Carbon and nitrogen tracers of land use effects on net ecosystem metabolism in mangrove estuaries, southwest Florida

Four estuaries in southwest Florida with different land-use characteristics in their watersheds were chosen to investigate the effects of anthropogenic land use on estuarine biogeochemical cycling. We compared. C:N ratios, concentrations of dissolved inorganic carbon (DIC), chlorophyll-a (chl-a) and particulate organic carbon (POC), stable isotope ratios of DIC (delta C-13(DIC)) and POC (delta C-13(POC)), and nitrogen isotope ratios of particulate organic nitrogen (delta N-15(PON)) among these estuaries. Values of delta C-13(DIC) ranged from -14.1 to +0.9 parts per thousand. The more negative values occurred upstream, resulting from DIC inputs derived from both the degradation of organic carbon and dissolution of carbonates. Upstream DIC concentrations were as high as 8066 mu mol L-1, suggesting high respiration rates. Further, a comparison of DIC values to a conservative mixing model indicates net heterotrophic metabolic state in all four estuaries. Supporting this interpretation, low delta C-13(POC) values suggest that terrestrial plants were the main source of POC in the upstream sampling points. However, C:N ratios ranged from 7.2 to 13.4, and were consistent with the decomposition of both terrestrial and aquatic sources. Chl-a concentrations were variable and typically below 20 mu g L-1, indicating moderate to low levels of autotrophy in all estuaries. Elevated chl-a concentrations indicative of increased primary productivity occurred at intermediate salinities, and were possibly caused by the mixing front at mid-estuary locations. There were no apparent differences in delta N-15(PON) among estuaries, suggesting that the N sources to these estuaries are comparable. The combined results show no differences between near-natural and anthropogenically influenced estuaries, indicating a minimal effect of anthropogenic activities on the parameters measured, possibly as a result of the filtering capacity of the extensive surrounding mangrove vegetation. (C) 2016 Elsevier Ltd. All rights reserved.