Oyster delta N-15 as a Bioindicator of Potential Wastewater and Poultry Farming Impacts and Degraded Water Quality in a Subestuary of Chesapeake Bay
Anthropogenic nitrogen contributes to water quality degradation, but it is difficult to distinguish sources once they are mixed in coastal ecosystems. Natural abundances of stable nitrogen isotopes (delta N-15) were measured in oyster (Crassostrea virginica) tissues (muscle, gills, and mantle) during summer 2006 to summer 2008 to identify nitrogen sources in Monie Bay (a subestuary of Chesapeake Bay) receiving freshwater inputs from three tributary creeks. The creeks (estimated flushing times: 3.5, 5.7, and 37.2 d) vary in size and potential nitrogen sources: septic systems and poultry operations (Monie Creek), crop fertilizer (Little Monie Creek), and wetlands, forest, or both (Little Creek). Grand mean oyster tissue delta N-15 values (11.8 +/- 0.4 parts per thousand in muscle, 10.4 +/- 0.4 parts per thousand in gills, and 10.5 +/- 0.3 parts per thousand in mantle) indicated a mixture of human and animal sources. Potential nitrogen loss from denitrification (15.1-24.5%) likely did not substantially modify isotopic values, and delta N-15 values were greater than would be expected from atmospheric sources, refuting these alternative explanations. Though dilute, spatial patterns supported the inference that human waste, poultry waste, or both entered Monie Bay from its watershed and the adjacent Wicomico River watershed (via mixing). Calculated nitrogen generation from poultry manure in the watershed (containing 2.5 x 10(3) people) was 2.9 x 10(4) to 1.0 x 10(6) kg of total nitrogen (TN) per year (equivalent to 6.8 x 10(3)-2.3 x 10(5) people), whereas throughout Delmarva Peninsula (containing 1.2 x 10(6) people) it was 3.9 x 10(6) to 1.3 x 10(8) kg TN y(-1) (equivalent to 9.0 x 10(5)-3.1 x 10(8) people). Conservatively estimated (based on 0.038 kg chicken(-1) y(-1)), poultry in the Monie Bay watershed generated an amount of nitrogen equivalent to that generated by 263% of the human population. Throughout Delmarva Peninsula, poultry generated an amount of nitrogen equivalent to that generated by 76% of the human population. Estuaries commonly receive nutrients from both inside and outside their watersheds, and oyster delta N-15 values elucidated this process locally.
Keywords: Bioindicators, nitrogen sources, water quality, stable nitrogen isotopes, oysters, poultry manures, land use, land cover