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Empirical Relationships Between Nitrogen Loading and Ecosystem Response in Buzzards Bay Embayments: Is There Transferability for TMDLs Elsewhere?

Joseph E. Costa 1, Ph.D.

1 Buzzards Bay Project National Estuary Program, MCZM, East Wareham, MA 02538

In 1990, the Buzzards Bay Project National Estuary Program developed a total maximum annual load (TMAL) strategy to manage anthropogenic nitrogen inputs to protect and restore water quality and living resources in coastal embayments. The approach was based on empirical comparisons between estimated loadings and ecosystem response in Buzzards Bay embayments and published studies. Nitrogen loads were estimated with GIS data and well defined loading assumptions for land uses and sewage disposal. Proposed embayment-specific loading limits were defined using depth, volume, and hydraulic turnover time, and consideration of regulatory water quality classifications. The approach was evaluated and revised using data collected through a citizen based water quality-monitoring program. Mean summertime total nitrogen, eelgrass coverage, and a Eutrophication Index showed the best correlation with estimates of loading adjusted for hydraulic turnover.

This simplified strategy had limited management success, in part because of difficulties in regulating non-point source (NPS) pollution, but led to a more ambitious effort to evaluate 89 Massachusetts embayments using more detailed embayment circulation and ecosystem response models. This ongoing Massachusetts Estuaries Project will likely cost more than $15 million and take more than a decade to complete, and will be used to support the regulatory imposition of nitrogen TMDLs on local government.

During the past two decades, ecosystem models have advanced considerably, but local regulatory tools for controlling NPS nitrogen changed little. Scientific debate about ecosystem models will continue, but ultimately, regulators make decisions using the best existing scientific information. TMDLs based on existing conditions and empirical relationships between loading and ecosystem response among similar embayments can be an important start.

Keywords: Buzzards Bay, nitrogen management, eelgrass, TMDLs, water quality, regulatory strategies

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