Deleterious effects of nitrogen loading in the Chesapeake Bay and significant inputs from septic systems and sewage treatment plants have been well documented. This project demonstrates that the native eastern oyster, Crassostrea virginica, is a suitable organism for use in the Chesapeake Bay and its tributaries as a biological indicator of these two nitrogen sources using nitrogen stable isotope techniques.

Tissues from the adductor muscle, gills, and mantle were sampled for isotopic nitrogen analysis. Oysters were sampled at two sites: in the Choptank River at Shoal Creek with proximity to the Cambridge sewage treatment plant, and the South River at Glebe Bay surrounded by low density residential land use with septic systems. Relative ratios of nitrogen were assessed and comparisons across land use, total dissolved nitrogen, and chlorophyll a were conducted. As expected, δ¹⁵N varied across tissues. Muscle showed higher δ¹⁵N values (δ¹⁵N 16.3 in the Choptank and 14.0 in the South Rivers) than either the mantle (δ¹⁵N 14.0 in the Choptank and 12.4 in the South River) or the gills (δ¹⁵N 14.3 in the Choptank and 12.6 in the South River). Furthermore, higher relative ratios of isotopic nitrogen were found in the Choptank River (δ¹⁵N 14.8 with standard error 0.16) than in the South River (δ¹⁵N 13.0 with standard error 0.11). Over the time period where nitrogen was available for the oysters, the two sites had similar total dissolved nitrogen and chlorophyll a levels, so δ¹⁵N values were not skewed. δ¹⁵N values in this study are comparable to those found in the literature for macroalgae in the Choptank and oysters found in the Maroochy River, Australia.

Variability between oysters was low for all tissues at both sites. Therefore, sampling only a few oysters per site is necessary, making sampling efforts more efficient and able to span a broader spatial scale. Developed methodologies utilizing these techniques can be incorporated in established monitoring programs, e.g. that of the Chesapeake Bay Program. Eastern oysters are a regional, culturally identifiable, symbol of the bay's declining health. Utilizing this easily recognizable natural resource as an indicator of nutrient inputs will facilitate and enhance effective communication of subsequent nutrient monitoring results. Such monitoring efforts can then serve as baseline evidence for evaluation of current nutrient management practices and guide future efforts. Nutrient source monitoring may be further integrated with additional environmental variables, such as precipitation, to create predictive spatial models of nutrient sources.

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