Oyster deltaN-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 (deltaN-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 deltaN-15 values (11.8 ± 0.4‰ in muscle, 10.4 ± 0.4‰ in gills, and 10.5 ± 0.3‰ 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 deltaN-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 10ˆ3 people) was 2.9 x 10ˆ4 to 1.0 x 10ˆ6 kg of total nitrogen (TN) per year (equivalent to 6.8 x 10ˆ3–2.3 x 10ˆ5 people), whereas throughout Delmarva Peninsula (containing 1.2 x 10ˆ6 people) it was 3.9 x 10ˆ6 to 1.3 x 10ˆ8 kg TN yˆ1 (equivalent to 9.0 x 10ˆ5–3.1 x 10ˆ8 people). Conservatively estimated (based on 0.038 kg chickenˆ-1 yˆ-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 deltaN-15 values elucidated this process locally.

Keywords: Bioindicators, nitrogen sources, water quality, stable nitrogen isotopes, oysters, poultry manures, land use, land cover