Stable nitrogen isotopes as indicators of anthropogenic activities in small freshwater systems

Abstract

Stable nitrogen isotope ratios (δ 15 N) were measured in fish, mussel, and sediment samples taken from 17 small freshwater sites to examine food chain length and trophic position across sites affected by differing levels of anthropogenic activity. Both shoreline development and fish species composition varied greatly among sites, and a range of up to 11.2‰ was found for the δ 15 N values of largemouth bass (Micropterus salmoides). Fish δ 15 N values were baseline corrected using unionid mussel (Elliptio complanata) δ 15 N values. Predators, such as largemouth bass and chain pickerel (Esox niger), exhibited normalized δ 15 N values that were less variable than those of benthic-feeding fishes. Relationships between δ 15 N and dissolved inorganic nitrogen concentrations indicated that δ 15 N was a valid descriptor of eutrophication at sites with low dissolved inorganic nitrogen concentrations. The fraction of residential land in buffer zones surrounding sites was correlated with fish δ 15 N, indicating that urban development, and presumably human wastewater, resulted in elevated δ 15 N values in these small freshwater systems.