Gulf of Mexico Hypoxia Assessment Plan Committee on Environment and Natural Resources Hypoxia Work G

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Gulf of Mexico Hypoxia Assessment PlanCommittee
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.)

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Objectives

• Evaluate the effect of nutrient-source reductions
  that may be implemented in the Mississippi River
  Basin on
• Water quality in the drainage basin itself
• Water quality in the nearshore Gulf waters

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Study Questions

• What are the effects of reducing MRB nutrient
  loads on
• Nutrient concentration in the flowing waters of
  the basin?
• Water quality and ecological condition in the
  flowing waters of the basin?
• 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?

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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

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Gulf of Mexico Approach

• Develop deterministic water quality model of the
  Louisiana Inner Shelf portion of the Gulf of
  Mexico
• WASP model
• Relatively simple kinetic complexity
• Externally specified hydrodynamics
• Apply model to investigate relationship between
  nutrient load reductions and dissolved oxygen/
  chlorophyll a concentrations on the LIS

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Sediment Flux
Boundary Conditions
Advection and Dispersion
External Source Loads
Light
Temperature
Water Column
Denitrification
Organic C
NO2 NO3
NH3
SRP
Reaeration
Denitrification
Settling
Organic P
OrganicN
Nitrification
Oxidation
Settling
Settling
Photosynthesis
Phyto- plankton
Zoo- plankton
Dissolved Oxygen
Grazing
Respiration
Respiration / Decay
Settling
SOD
Sediment
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Watershed Findings

• Nutrient loss processes from agricultural lands
  differ between N and P
• N Subsurface drainage
• P Erosion
• Water quality standards violations are rare, but
  proposed nutrient criteria routinely exceeded
• River productivity correlated to phosphorus

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Gulf Findings

• Dissolved oxygen and chlorophyll a on the LIS
  appear to be sensitive to changes in nutrient
  loads
• Nitrogen more important than phosphorus
• A 20-30 reduction in TN loads could increase
  bottom water dissolved oxygen by 15 to 50
• Sensitivity analyses conducted to determine
  primary sources of uncertainty
• seaward boundary conditions underwater light
  attenuation sediment oxygen demand variability
  in hydrometeorology

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Average Dissolved Oxygen Responses1985 - N
Reductions
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Average Dissolved Oxygen ResponsesN Reductions -
All Boundaries Reduced
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Dissolved Oxygen Sensitivity Analyses1990
Conditions
450
30
400
-30
350
300
250
200
150
Percent of Baseline
100
50
0
-50
-100
Extinction
Saturation
Carbon
Water
Sediment
Extinction
Saturation
Carbon
Water
Sediment
Coefficient
Light
Chlorophyll
Column
Oxygen
Coefficient
Light
Chlorophyll
Column
Oxygen
Intensity
Ratio
Oxygen
Demand
Intensity
Ratio
Oxygen
Demand
Demand
Demand
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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.

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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

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Basin Research Recommendations

• Studies on improved management practices to
  minimize off-site impacts of agricultural
  production
• Research on impacts of large confined-animal-feedi
  ng-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

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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

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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

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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
  complexityfrom spreadsheet to complex simulation
  modelsto 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

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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

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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

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Gulf Research Recommendations

• Emphasis should be placed on better defining
  physical, chemical and biological processes
• Primary productivity
• Indigenous species, light dependency
• Factors controlling underwater light attenuation
• Fate pathways for organic carbon
• Cycling and transformation of nutrients, carbon
  and oxygen
• Sediment processes and sediment-water
  interactions
• Shifts in phytoplankton species abundance

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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

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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