotopes - + light heavy 0 -10 +30

32. 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

33. So – How to Uptake Length (Addition vs. Isotope) Compare?

34. 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

35. What Happens to Uptake Length as we Add Nutrients • Sequential steady state additions (Earl et al. 2006)

36. Back-Extrapolating From Nutrient Additions • Multiple additions (Payn et al. 2005) result in a curve from which ambient (background) uptake rate can be inferred

37. Laborious but Fruitful(back extrapolation to negative ambient)

38. Lazy People Make Science Better • Use a single pulse co-injection to get at multiple concentrations in one experiment (Covino et al. 2010)

39. 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

40. Data

41. 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

42. 19_16.jpg

43. Invertebrates and Spiraling Length

44. 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

45. 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

46. Local Nitrogen Enrichment Arthur et al. 2006 • 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)

47. Mission Springs Chassowitzka (T. Frazer) Mill Pond Spring WeekiWachee WeekiWachee 1950’s 2001

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

49. 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 uncorrelatedwith [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)

50. 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