Fertig, Benjamin MeirThis dissertation demonstrates that stable nitrogen isotope signatures (&delta;<super>15</super>N) in oysters (<italic>Crassostrea virginica</italic>) 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 &delta<super>15</super>N 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 &delta<super>15</super>N 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 &delta<super>15</super>N increases in seston (summer) and oysters (winter). At the small scale (10s of km<super>2</super>), 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 10<super>4</super> to 2.4 x 10<super>6</super> kg N yr<super>-1</super>), not sewage (2.0 x 10<super>5</super> kg N yr<super>-1</super>) or septic (1.1 x 10<super>5</super> kg N yr<super>-1</super>). This has large implications for Delmarva Peninsula: home to 4,630 poultry feeding houses (generating 3.9 x 10<super>6</super> to 1.3 x 10<super>8</super> kg N yr<super>-1</super>) and 1.2 x 10<super>6</super> people (combined sewage and septic generating 3.7 x 10<super>6</super> kg N yr<super>-1</super>), thus a poultry:human nitrogen generation ratio of 1:1 to 91:1. At the medium spatial scale (100s of km<super>2</super>), water quality in Maryland's Coastal Bays was susceptible to runoff. Macroalgae &delta<super>15</super>N (<italic>Gracilaria</italic> sp.) responded rapidly (4 days) over 100s of km<super>2</super>, while oyster &delta<super>15</super>N responded slowly (2 months) over 10s of km<super>2</super>. Broadly, in Chesapeake Bay (large scale, 10,000s of km<super>2</super>), oyster &delta<super>15</super>N was correlated to land use, stream and tributary water quality, and it reflected tributary wastewater plumes. The overall oyster &delta<super>15</super>N 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 &delta<super>15</super>N signals, potentially before oyster assimilation (via plankton). Nevertheless, oyster &delta<super>15</super>N is a powerful tool for indicating nitrogen sources across spatial and temporal scales.DissertationBiology, EcologyEnvironmental SciencesBiogeochemistryanthropogenic pollution detectionChesapeake Bayeastern osyter (Crassostrea virginica)land-estuary interfacespatial analysisstable nitrogen isotopes