Stable isotope compositions of aquatic flora as indicators of wetland eutrophication

Abstract Carbon and nitrogen stable isotopes (δ 13 C and δ 15 N) have been used as indicators for environmental changes in aquatic ecosystems. In this study, δ 13 C and δ 15 N values of select flora taxa were evaluated as indicators of human-induced eutrophication in an Everglades wetland which has been impaired by the source and quality of inflow and the alteration of hydrological pattern. Results showed that δ 13 C of cattail ( Typha domingensis Pers.) increased with the decrease in total phosphorus (TP) concentration. Limited data for sawgrass ( Cladium jamaicenses Crantz) collected at three study sites along the nutrient gradient also show a similar change in δ 13 C. However, δ 13 C of periphyton and bladderwort ( Utricularia sp.) decreased with the decrease in TP concentration. The elevated δ 13 C values of emergent vascular plants (cattail and sawgrass) were likely the results of decreases in stomatal conductance, carboxylation and 13 C fractionation under nutrient stress. This is supported by corresponding decreases in photosynthetic rate of cattail along the decreasing nutrient gradient. In contrast, δ 15 N values of all flora taxa increased with the increase in TP concentration. Findings from this study indicate that δ 13 C of emergent plants is a sensitive indicator for nutrient stress while δ 15 N of major flora species may serve as a robust indicator for wetland eutrophication.