Waquoit Bay Watershed Ecological Risk Assessment: the Effect of Land-Derived Nitrogen Loads on Estuarine Eutrophication

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A watershed ecological risk assessment of Waquoit Bay, located on the south coast of Cape Cod, MA, was performed for managers to better understand the environmental impacts of human activities. An interdisciplinary and interagency workgroup identified all the stressors of concern (chemical pollution, pathogens, altered freshwater flow, nutrient enrichment/eutrophica-tion, physical alteration of habitat, and fishing/shellfishing) and selected assessment endpoints (estuarine percent eelgrass cover, finfish diversity and abundance, scallop abundance, anadromous fish reproduction, wetland bird and piping plover habitat distribution and abundance, and tissue contamination of fish and shellfish). The workgroup later decided to focus on nutrient enrichment and its impacts on percent eelgrass cover and scallop abundance.

A nitrogen loading model (NLM) was used to estimate the amount and sources of nitrogen entering the watershed, and an estuarine loading model (ELM) was used to estimate the nitrogen available to producers in shallow estuaries. The NLM indicates that atmospheric deposition is the largest source of nitrogen, but because more atmospheric nitrogen than wastewater nitrogen is intercepted in the watershed, wastewater becomes the largest contributor of nitrogen reaching the bay. By comparing increases in nitrogen loads to losses in the area of eelgrass cover over the last 60 years, it appears that eelgrass disappears once nitrogen loads reach 20 kg/ha/yr. Both the increase in nitrogen load and the decrease in eelgrass can be correlated to decreases in the annual harvest of scallops. The models provide the opportunity for managers to assess a variety of options to reduce nitrogen loads to their estuaries and to achieve the loads that could allow the return of eelgrass to the target area.


U.S. EPA. Waquoit Bay Watershed Ecological Risk Assessment: the Effect of Land-Derived Nitrogen Loads on Estuarine Eutrophication. U.S. Environmental Protection Agency, Office of Research and Development, National Center for Environmental Assessment, Washington Office, Washington, DC, 600/R-02/079, 2002.