Responses of nitrogen stable isotopes in fish to phosphorus limitation in freshwater wetlands

Human-induced eutrophication has altered ecological processes in aquatic ecosystems. Detection of ecological changes is a prerequisite for protecting ecosystems from degradation. In this study, nitrogen stable isotopes (δ15N) in fish are evaluated as indicators of environmental changes in south Florida wetlands. Stable nitrogen isotope (δ15N) data of select fish species and water quality collected from the Florida Everglades between the 1990s and 2000s were used to assess the relationship between total phosphorus concentrations and δ15N ratios. The δ15N ratios in nine of ten select fish species increase significantly as total phosphorus concentration in the surface water increases. There were significant relationships between total nitrogen concentration in the surface water and δ15N ratios in several fish species. The pattern of changes in δ15N ratios along nutrient gradients suggests that increased eutrophication is recorded as the δ15N ratios in fish. The lack of human wastewater loading, the dominance in agricultural runoff and the high TN:TP ratio suggest that phosphorus is the limiting factor driving ecosystem productivity and the changes of δ15N ratios in fish. Results from this analysis demonstrate that δ15N ratios in fish integrate biotic responses to eutrophic process over time and could be a robust indicator for early ecological changes.