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

Abstract

We tested the usefulness of δ 15 N values in the organic matrix of whole shells from Merce- naria mercenaria as tracers of anthropogenic nitro- gen inputs to coastal ecosystems. Low and high strin- gency acidification methods were used to define parameters for reliable δ 15 N determination in shell material for comparison with δ 15 N values in soft tis- sues. δ 15 N values in shell from transplanted and native clams reflected %-wastewater contribution to estuaries, but were 2.3 to 2.5% lighter than δ 15 N values in soft tissues. Accuracy of δ 15 N values in shell material depended on recovering a sufficient quantity of organic N from shell (~70 μg) and was not altered by acidification method. Reliable δ 15 N values were obtained with as little as 80 mg of shell and using 100 μl of acid, but higher stringency methods (treating more shell with more acid for longer duration) typically yielded more N for sub- sequent 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 δ 15 N values from shell material than acidification effects. Differ- ences between δ 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