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

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’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

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

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 Time Inorganic forms Organic forms
length (S)
Uptake
length
(Sw)
Turnover
length
(So) + =

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 1st order (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. 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

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

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. 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

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
-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 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
The Floridan Aquifer (our primary water source) is:
Vulnerable to nitrate contamination
Locally enriched as much as 30,000% over background (~ 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

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

48. 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

49. 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)

50. Field Measurements: Nitrate vs. Algae in Springs
Fall 2002 (closed circles)
Spring 2003 (open triangles)
From Stevenson et al Ecological condition of algae and nutrients in Florida Springs DEP Contract #WM858
No useful correlation between algae and nitrate concentration

51. Visualizing the Problem
Silver Springs (1,400 ppb N-NO3)
Alexander Springs (50 ppb N-NO3)

52. Synthesis of Ecosystem Productivity: Nitrate vs. Metabolism in Springs
Data Sources:
- WSI (2010)
- WSI (2007)
- WSI (2004)
- Cohen et al. (2013)

53. Slight Digression - Nutrient Contamination Broadly in Florida
Source: USEPA (

54. 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?

55. 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

56. 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.

57. 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?

58. 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

59. 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

60. 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

61. Estimating Ua from Diel Nitrate Variation (Ichetucknee River, 5 km downstream of headspring)
YSI Multiprobe
Submersible UV Nitrate Analyzer (SUNA)

62. 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

63. 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)

64. Inducing N Limitation in Spring Runs [some were, many springs were not N limited at 0.05 mg/l]

65. Autotrophic Uptake Length Globally

66. 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

67. So – Why All the Algae?

68. 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

69. 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

70. 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

71. 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)

72. Flow and DO Affect Grazers

73. Observational Support: Grazer Control Algal Biomass Accrual
Gastropod biomass (g m-2)
Algae biomass (g m-2)
y = 2350x 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)

74. 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 A) B)

75. Qualitative Confirmation: Gastropods Control Algal Biomass

76. Quantitative Confirmation

77. 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.

78. 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)

79. 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)

80. DO Management Thresholds?

81. Experimental Manipulation of DO

82. Short Term DO Effects (2-day pulses of hypoxia)
DO dramatically controls snail grazing rates

83. Behavioral and Mortality Responses

84. Complex Ecological Controls?
Heffernan et al. (2010)

85. 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