Nitrogen stable isotopes in the shell of Mercenaria mercenaria trace wastewater inputs from watersheds to estuarine ecosystems

We tested the usefulness of 15 N values in the organic matrix of whole shells from Mercenaria mercenaria as tracers of anthropogenic nitrogen inputs to coastal ecosystems. Low and high stringency acidification methods were used to define parameters for reliable 15 N determination in shell material for comparison with 15 N values in soft tissues. 15 N values in shell from transplanted and native clams reflected %-wastewater contribution to estuaries, but were 2.3 to 2.5% lighter than 15 N values in soft tissues. Accuracy of 15 N values in shell material depended on recovering a sufficient quantity of organic N from shell (~70 g) and was not altered by acidification method. Reliable 15 N values were obtained with as little as 80 mg of shell and using 100 l of acid, but higher stringency methods (treating more shell with more acid for longer duration) typically yielded more N for subsequent stable isotope analysis. Conversely, higher concentrations of acid reduced N recovery. These results suggest that the content of N recovered was of greater concern to obtaining reliable 15 N values from shell material than acidification effects. Differences between 15 N values in shell material and soft tissues likely reflected differences in N assimilation among tissues. In combination with other analyses, this method may be applied to refine modern and historical trophic assessments and discern natural from anthropogenic influences on coastal ecosystems