| 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 health. Estuaries were sampled for salinity, dissolved oxygen (DO), concentration of dissolved inorganic carbon (DIC), stable carbon isotope ratios of DIC (delta13CDIC), stable carbon isotope ratios of particulate organic carbon (delta13C POC), concentration of particulate organic carbon (POC), nitrogen isotope ratios of particulate organic nitrogen (delta15NPON ), C/N ratios, and chlorophyll-alpha concentrations (Chl-alpha). delta 13CDIC values ranged from -14.09 to +0.85‰. The more negative values occurred upstream, and resulted from DIC inputs derived from the degradation of organic carbon and the dissolution of carbonates. The more positive values occurred downstream, and were typical of marine values. DIC concentrations ranged from 1461 to 8066 mumol/L. Higher concentrations occurred upstream and support the interpretation that high respiration rates were occurring in the upstream sampling points. A comparison of DIC values to a conservative mixing model indicates that the input of terrestrial organic matter and its degradation created a net heterotrophic metabolic state in all four estuaries. Supporting this interpretation, delta13C POC values (-36.53 to -20.11‰) suggest that terrestrial plants were the main source of POC in the upstream sampling points, while aquatic plants were the main contributor of POC in the downstream sampling points. delta 15NPON values range from +1.52 to +5.60‰, which is consistent with natural sources of nutrients. C/N ratios ranged from 7.2 to 13.4, and are consistent with both terrestrial and aquatic sources. Chl-alpha concentrations were variable and were typically below 20 mug/L, indicating moderate to low levels of autotrophy in all estuaries. Elevated Chl-alpha concentrations indicative of increased primary productivity occurred at intermediate salinities (mid-estuary). It is possible the mixing front at mid-estuary locations influenced Chl-alpha concentrations. Results from this study show no apparent difference between the estuaries, including the control site, thus indicating that anthropogenic activities had little effect on the parameters measured. |
