Linking stable isotopes in resuspended particulate material from lakes to anthropogenic development in watersheds

Background/Question/Methods

Current large scale ecological assessments of lakes and their surrounding watersheds rely on probabilistic sampling of selected sites and extrapolation of these characteristics to untested sites. While allowing broad geographic scale coverage, the accuracy of such assessments for predictions at untested sites is unclear. The present study was undertaken to determine whether measurements of stable isotope ratios in resuspended particulate organic material (RPOM) of sediments would link to values in aquatic biota and provide a stable isotope baseline indicator for evaluating lake condition and the influence of anthropogenic development in the surrounding watershed. RPOM samples were taken from the resuspended surface material of cores collected over a two year period from the shallow (< 2 m) littoral zone of 45 sites in Rhode Island, USA. Sites were selected to represent a wide range of anthropogenic influence. We compared values of ratios of stable isotopes of nitrogen (δ¹⁵N) and carbon (δ¹³C) between RPOM and largemouth bass (LMB), the top level aquatic predator from these sites. Temporal and spatial variability of δ¹⁵N at sites were determined, and the linkage between the δ¹⁵N ratio of RPOM and land use in the surrounding watersheds was evaluated.

Results/Conclusions

Significant relationships were found between RPOM and LMB for δ¹⁵N (p < 0.001, r² = 0.84, n = 45) and δ¹³C (p < 0.001, r² = 0.54, n= 45). These results show RPOM values reflect processes in the aquatic food webs of sites.  The year to year differences of δ¹⁵N in RPOM from sites were small (absolute value of mean difference among all sites = 0.49 ‰). Spatial variability was examined by sampling at different locations in the littoral zone around the perimeter of selected sites. Statistically significant differences in δ¹⁵N in RPOM were found within some sites, but these differences were relatively small (< 1.5‰) and did little to change results of among site comparisons. Comparison of δ¹⁵N in RPOM with GIS land use data from the surrounding watersheds showed significant positive relationships with percentage of urban and residential land within 100m buffer zones surrounding sites (p< 0.001, r² = 0.55). A similar comparison with the number of houses within the buffer zone was significant, but the r-squared was lower (p < 0.01, r² = 0.21). These comparisons show that this stable isotope-based measure is useful for assessing anthropogenic influence within site waters and linking these measures to land use in the surrounding watersheds.