Citation:

Lake, James L., Jonathan R. Serbst, A. Kuhn, M. Charpentier, AND Nathan J. Smucker. Evaluation of Nitrogen and Carbon Stable Isotopes in Filter Feeding Bivalves and Surficial Sediment for Assessing Aquatic Condition in Lakes and Estuaries. WATER. MDPI, Basel, Switzerland, 14(22):3712, (2022). https://doi.org/10.3390/w14223712

Impact/Purpose:

Inputs of nitrogen nutrients from human development in watersheds of streams, lakes and estuaries can have significant negative impacts on the proper functioning of these aquatic systems. Measurements of aquatic nutrients have been used to access the sources and consequences of nutrients enrichment, but at many aquatic sites nutrients are highly variable over time and distance from sources. Changes in the ratio of stable isotopes of two isotopes of nitrogen in filter feeding bivalves, like mussels and clams, have been related to the extent of inputs of nitrogen nutrients resulting from the variety and typesof different land use in the surrounding watersheds. However, filter feeding bivalves are often not available in aquatic sites. The present study examined whether the measurement of nitrogen stable isotopes in sediments can be used as a substitute for the findings obtained with bivalves. We found a good correspondence among the isotope results from bivalves and sediments. And strong relationships of stable isotope ratios in sediments with measures of human development (e.g. % of urban residential development, % impervious surfaces, human population) in the watersheds of lakes and estuaries. These results indicate that nitrogen stable isotopes in sediment provides a useful measure for assessing and ranking aquatic sites regarding the extent of anthropogenic influence from surrounding watersheds.

Description:

Excessive inputs of nitrogen from anthropogenic activities in watersheds can cause detrimental effects to aquatic sites. Here, we (1) tested the use of stable isotopes of nitrogen (δ15N) and carbon (δ13C) in benthic organic matter as proxies for isotope ratios in harder to sample, and often spatially sparse, filter feeding bivalves in lakes and estuaries and (2) evaluated the utility of these stable isotope ratios in benthic organic matter to assess impacts from anthropogenic land development of watersheds.  The relationship of nitrogen stable isotopes in benthic organic material (BOM) isolated from surficial sediment (δ15NBOM) with δ15N in filter feeding Unionid (UN) mussels (Elliptio complanata) (δ15NUN) from lakes was significant as was a similar relationship of δ15NBOM with hard-shell clams (Mercenaria mercenaria, MM) (δ15NMM) from estuaries. Comparisons of these two relationships showed no significant differences in slopes, but differences in intercepts.  In lakes, the relationship of δ13CBOM with δ13CUN was significant, but in estuaries the relationship of δ13CBOM with δ13CMM was not significant. Values of δ15NBOM and δ15NUN were significantly and positively correlated with increasing amounts of impervious surface, urban land cover, and human populations in watersheds surrounding lakes. However, in estuaries no δ15NMM correlations with environmental variables were significant and the only significant correlation was with δ15NBOM, which increased with greater Log % impervious surface in the watersheds. Correlations of δ13CBOM, δ13CUN, and δ13CMM with land use and human population were mostly non-significant or weak. Overall, these results show that δ15NBOM can serve as a proxy for δ15N 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.

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