Crassostrea virginica shells record local variation in wastewater inputs to a coastal estuary

We measured delta N-15 values in the acid-insoluble organic portion of shells from Crassostrea virginica transplanted at increasing distance from a major wastewater treatment plant in Mobile Bay, Alabama. To determine whether delta N-15 in shell material recorded local spatial variation in wastewater influence, we compared delta N-15 in newly deposited oyster shell to delta N-15 values in wastewater effluent and in suspended particulate matter in receiving waters. We compared delta N-15 values in shell to delta N-15 in adductor muscle and whole tissues to determine the isotopic relationships between shell and soft tissues for this previously untested species. delta N-15 values in oyster shell reflected differences in wastewater influence relative to distance from the wastewater treatment plant within 38 d of transplanting. delta N-15 values in shell were enriched by 2.4 parts per thousand compared to available foods, consistent with a trophic shift from food source to consumer. delta N-15 values in shell also were significantly correlated with delta N-15 values in soft tissues, but were enriched by 1.9 parts per thousand compared to whole tissues and 0.8 parts per thousand compared to adductor muscle. Overall, delta N-15 values in oyster shell were a better proxy for adductor muscle than whole tissues. If applied with care, oyster shells provide an ecologically and commercially meaningful wastewater detection tool that may be effective over relatively short spatial and temporal scales. delta N-15 values in shell material may be particularly useful to enable anthropogenic source tracing and refine food web reconstructions in areas with remnant shells where oysters or other bivalves have been severely depleted or already lost.