Evaluation of Nitrogen and Carbon Stable Isotopes in Filter Feeding Bivalves and Surficial Sediment for Assessing Aquatic Condition in Lakes and Estuaries

Excessive inputs of nitrogen from anthropogenic activities in watersheds can cause detrimental effects to aquatic ecosystems, but these effects can be difficult to determine based solely on nitrogen concentrations because of their temporal variability and the need to link human activities to ecological responses. Here, we (1) tested the use of stable isotopes of nitrogen (δ(15)N) and carbon (δ(13)C) in benthic organic matter (BOM) as proxies for isotope ratios of filter feeding bivalves in lakes and estuaries, which can be used as indicators but are harder to sample and often spatially sparse, and (2) evaluated if stable isotope ratios in benthic organic matter could be used to assess impacts from anthropogenic land development of watersheds. The δ(15)N in BOM isolated from surficial sediment (δ(15)N(BOM)) was significantly correlated with δ(15)N in filter feeding unionid mussels (Elliptio complanata, δ(15)N(UN)) from lakes and with hard-shell clams (Mercenaria mercenaria, δ(15)N(MM)) from estuaries. In lakes, δ(13)C(BOM) was significantly correlated with δ(13)C(UN), but δ(13)C(BOM) was not significantly correlated with δ(13)C(MM) in estuaries. Values of δ(15)N(BOM) and δ(15)N(UN) were significantly and positively correlated with increasing amounts of impervious surface, urban land cover, and human populations in watersheds surrounding lakes. In estuaries, δ(15)N(BOM) was only significantly and positively correlated with greater percent impervious surface in the watersheds. Correlations of δ(13)C(BOM) in lakes and estuaries, δ(13)C(UN), and δ(13)C(MM) with land use and human population were mostly non-significant or weak. Overall, these results show that δ(15)N(BOM) can serve as a proxy for δ(15)N of filter feeding bivalves in lakes and estuaries and is useful for assessing anthropogenic impacts on aquatic systems and resources. Our study area was limited in size, but our results support further studies to test the application of this sediment stable isotope-based technique for assessing and ranking aquatic resources across broad geographical areas.