Topic 4: Effects of Nutrient Load Reductions David W. Dilks (Presenter)

1. 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.)

2. 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

3. 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?

4. 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

5. 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

6. External Source Loads Advection and Dispersion Sediment Flux Boundary Conditions Temperature Light 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

7. 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

8. 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

10. Average Dissolved Oxygen Responses1985 - N Reductions

11. Average Dissolved Oxygen ResponsesN Reductions - All Boundaries Reduced

12. Dissolved Oxygen Sensitivity Analyses1990 Conditions 450 +30% 400 -30% 350 300 250 200 150 Percent of Baseline 100 50 0 -50 -100 Extinction Extinction Saturation Saturation Carbon: Carbon: Water Water Sediment Sediment Coefficient Coefficient Light Light Chlorophyll Chlorophyll Column Column Oxygen Oxygen Intensity Intensity Ratio Ratio Oxygen Oxygen Demand Demand Demand Demand

13. 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.

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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