Nitrogen stable isotopes in the shell of Mercenaria mercenaria trace wastewater inputs from watersheds to estuarine ecosystems
Carmichael, Ruth H.
Hattenrath, Theresa K.
Michener, Robert H.
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We tested the usefulness of δ15N 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 δ15N determination in shell material for comparison with δ15N values in soft tissues. δ15N values in shell from transplanted and native clams reflected %-wastewater contribution to estuaries, but were 2.3 to 2.5% lighter than δ15N values in soft tissues. Accuracy of δ15N 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 δ15N 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 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 δ15N values from shell material than acidification effects. Differences between δ15N 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
Author Posting. © Inter-Research, 2008. This article is posted here by permission of Inter-Research for personal use, not for redistribution. The definitive version was published in Aquatic Biology 4 (2008): 99-111, doi:10.3354/ab00106.
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