The use of ostracode metal/calcium ratios (Mg/Ca, Sr/Ca, and Na/Ca) was investigated to reconstruct the salinity history of central Florida Bay from its sedimentary record. First, provisional partition coefficients (K_D-Mg and K_D-Sr ) were developed from the average me/Ca ratio for modern Florida Bay water and 171 modern shells of Loxoconcha matagordensis at a mean Florida Bay water temperature of 26.5 °C. Trends in Mg/Ca ratios for L. matagordensis and Peratocytheridea setipunctata from radiometrically dated sediment cores from Russell Bank, Park Key, and Bob Allen Keys appear to provide a reasonable estimate of Florida Bay salinity history for the past 130 years. The most complete Mg/Ca record from Russell Bank extends back to 1875 and shows a good correspondence between estimated paleosalinity and measured annual rainfall in south Florida. Paleosalinity estimates for the Russell Bank area range from ~13 to ~55 ppt. Estimates for the post-1950's period are comparable to instrumental measurements of salinity near the Russell Bank site.

The overall record indicates a decadal-scale variability in salinity in which large amplitude shifts in ostracode Mg/Ca ratios with high Mg/Ca (higher salinity) values corresponding to periods of low rainfall from ~1895 to ~1920 and from 1940 to present. The intervening 20 years (~1920-40) are characterized by relatively low amplitude shifts in Mg/Ca and rainfall. Since 1950, periods of high salinity (~45 to ~55 ppt) occur during each decade. Prior to 1940, salinity never exceeded ~45 ppt and generally remained below 40 ppt, despite evidence in the rainfall record of dry intervals in the early 1900's equivalent to those of the last 50 years. Salinity minima associated with the decadal-scale oscillations range from ~15 to ~25 ppt. Four of the five extreme low values (~1915, ~1940, ~1983, ~1998) appear to correspond with times of strongly negative values of the Southern Oscillation Index, that is, El Nino Conditions that bring anomalously high winter rainfall in the southeastern U.S.

These results support the hypothesis that seasonal and decadal-scale salinity fluctuations are a natural part of the Florida Bay Ecosystem, and that these fluctuations are largely a function of natural variability of regional climate (rainfall). The relatively low peak salinity values associated with apparent dry periods in the early 1900's suggests that, since ~1950, anthropogenic factors played a role in the magnitude of the salinity fluctuations in more recent times.

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(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report (PDF, 8.7 MB))

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