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. - Highlights: • We used carbon and nitrogen isotope ratios in fish to indicate inputs to estuaries. • Fish nitrogen isotope ratios (δ¹⁵N) were significantly different among all systems. • δ¹⁵N in fish was significantly correlated with watershed human population density. • Narragansett Bay δ¹⁵N values indicated isotope fractionation in nutrient-rich areas.