topic 4: effects of nutrient load reductions david w. dilks (presenter)
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Gulf of Mexico Hypoxia Assessment Plan
Committee on Environment and Natural ResourcesHypoxia Work Group
Topic 4: Effects of Nutrient Load Reductions
David W. Dilks (Presenter)
Patrick L. Brezonik (Watershed P. I.)
Victor J. Bierman, Jr. (Gulf of Mexico P. I.)
Objectives
Evaluate the effect of nutrient-source reductions that may be implemented in the Mississippi River Basin on:– Water quality in the drainage basin itselfWater quality in the drainage basin itself
– Water quality in the nearshore Gulf watersWater quality in the nearshore Gulf waters
Study Questions
What are the effects of reducing MRB nutrient loads on
– Nutrient concentration in the flowing waters of the basin?Nutrient concentration in the flowing waters of the basin?
– Water quality and ecological condition in the flowing waters Water quality and ecological condition in the flowing waters of the basin?of the basin?
– Dissolved oxygen and chlorophyll on the LIS? Dissolved oxygen and chlorophyll on the LIS?
What magnitude of reductions in nutrient loadings might be necessary to improve present water quality conditions, especially seasonal hypoxia?
Watershed Approach
Consider both nitrogen and phosphorus
Case study on Minnesota River examining effectiveness of improved management practices
Examine nutrient retention in the flowing waters of the Mississippi River Basin
Examine benefit of lower nutrient concentrations on ecosystem and water quality
Gulf of Mexico Approach
Develop deterministic water quality model of the Louisiana Inner Shelf portion of the Gulf of Mexico– WASP modelWASP model
– Relatively simple kinetic complexityRelatively simple kinetic complexity
– Externally specified hydrodynamicsExternally specified hydrodynamics
Apply model to investigate relationship between nutrient load reductions and dissolved oxygen/ chlorophyll a concentrations on the LIS
DissolvedOxygen
OrganicN
OrganicC
NH3NO2 +NO3
OrganicP
SRP
Phyto-plankton
Zoo-plankton
Sediment
LightTemperatureSediment
FluxBoundary
ConditionsAdvection and
DispersionExternal Source
Loads
Water Column
Denitrification Settling
Settling
Oxidation
Nitrification
Denitrification
Reaeration
Respiration / Decay
Respiration
Photosynthesis
SOD Settling
Settling
Grazing
Watershed Findings
Nutrient loss processes from agricultural lands differ between N and P– N: Subsurface drainageN: Subsurface drainage
– P: ErosionP: Erosion
Water quality standards violations are rare, but proposed nutrient criteria routinely exceeded
River productivity correlated to phosphorus
Gulf Findings
Dissolved oxygen and chlorophyll a on the LIS
appear to be sensitive to changes in nutrient loads
– Nitrogen more important than phosphorusNitrogen more important than phosphorus
– A 20-30% reduction in TN loads could increase bottom A 20-30% reduction in TN loads could increase bottom
water dissolved oxygen by 15 to 50%water dissolved oxygen by 15 to 50%
Sensitivity analyses conducted to determine primary
sources of uncertainty:
– seaward boundary conditions; underwater light seaward boundary conditions; underwater light
attenuation; sediment oxygen demand; variability in attenuation; sediment oxygen demand; variability in
hydrometeorologyhydrometeorology
Average Dissolved Oxygen Responses1985 - N Reductions
Average Dissolved Oxygen ResponsesN Reductions - All Boundaries Reduced
Dissolved Oxygen Sensitivity Analyses1990 Conditions
ExtinctionCoefficient
SaturationLight
Intensity
Carbon:Chlorophyll
Ratio
WaterColumnOxygenDemand
SedimentOxygenDemand
-100
-50
0
50
100
150
200
250
300
350
400
450
Perc
ent
of
Base
line
ExtinctionCoefficient
SaturationLight
Intensity
Carbon:Chlorophyll
Ratio
WaterColumnOxygenDemand
SedimentOxygenDemand
+30%
-30%
Basin Monitoring Recommendations
Routine monitoring programs by local, state and federal agencies are essential should be continued
Additional monitoring sites are needed in the Upper Mississippi main channel to evaluate nutrient retention/loss in the lock and dam system.
Monitoring needs to be expanded in the Lower Mississippi to clarify the extent of nutrient retention.
Basin Monitoring Recommendations
Better monitoring is needed at fine spatial scales to establish effects of changes in land management on nutrient loads
Long-term, intensive monitoring/research sites should be established at the field/minor watershed scale
Monitoring fertilizer use patterns is critical for targeting improvements in management practices within identified problem areas
Basin Research Recommendations
Studies on improved management practices to minimize off-site impacts of agricultural production
Research on impacts of large confined-animal-feeding-operations (CAFOs) and ways to minimize
Better information on rates of nitrification and denitrification and factors affecting these processes
Information on mechanisms of P retention and factors affecting these processes
Basin Research Recommendations
Assess whether the mechanism(s) causing shifts towards dominance of plankton by blue-green algae in eutrophic rivers are the same as or different from those causing blue-green blooms in lakes
N- vs. P-limitation should be assessed by bioassays and algal tissue analysis
Importance of light vs. nutrient limitation of algal growth in rivers needs to be assessed
Critical nutrient concentrations and loading rates need to be developed for flowing waters
Basin Modeling Recommendations
Further development and field testing of the SWAT-based, national- scale model for nutrient export and transport should be pursued
Further development of regression-based models relating nutrient-related variables to stream trophic state and nutrient loading from the watershed
Further modeling efforts are needed in extending the chemical reactor modeling approach of Vollenweider to rivers, for both N and P
The N/P ratio hypothesis needs further clarification
Basin Modeling Recommendations
Models for algal growth in rivers should focus on peak biomass, not only on mean annual biomass
Models should be developed at different levels of complexity–from spreadsheet to complex simulation models–to relate watershed export and stream nutrient concentrations and transport in the MRB
Models to predict effects of changes in river nutrient levels on fish yield or fish species composition are lacking, and should be developed
Gulf Monitoring Recommendations
Future monitoring design should be driven by management questions, and should be based on a quantitative ecosystem model
Monitoring should be conducted on a hierarchy of spatial scales, temporal scales and parameters
There is a basic need for physical oceanographic data on water movements
Gulf Monitoring Recommendations
Data needed on light attenuation and other correlated parameters
In-situ measures of primary productivity
Comprehensive data specifying external model forcing functions
Gulf Research Recommendations
Emphasis should be placed on better defining physical, chemical and biological processes– Primary productivityPrimary productivity
Indigenous species, light dependency
– Factors controlling underwater light attenuationFactors controlling underwater light attenuation
– Fate pathways for organic carbonFate pathways for organic carbon
– Cycling and transformation of nutrients, carbon and Cycling and transformation of nutrients, carbon and oxygenoxygen
– Sediment processes and sediment-water interactionsSediment processes and sediment-water interactions
– Shifts in phytoplankton species abundanceShifts in phytoplankton species abundance
Gulf Modeling Recommendations
The water quality model should be directly coupled with a hydrodynamic model
The temporal domain should be extended to include continuous representation of water quality conditions
The spatial domain should be extended to include entire Gulf of Mexico
Gulf Modeling Recommendations
The horizontal and vertical spatial resolution should be refined
A sediment diagenesis submodel is needed
The model should be expanded to include multiple phytoplankton groups and silicon
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