Carbon and nitrogen isotope ratios of juvenile winter flounder as indicators of inputs to estuarine systems

Stable carbon and nitrogen isotope ratios were measured in young-of-the-year (YOY) winter flounder, Pseudopleuronectes americanus, collected from several Rhode Island, USA estuarine systems. These included three coastal lagoons, an estuarine river and Narragansett Bay. The δ13C trends observed along transects in several systems showed isotopically depleted terrestrial signals in the upper reaches of the estuaries. Significant differences (P < 0.05) in δ15N were observed among all estuarine systems and these differences correlated (P < 0.01) with human population densities in the watersheds. Although Narragansett Bay has a strong north-south gradient in nutrient concentrations this trend was not reflected in flounder δ15N. The northernmost station with the highest nutrient concentrations unexpectedly had significantly lower δ15N values. Depleted δ15N values at this nutrient-rich station may indicate that concentration-dependent fractionation needs to be considered when using nitrogen isotope ratios in biota to monitor anthropogenic nitrogen inputs in systems with high nitrogen loadings.