Biological linkages between landuse and water quality: the use of nitrogen stable isotopes for diagnosing the cause and source of water pollution.

M.S. Pearson, M.F. Moffett, T.M. Jicha, L.E. Anderson, and C.M. Elonen. USEPA-Mid-Continent Ecology Division, Duluth, MN 55804

A shortcoming in developing nutrient TMDLs is accurately linking the causes and sources of anthropogenic nutrients at the landscape scale. Previous research of others has shown that anthropogenic inputs of nitrogen to a watershed enhances the proportion of ¹⁵N to ¹⁴N assimilated by aquatic organisms. We tested the hypothesis that fish and invertebrates exposed to high nutrient loads would assimilate higher proportions of ¹⁵N relative to organisms from more pristine aquatic systems. Twenty riverine coastal wetlands were sampled along the shores of Lake Michigan in 2001 as part of a USEPA Regional Environmental Monitoring and Assessment Program. Fish, invertebrates, aquatic plants, periphyton, and seston were collected for ¹⁵N stable isotope analyses. Initial results show a positive correlation between the proportion of ¹⁵N in Amphipoda and wetlands with higher nitrogen loadings. Centrarchid fish show a similar trend across the nutrient gradient, although not as distinct. These results indicate that ¹⁵N stable isotope measurements in aquatic organisms provide a biological measure of water quality degradation and can be used to more accurately define causes and sources of nutrient pollution. This abstract does not necessarily reflect USEPA policy.