Loads and Transformations
Matt Cohen
SFRC/IFAS
Ichetucknee River
L.V. Korhnak
Goals
• Synthesize known knowns
– Are there “knowns” that should be downgraded to “thinks”?
• Identify known unknowns
– What do we know we don’t know?
– Research priorities?
• Unknown unknowns?
“Known” Knowns
1. Nitrogen loads to many springs have increased
•
•
•
Predominantly from “unconfined” regions
P loads are unchanged
Other factors have changed
2. Nitrate is effectively inert in the sub‐surface
•
P is rapidly attenuated
3. Isotopic evidence suggests fertilizer origin for N
4. Travel times for water average years‐to‐decades
5. Springsheds have logarithmic “capture zones”
•
Sets priorities for land acquisition and protection
6. Applied N loads and resulting spring fluxes indicate significant landscape assimilative capacity
1. N has increased
Strong 2004
Hornsby et al. 2002, 2003, 2004, 2005
Research Need: Why Are Nitrate Concentrations Going Down Locally?
Time series of flow and nitrate concentration in Wekiva Springs
Trends in nitrate concentrations in springs of the SRWMD (Upchurch et al. 2007).
2. N Inert in Subsurface
• Low assimilatory removal
– Limited biological activity
• Low dissimilatory removal
– Limited carbon availability
Katz et al. 2004
• Dissolved N2 in spring water is consistent with atmospheric equilibrium during recharge
3. N of fertilizer origin
Champion and Starks 2001
Research Need: Improve Poorly Constrained Isotope Source Model
• End‐members (fertilizer 15N < 6‰, organic 15N > 10‰) are potentially confounded
– Biological N fixation ~ ‐3‰
– Nitrification (ammonia to nitrate)
lowers δ15N if incomplete
• What is required is path‐dependent measurements of isotopic fractionation
– Improve the specificity of “sourcing” by better constraint on end‐members
4. Water fluxes of different ages
Katz 2004
Research Need: Time Domain of Springshed Residence Times
• Time domain uncertainty
– Nitrate serially uncorrelated at current sampling regime
– Isotope measurements apparently highly variable in time
– Strong disagreement between dye trace and age‐date measurements
• Time‐series of age measurements can provide improved understanding of water mixture at vent
5. Variable Capture Zones
Phelps 2004
6. Landscape assimilation
Respectively:
Chelette et al. 2002
Katz et al. 1999
MACTEC 2007
Phelps 2004
Jones et al. 1996
Research Need: Where did the N go?
• NE rivers export ~ 20% of the load
• Springs appear to export ~ 10% of load
– Where does it go?
– Why is the landscape less vulnerable than expected?
– Can sinks be enhanced?
Van Breeman et al. 2002
Known Unknowns
1. Load, transport & subsurface reactivity
A. Phosphorus (exchange saturation front?)
B. Micronutrients
C. Carbon, BOD and dissolved oxygen
2. Temporal dynamics
A. Nitrate loading
B. Flow vs. Age vs. basic water chemistry (DO, conductivity)
3. Sinks for nutrients
A. Assimilatory vs. dissimilatory for N (conduits, channels)
B. Biotic vs. abiotic for P
C. Saturation of attenuation
Research Need: Improve Incomplete N Budgeting
• Biological N fixation is unaccounted for
– 25‐35% of contemporary budget in NE USA
– 10‐20 kg/ha/yr
– 5x higher than atmospheric deposition in Florida (4 kg N/ha/yr)
• Systems boundaries are poorly defined
– Cross‐boundary flows are generally neglected in place of loading rates
Van Breeman et al. 2002
Research Need: Improve Incomplete P Budgeting
• P Sources
– What fraction derives from Hawthorn apatite?
• P and F are significantly associated (r = +0.41)
– Essentially no information on P concentrations along karst flowpaths
– No change = No worry
• P sinks
– Not infinite
– Non‐linear breakthrough
– Functionally permanent source after breakthrough
Research Need: Move Beyond a Singular Focus on N (or indeed nutrients)
• Other attributes of springs have changed
– Mechanisms are speculative and multiple
• Changing ages of water with drought, pumping
• Landscape loading of BOD
– Effects are plausible, but only crudely demonstrated
• DO effects on grazers
• Flow effects on residence times and shear environment
Research Need: DO and Grazers
20
15
Count
• ONLY parameter covarying with algae cover in Stevenson (2004) report
• Habitat exclusion ~ 2.0 mg/L
10
m5
– Insect grazers
– Fish
0
20
2002
15
Count
• Increased algal biomass in response to reduced herbivory
• One septic tank deoxygenates (5 to 0 mg/L) ~ 60 million gallons/yr of UFA
1972
10
m5
0
0‐1
1‐2
2‐3
3‐4
4‐5
Dissolved Oxygen (mg/L)
5 +
Unknown Unknowns…
• Adaptive management
– Unique opportunity in springs because of potential examination of multiple explanations
– Willingness to view prevailing paradigm as hypotheses
– Bold experiments to expedite learning
– Managing for surprise and uncertainty
Thanks.
[email protected]
Mill Pond
L.V. Korhnak