HYDROLOGIC EXCHANGE AND CARBON CYCLING IN VARIABLY ALTERED SUB-TROPICAL ESTUARIES OF SOUTHWEST FLORIDA, USA

The objective of this study was to investigate the effects of watershed and channel modifications on physical, chemical, and isotopic characteristics of three estuaries in southwest Florida, USA. Each estuary in the study has variable watershed and estuarine morphologic alteration, affecting the volume and timing of inland water delivery, impacting circulation, and altering carbon cycling. The Blackwater River estuary is mostly pristine and has minimal alteration. The headwaters of Faka Union are canalled and channel regions have been extensively dredged and straightened. The Henderson Creek estuary watershed is altered by residential and agricultural activities. The headwater input to both Faka Union and Henderson Creek estuaries is controlled via weirs. Water temperature, pH, salinity, alkalinity, dissolved inorganic carbon (DIC), d¹⁸O, and d¹³C_DIC, for Blackwater River was similar for low, rising, and high tides, suggesting that the estuary is well mixed. However, well mixed characteristics were not observed for Henderson Creek and Faka Union. Additionally, spatial analyses of salinity, DIC, and d¹³C_DIC and mixing models based on d¹⁸O and salinity illustrate steep vertical gradients in the headwaters of both Faka Union and Henderson Creek estuaries. In Faka Union differences resulted primarily from channel modification, while vertical gradients in Henderson Creek are linked to reduced freshwater influx. Carbon isotope ratios of DIC suggest differences in productivity among the estuaries. DIC was highest in Faka Union (~7000 m mol kg^-1) and lowest in Blackwater River (~1000 m mol kg^-1), suggesting higher biological productivity in Faka Union and Henderson Creek estuaries. Higher productivity may relate to greater nutrient input from the watersheds of these estuaries. The results of this study show that watershed and channel modifications can significantly impact estuarine mixing and carbon cycling with significant implications for ecosystem stability.