This dissertation demonstrates that stable nitrogen isotope signatures (δ15N) in oysters (Crassostrea virginica) can identify anthropogenic nitrogen sources (a cause of degraded water quality) at multiple spatial scales in Chesapeake Bay and Maryland's Coastal Bays. Fieldwork, monitoring and land use data, spatial analyses, and modeling techniques were employed. Due to minimal tissue &delta15N variations between individuals as replicates (standard error < 0.5 /), a sample size of five individuals optimally balanced error with effort. Transplantation verified convergence of oyster &delta15N after changes in nitrogen source while modeling quantified temporal integration (four months for muscle, two to three months for gill and mantle) and measurements over two years demonstrated seasonal &delta15N increases in seston (summer) and oysters (winter). At the small scale (10s of km2), oyster tissues in Monie Bay's creeks (varying by watershed land use) were dominated by anthropogenic nitrogen transported to Monie Bay from Wicomico River whose watershed inputs were predominantly manures (6.8 x 104 to 2.4 x 106 kg N yr-1), not sewage (2.0 x 105 kg N yr-1) or septic (1.1 x 105 kg N yr-1). This has large implications for Delmarva Peninsula: home to 4,630 poultry feeding houses (generating 3.9 x 106 to 1.3 x 108 kg N yr-1) and 1.2 x 106 people (combined sewage and septic generating 3.7 x 106 kg N yr-1), thus a poultry:human nitrogen generation ratio of 1:1 to 91:1. At the medium spatial scale (100s of km2), water quality in Maryland's Coastal Bays was susceptible to runoff. Macroalgae &delta15N (Gracilaria sp.) responded rapidly (4 days) over 100s of km2, while oyster &delta15N responded slowly (2 months) over 10s of km2. Broadly, in Chesapeake Bay (large scale, 10,000s of km2), oyster &delta15N was correlated to land use, stream and tributary water quality, and it reflected tributary wastewater plumes. The overall oyster &delta15N gradient (16.0 / in Eastern Bay, 8.3 / in Lynnhaven River) decreased with flushing time, with increased salinity, and with increased shell height. Denitrification remains potentially confounding as it elevates nitrate &delta15N signals, potentially before oyster assimilation (via plankton). Nevertheless, oyster &delta15N is a powerful tool for indicating nitrogen sources across spatial and temporal scales.