The Role of Estuarine Eutrophication Models in Nutrient Criteria Development

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Abstract:

The Florida Department of Environmental Protection (FDEP) is currently in the process of developing numeric nutrient criteria for its bays and estuaries for submittal to the United States Environmental Protection Agency (USEPA). In order to avoid implementing generic nutrient criteria, FDEP has encouraged scientists, water quality managers and engineers familiar with specific water bodies to compile and develop water body specific information that can be used to develop more scientifically based criteria for individual water bodies. This paper summarizes the efforts to develop an estuary model framework that can be used to assess and develop nutrient criteria for the Pensacola Bay system, which includes Escambia Bay, Blackwater Bay, East Bay, and Pensacola Bay, located in the Florida Panhandle.

The methodology for developing proposed nutrient endpoints during this effort was to use an estuary modeling framework to assess how nutrient response variables (e.g., chlorophyll-a, dissolved oxygen, light transparency for seagrass needs) respond to changes in nutrient loadings, and to determine what nutrient loads to the system are required to protect the ecological health as indicated by these nutrient response variables. The effort involved using an existing three-dimensional, time-variable model of the Pensacola Bay system that was calibrated for two, low-flow months during 1997 and expanding it for calibration/validation to the calendar years 1997 and 1998. These years represent an average flow year (1997) and a high flow year (1998) based on USGS flow records on the Escambia River at Century, FL from 1935-2009. Both years also included sustained periods of summer low-flow that represent critical conditions in the bay.

The models used for the assessment include the hydrodynamic model, Estuarine, Coastal and Ocean Model with sediment transport (ECOMSED) (Blumberg and Mellor 1987), and the water quality model, Row Column AESOP (RCA) (HydroQual 1999), which is similar to USEPA's WASP model (originally developed by HydroQual personnel). The sediment transport component of ECOMSED was not used. The RCA model was expanded to include color as a state-variable in order to assess the light conditions necessary to establish and maintain submerged aquatic vegetation (SAV). The original version of the water quality model was also expanded to include a sediment nutrient flux model, which completes the mass balance with the water column and allows for the direct calculation of sediment oxygen demand (SOD) and nutrient fluxes rather than the need to assign these fluxes and adjust them for projection analyses.

Potential parameters that can be used to develop nutrient response criteria include: chlorophyll-a, dissolved oxygen and Secchi depth (measure of light transparency), so it was important that the model was able to assess each of these parameters accurately. Using these nutrient response criteria, protective nutrient loads to the bay system can be developed. Escambia Bay is a highly stratified system due to a large influx of fresh water from the Escambia River. Thus, it was also very important to use a hydrodynamic model to accurately reproduce the advection, dispersion, and vertical mixing processes within the Pensacola Bays system.

The model was calibrated to 1997 and 1998, and then this calibration was used as a baseline to represent existing conditions. The baseline was used to compare against a projection scenario that was meant to simulate “natural background” conditions assuming no point source discharges, and land use consisting only of forest.

The model was developed as a tool that can be used to develop site specific nutrient criteria for the Pensacola Bays system. A USEPA document (Hagy et al., 2008) that specifically addresses nutrient criteria in the Pensacola Bays system was used for comparison purposes against the model results for the existing and natural background conditions. Both Hagy et al. and the modeling results point to existing conditions as a reasonable starting point for the development of nutrient criteria in the Pensacola Bays system.
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