Estuarine Nutrient Criteria: An Effects Based Approach

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 (EPA). 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. To this end, an effects based nutrient criteria development approach has been proposed for the Pensacola Bay system on the Florida panhandle. That is, nutrient criteria development efforts were focused on the response variables (e.g., chlorophyll-a, dissolved oxygen (DO), seagrass light requirements) for protecting the ecological health of the bay system rather than solely relying on nutrient concentrations, as has been used in many other nutrient criteria development efforts.The nutrient criteria development effects based approach used dissolved oxygen standards and seagrass light requirements to determine acceptable bay chlorophyll-a levels and ultimately protective nutrient loads. The linkage between nutrient loading and nutrient effects was accomplished through the development of an estuary model that included a hydrodynamic model to represent tidal circulation in the bay system and a eutrophication model to link nutrient loads to nutrient effects. A companion paper that is being submitted is presenting the development of these models and the importance of using the models in nutrient criteria development. A recent EPA Gulf Breeze Report on Pensacola Bay, proposed nutrient and nutrient-related water quality criteria for Pensacola Bay using these same three nutrient response variables: chlorophyll-a; secchi depth (measure to determine bottom light for seagrass requirements); and DO. In establishing the nutrient criteria endpoints, a determination of the spatial area (compliance zone), averaging period (annual or seasonal average), and compliance frequency for applying the developed criteria were determined. In addition, an estimated “natural background” model scenario was completed for comparison to the existing conditions. The “natural background” projection scenario was developed assuming no point source discharges and conversion of agricultural, rangeland, urban and barren land uses to a forest land use.In order to assess nutrient loading to the bay system, a loading hindcast was completed with river flow records so that existing loads can be put into perspective with the historical variability. Based on preliminary information in the system (e.g., biological health, seagrass distribution, water quality), existing loads seem to support a healthy bay system and could be estimated as protective loads to the bay. One option for developing a protective nutrient load to the bay is to develop a never to exceed load (95th, 99th percentile load). This would allow the regulatory agencies to establish compliance with the criteria each year rather that waiting many years to establish a long term average and a specific number of years (e.g., three) to determine if there is more than one exceedance of the annual geometric mean. In addition, a never to exceed load would also allow existing load variability due to the hydrology of the river to be accounted for in setting a protective nutrient load to the bay.This paper presents a summary of the EPA proposed nutrient criteria development efforts in Florida, estuarine nutrient criteria development efforts in other Florida estuaries, and the site-specific process applied in the Pensacola Bay system. It will also present how the effects based approach to nutrient criteria development can be applied to other water bodies and is a valid and scientifically justified approach for developing nutrient criteria. That is, nutrient effects must be considered when developing nutrient criteria based on responses and more importantly in determining a protective nutrient load to a water body.