Isotopic values in oysters indicate elemental sources constrained by multiple gradients

Abstract Stable nitrogen and carbon isotopes ( δ 15 N and δ 13 C) and elemental content (% nitrogen, % carbon) in oysters ( Crassostrea virginica ) grown by a network of 132 citizen–scientists (11,600 km 2 , 87.9 km 2  site −1 ) were examined to test effects of land use, salinity, flushing time, and oyster size on bioindication of human and/or animal nitrogen sources. Oyster δ 15 N sampled from shallow waters sites throughout Chesapeake Bay and its tributaries exhibited nested spatial patterns: (1) decreasing toward the mouth of Chesapeake Bay (1000s km 2 ) and (2) decreasing, increasing, and not changing toward tributary mouths (100s km 2 ). Distinct isotopic ‘signatures’ in tributaries were associated with the composition of land use, water quality in tributaries and freshwater streams, human and/or animal nitrogen sources, and marine vs. terrestrial nitrogen and carbon sources. Yet at 1000s km 2 , oyster δ 15 N varied with flushing time, salinity, and bioindicator size, thus constraining the upper extent for inferring nitrogen sources from bioindicator δ 15 N to the scale of gradients in these confounding physical and biological factors. Nevertheless, at 100s km 2 isotopic ‘signatures’ can be used to infer nutrient sources and transport mechanisms and might have implications for fishery management/enforcement. Ultimately, δ 15 N and δ 13 C in bioindicators distributed to citizen–scientists may add substantial value to existing and ongoing programs, networks, monitoring and databases, and might have some use for imputing data gaps where intensive water quality monitoring is lacking.