Stable isotopes and mercury in stream macro-invertebrates: Links to watershed characteristics

Background/Question/Methods

Much of the landscape of New England consists of small streams in watersheds which drain into lakes, ponds or reservoirs and then onward to rivers and the sea. Evaluating the distribution and abundance of two important contaminants, one input largely from human wastewater (nitrogen) and the other from atmospheric deposition (mercury), across the watersheds in this region is problematic because of the great number and variety of streams, the variety of inputs and the possible transformations that these contaminants can undergo. This study sampled  and analyzed nitrogen stable isotope ratios (δ¹⁵N) and total mercury concentrations ([THg]) in insect larvae from 35 stream sites in Rhode Island, USA which were selected to encompass a gradient from almost pristine to heavily impacted by anthropogenic activities. Abundance data for insect groups and comparisons of δ¹⁵N and [THg] data from insect groups within and among sites were used to determine groups most suitable for use as indicators of anthropogenic impact and for development of relationships with land use in the surrounding watersheds.

Results/Conclusions

Although the distributions of larval insects varied among sites, darner dragon flies (darners) and dobson flies (hellgrammites), both predators, were present at the most sites. We used the δ¹⁵N values of two detritivores (northern case maker caddisflies and crane flies) to establish baselines, and related these values to those of the two predators to calculate trophic positions. Thirteen sites contained all four organisms and comparisons of trophic positions of the two predators calculated with either baseline organism were statistically similar and agreed well with values from the literature. The [THg] increased from baseline organisms to predators within sites; among sites the magnitude of increase was negatively related to site pH. Because δ¹⁵N can reflect the level of anthropogenic influence, we developed relationships between δ¹⁵N in organisms and % residential land in 50m x 500m buffer zones surrounding sites. Darners, hellgrammites and northern case makers showed significant relationships, r² = 0.49, 0.48, 0.47 respectively, but the relationship for crane flies was not significant. Overall, the frequency of occurrence and strength of these relationships supports the use of dragon flies and/or hellgrammites as indicators of anthropogenic influence at stream sites. In addition, construction of trophic positions with δ¹⁵N appears to be a useful approach for examining movement of THg and, potentially, other contaminants within these systems.