Ecohydrology Fall 2017 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 ID: 757081
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Slide1
Stream Nutrient Processing: Spiraling, Removal and Lotic Eutrophication
Ecohydrology
Fall
2017Slide2
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…
ecohydrologySlide3
Global Ratios of Supply and Demand – Aquatic EcosystemsSlide4
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 limitsWhat limits productivity in forests? Crops? Lakes? Pelagic ocean?
Justus von Liebig
(GPP)Slide5
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 interactionsSorption, co-precipitation in many minerals (apatite), solubility that is redox sensitiveSlide6
Phosphorus Cycle
http://arnica.csustan.edu/carosella/Biol4050W03/figures/phosphorus_cycle.htmSlide7
Nitrogen Cycle
Includes major atmospheric pool - N
2
.
N fixers use
atmospheric supply directly
(prokaryotes
).Energy-demanding process; reduces to N
2 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)Slide8Slide9
Global Nitrogen Enrichment
Humans have massively amplified global N cycle
Terrestrial Inputs
1890: ~ 150
Tg
N yr
-1
2005: ~ 290+
Tg N yr-1River Outputs1890: ~ 30
Tg N yr-12005: ~ 60+ Tg N yr
-1N frequently limits terrestrial and aquatic primary productionEutrophication
Gruber and Galloway 2008Slide10
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 geometryCan be saturated
Boyer et al. 2006
Van
Breeman
et al. 2002Slide11
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 prioritySlide12
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 SPIRALSSpiraling Length is the length of a stream required for a nutrient atom to complete a cycle (mineral – organic – mineral).Uptake (assimilation + other removal processes)
RemineralizationSlide13
Nutrient Spiraling in StreamsSlide14
2) Cycling in open
ecosystems
[
creates spirals]
Longitudinal Distance
Inorganic
forms
Organic
forms
1) Cycling in
closed
systems
Advective flow
Nutrient Cycling vs. SpiralingSlide15
Distance
Time
Uptake
length
(
S
w
)
Turnover
length
(
S
o
)
+
Spiral
length (S)
=
Inorganic
forms
Organic
forms
Components of a SpiralSlide16
From : Newbold (1992)
Nutrient SpiralingSlide17
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 ratesUsually assumed
1st order (exponential decline)Constant mass loss FRACTION per unit distanceSlide18
Constant Fractional Loss
Basis for exponential decline
dF
/dx = -
k
L
* 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:Slide19
1/k =
S
w
Longitudinal distance
Tracer abundance
Field data
Best-fit regression line using:
F
x
= F
0
e
-kx
where:
F
x
= tracer flux at distance x
F
0
= tracer flux at x=0
x = distance from tracer addition
k = longitudinal loss rate (fraction m
-1
)
1/k =
S
w
Uptake Length (
S
w
)Slide20
Turnover Lenth (SB
)
Distance that a nutrient atom travels in organic (biotic form) before being
remineralized
to the water column
Hard to measure directly
R
egeneration 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 (
v
B
= u * X
S
/X
B
)Slide21
Time
Longitudinal distance
Advective flow
Uptake length (
S
w
)
Turnover length (
S
o
)
Spiral Length in Headwater Streams
(dominated by uptake length)Slide22
Open Controversy
Headwater systems have short uptake lengths
Direct (1
st
) 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 lengthUptake 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 riversSlide23
Linking Uptake Length to Associated Metrics
Uptake velocity (
v
f
; rate at which solutes move towards the benthos; measure of uptake
efficiency
relative to supply) [L T
-1]v
f = u * d / Sw = u * d *
kLUptake rate (U; measure of flux per unit area from water column to the benthos) [M L-2
T-1]U = vf * CSlide24
Solute
Spiraling
Metric
Triad
Spiraling Metric TriadSlide25
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 availabilitySlide26
Nutrient availability
U
U
max
C
C + K
m
U =
Linear
Transitional
Saturated
U
max
C + K
m
v
f
=
v
f
S
w
vd
U
max
S
w
=
C
vd K
m
U
max
+
M-M Kinetics for U
provides predictions for
S
w
and
V
f
profilesSlide27
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 availabilityChanges in availability affects uptake kinetics
Ergo – adding nutrients (changing the concentration) changes the thing we’re trying to measureSlide28
Mulholland et
al. (2002)
Enrichment Affects KineticsSlide29
Alternative Approach
Add isotope tracer (
15
N)
Isotope are forms of the same atom (same atomic number) with different atomic mass (different number of neutrons)
Two isotopes of N,
14
N (99.63%) and 15N (0.37%)We can change the isotope ratio (15N : 14N) a LOT without changing the N concentrationTrace the downstream progression of the 15
N enrichment to discern processes and ratesSlide30
‰
‰
‰ Notation
The “per mil”
or
“‰” or “
δ
” notation
R is the isotope ratio (
15
N:
14
N)
Reference standard (
R
std
) for N is the atmosphere (by definition, 0‰)
More
15
N (i.e., heavier) is a higher
δ
valueSlide31
light
heavy
0
+
-
-10
+30
Natural Abundances of IsotopesSlide32
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 (
ε
Fix =
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 (
ε
Den
=
δ
NO3
–
δ
N2
=
5 to 40‰)
Note that where denitrification happens, it yields nitrate that “looks” like its from organic waste and septic tanksSlide33
So – How to Uptake Length (Addition vs. Isotope) Compare?Slide34
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 methodsSlide35
What Happens to Uptake Length as we Add Nutrients
Sequential steady state additions
(Earl et al. 2006)Slide36
Back-Extrapolating From Nutrient Additions
Multiple additions (
Payn
et al. 2005) result in a curve from which ambient (background) uptake rate can be inferredSlide37
Laborious but Fruitful(back extrapolation to negative ambient)Slide38
Lazy People Make Science Better
Use a single pulse co-injection to get at multiple concentrations in one experiment (
Covino
et al. 2010)Slide39
Method Outline
Add tracers in known ratio
Measure the change in ratio with concentration; the ratio at each time yields an uptake length (
S
w
) which can be indexed to concentration
U can be obtained from
Sw
from the triad diagram (U = u*d*C/Sw = Q*C/w*S
w)Fit to Michaelis-Menten kinetics and back extrapolate to ambientSlide40
DataSlide41
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 elementsIn a “pure” limitation, uptake length goes to zero and all downstream transport occurs via
organic particlesCONCENTRATION GOES TO ZERO @ LIMITATIONAny biota that accelerate remineralization (e.g., shorten turnover length) amplify productivityInvertebrates accelerate remineralizationSlide42
19_16.jpgSlide43
Invertebrates and Spiraling LengthSlide44
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 impactsChanges in color, clarity, aromaSlide45
Typical Symptoms: Alleviation of Nutrient Limitation
(GPP)
Phosphorus limitation in shallow temperate lakes
Nitrogen limitation in estuarine systems
V. Smith, L&O 2006
V. Smith, L&O 1982Slide46
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 pollutionFlorida has world’s highest density of 1st magnitude springs (> 100 cfs)
Arthur et al. 2006Slide47
Weeki
Wachee
2001
1950’s
Mission Springs
Chassowitzka
(T. Frazer)
Weeki
Wachee
Mill Pond SpringSlide48
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. 2007Slide49
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)Slide50
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 concentrationSlide51
Alexander Springs
(50
ppb N-NO3)
Visualizing the Problem
Silver Springs (1,400
ppb N-NO3)Slide52
Synthesis of Ecosystem Productivity:
Nitrate vs. Metabolism in Springs
Data Sources:
- WSI (2010)
- WSI (2007)
- WSI (2004)
- Cohen et al. (2013)Slide53
Slight Digression - Nutrient Contamination Broadly in Florida
Source: USEPA (
http://iaspub.epa.gov/waters10/state_rept.control?p_state=FL&p_cycle=2002
) Slide54
Recent Developments – Numeric Nutrient Criteria
Nov 14
th
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-protectiveWhy?Slide55
Stressor – Response for Streams
No association found between indices of ecological condition and nutrient levels
Elected to use a reference standard where the 90
th
percentile of
unimpacted
streams is the criteriaSlide56
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.Slide57
Qualitative Insight: Comparing Assimilatory Demand vs. Load
Primary Production is very high
8-20 g O
2
/m
2
/d (ca. 1,500 g C/m
2/yr)N demand is basically proportional
0.05 – 0.15 g N/m2/dayN flux (over 5,000 m reach) is largeNow: 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 demandWhen does flux:demand become critical?Slide58
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/
U
a
) (
unitless)Requires arbitrary reach length to estimate demandAutotrophic uptake length (Sw,a) (length units)Consistent with nutrient spiraling theory (Newbold
et al. 1982)Ratio of flux to width-adjusted benthic uptakeSlide59
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,NFor P: Sw,a,PSlide60
Predicting GPP Response
Nutrient Limitation Assay (NLA)
Relative response (RR) of N
enrichment:control
Regressed vs. Concentration and
S
w,a,N
NLA Response Data from Tank and
Dodds
(2003); Analysis by Sean KingSlide61
Estimating U
a
from Diel Nitrate Variation
(Ichetucknee River, 5 km downstream of headspring)
Submersible UV Nitrate Analyzer (SUNA)
YSI
MultiprobeSlide62
Autotrophic Assimilation
[NO
3
-
]
[
NO
3
-
]
min
[NO
3
-]
max
Assumptions: No autotrophic
assimilation at
[NO
3
-]
max
Other processes constant (unknown)
Other N species constant (validated)
Diel Method for Estimating Autotrophic N Demand
Heffernan and Cohen 2010Slide63
Ua
Estimates Yield Reasonable C:N Stoichiometry at the Ecosystem Scale
NPP =
U
a
* 25.4
R2 = 0.67, p < 0.001
C:N Ratios
Vascular Plants ~ 25:1Benthic Algae ~ 12:1Net Primary Production (NPP) (mol C/m2
/d)N Assimilation (Ua) (mol N/m2/d)Slide64
Inducing N Limitation in Spring Runs
[some
were
, many springs
were not
N limited at
0.05 mg/l
]Slide65
Autotrophic Uptake Length GloballySlide66
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 basisSlide67
So – Why All the Algae?Slide68
Back to First Principles:
Controls on Algal Biomass
bottom up effects
top
down effects
Algae Biomass
Grazers
Flow Rates
Dissolved Oxygen
Nutrients
Light
mediating factorsSlide69
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 timesThe changes are understudied because of the plausibility of the N loading story
Data from Scott et al. 2004Slide70
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. 2007Slide71
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)Slide72
Flow and DO Affect GrazersSlide73
Observational Support: Grazer Control Algal Biomass Accrual
Liebowitz
et al.
(
in review)
A)
B)
C)
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).Slide74
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
Residual algae biomass
A)
B)Slide75
Qualitative Confirmation: Gastropods Control Algal BiomassSlide76
Quantitative ConfirmationSlide77
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.Slide78
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 disturbanceRecreational burden is 25,000 visitors/mo
at Wekiva Springs
Heffernan et al.
(2010)Slide79
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
)Slide80
DO Management Thresholds?Slide81
Experimental Manipulation of DOSlide82
Short Term DO Effects (2-day pulses of hypoxia)
DO dramatically controls snail grazing ratesSlide83
Behavioral and Mortality ResponsesSlide84
Complex Ecological Controls?
Heffernan et al.
(2010)Slide85
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