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