Reconstructing the history of eastern and central Florida Bay using mollusk-shell isotope records

Stable isotopic ratios of carbon and oxygen (delta(13)C and delta(18)O) from mollusk shells reflect the water quality characteristics of Florida Bay and can be used to characterize the great temporal variability of the bay. Values of delta(18)O are directly influenced by temperature and evaporation and may be related to salinity. delta(13)C values of delta(13)C are sensitive to organic and inorganic sources of carbon and are influenced by productivity. Analyses of eight mollusk species from five short-core localities across Florida Bay show large ranges in the values of delta(13)C and delta(18)O, and reflect the variation of the bay over decades. Samples from southwestern Florida Bay have distinct delta(13)C values relative to samples collected in northeastern Florida Bay, and intermediate localities have intermediate values. C-13 values of delta(13)C grade from marine in the southwest bay to more estuarine in the northeast. Long cores (> 1 m) with excellent chronologies were anal-I-Led from central and eastern Florida Bay. Preliminary analyses of Brachiodontes exustus and Transenella spp. from the cores showed that both delta(13)C and delta(18)O changed during the first part of the twentieth century. After a century of relative stability during the 1800s, delta(13)C decreased between about 1910 and 1940, then stabilized at these new values for the next five decades. The magnitude of the reduction in delta(13)C values increased toward the northeast. Using a carbon budget model, reduced delta(13)C values are interpreted as resulting from decreased circulation in the bay, probably associated with decreased freshwater now into the Bay. Mollusk shell delta(18)O values display several negative excursions during the 1800s, suggesting that the bay was less evaporitic than during the twentieth century. The isotope records indicate a fundamental change took place in Florida Bay circulation early in the twentieth century. The timing of the change Links it to railroad building and early drainage efforts in South Florida rather than to flood control and water management measures initiated after World War II.