Temporal and spatial differences in salinity and water chemistry in SW Florida estuaries: Effects of human-impacted watersheds

Abstract

Three estuaries near Naples, Florida with variably modified watersheds have been investigated to understand the chemical consequences of altering drainage patterns. Blackwater River (near natural drainage, control site), Henderson Creek (moderately modified watershed), and Faka-Union Canal (severe channelization) were sampled for temperature, salinity, δ18O, δ13C of dissolved inorganic carbon (DIC), molality of CO2 (ΣCO2), and Mg:Ca and Sr:Ca ratios between freshwater and marine water end members over a 17-mo period. Carbon isotope composition followed similar seasonal patterns as salinity. Freshwater and seawater end members are more negative than the global average, likely reflecting equilibration with local carbon sources derived from mangrove leaf litter and groundwater. δ13C responds to differences in primary productivity between estuaries. Henderson Creek has higher primary productivity than Blackwater River (probable due to higher sewage input and agricultural runoff) and has more positive δ13C and lower ΣCO2. δ18O is affected by seasonal input of freshwater from atmospheric precipitation, evaporation, and groundwater. Late summer and fall rains lower the δ18O of estuarine water, whereas evaporative conditions in the dry season elevate δ18O to values that can be more positive upstream than those from the Gulf of Mexico (estuarine inversion). Evaporation produces water in the Gulf of Mexico that is >1‰ more positive than the global sea surface average most of the year. The very negative δ18O values in Blackwater River and Henderson Creek likely reflect atmospheric and groundwater contribution. Mg:Ca and Sr:Ca ratios of Gulf water from all three estuaries are similar to global averages at low latitudes. Freshwater end members among estuaries are different in that Blackwater River has higher ratios, suggesting a groundwater contribution. Dolomitic rocks in the subsurface likely provide a source of Mg ions.