The impact of waste water from anthropogenic development in watersheds is one of the foremost issues facing aquatic ecological communities. Ratios of stable isotopes of nitrogen (15N/14N), herein (δ15N), in organisms have been used in assessments of anthropogenic development on aquatic resources in lakes and estuaries. However, comparisons using organisms (often filter feeding bivalves) can be limited by availability of organisms and constrained by the need to compare among species with potentially differing feeding strategies and physiologies. To determine whether δ15N in surficial sediments was a useful proxy for δ15N in bivalves we sampled surficial sediments and bivalves from 26 small lakes in Rhode Island, USA and a total of 9 estuaries in RI and Massachusetts. These sites were selected to represent a large range of anthropogenic development in their respective watersheds. Benthic organic material (BOM) was collected from sediment samples. The bivalves we collected were mussels (Elliptio complanata), herein UN, in lakes, and in hard-shell clams (Mercenaria mercenaria), herein MM, in estuaries. We developed regressions of δ15NBOM vs. δ15NUN in lakes and δ15NBOM vs. δ15NMM in estuaries. We then compared regressions of bivalves with BOM from estuaries and lakes and compared correlations of δ15NBOM and δ15N in bivalves with land cover variables from the surrounding watersheds.
Results/Conclusions
Significant relationships were found for δ15NBOM vs. δ15NUN in lakes (R2 = 0.83, p < 0.001, n = 26) and δ15NBOM vs. δ15NMM in estuaries (R2 = 0.87, p < 0.001, n= 9). Comparisons of these regressions showed no significant difference in slopes, but intercepts were significantly different. The patterns of correlation coefficients of both δ15NBOM and δ15NUN with land cover data from watersheds and 200 m buffer zones surrounding 26 lakes showed that either sample type was useful for indicating anthropogenic development. Similar results were found for δ15NBOM and δ15NMM and land cover variables in the 9 estuarine sites. The strongest correlations were found with land cover variables that related to the extent of anthropogenic development of land surrounding lakes and estuaries (positive correlations with % impervious surface, % urban residential and negative correlations with % forest). However, correlation coefficients were generally higher for lakes than for estuaries. Overall, these findings support the utility of δ15N in surficial sediments to represent ratios in bivalves and to provide a simple metric to assist in studies of the impact of anthropogenic development in watersheds on aquatic sites.