The Florida Bay environmental record, as indicated by the biotic remains within the sediments, is one of change. Analyses of three cores from Florida Bay and the fringe environments of the Bay show that fluctuations in salinity, substrate, and other critical physical, and chemical parameters, have occurred throughout the history of the bay. During this century, an increase in average salinity and an increase in benthic faunal diversity is recorded.

The principle objective of the U.S. Geological Survey's South Florida Ecosystem History projects is to use paleoecologic tools to reconstruct the history of Florida Bay, Biscayne Bay, and the terrestrial Everglades over the last 150-200 years, as well as the last few millennia. The data gathered from these projects are being compiled to develop a broad regional and temporal picture of changes in the south Florida ecosystem. At selected sites in Florida Bay, data are gathered on modern faunal and floral distributions and environmental preferences, such as salinity, dissolved oxygen, nutrients, circulation, substrate and seagrass conditions. These data are used as proxies for interpreting down-core environmental changes as indicated by the abundance and distribution of faunal and floral remains present in cores collected throughout Florida Bay and the south Florida ecosystem. The chronology of the cores is based primarily on 210Pb analyses; this absolute age control makes it possible to interpret the rate of change of such critical parameters as salinity and substrate, including seagrasses and sediment sources. In turn, changes in salinity patterns provide information on changing freshwater flow, sea level rise, and circulation patterns. In conjunction with geochemical analyses, information on nutrient supplies also can be obtained from the faunal and floral data.

Analyses of Core 6A from the Bob Allen mudbank, central Florida Bay, and Core T-24 from the mouth of Taylor Creek in Little Madeira Bay, eastern Florida Bay, are complete. The benthic fauna were examined at 2 cm intervals in these cores. The 210Pb age model of Core 6A gives a sedimentation rate of 0.86 - 0.06 cm/yr. Core 6A records a history of fluctuating salinity over the last 150 years, but from around the turn of the century a general increase in the average salinity has occurred. Benthic foraminifer and ostracode data indicate a significant shift occurred during the 1950's, from a period of relatively lower average salinities (~18 ppt) and relatively low amplitude fluctuations in the salinity, to a period of higher average salinities (~32 ppt) and greater fluctuations in the salinity. The mollusc data show similar patterns. During the period from around 1950 to the present the overall benthic faunal diversity and abundance increased. Patterns of pollen and dinocyst distribution within the core show shifts that correspond to the benthic faunal changes, indicating that the change was not a localized effect. Comparison of the benthic faunal record to precipitation records dating back to 1906 indicates no direct correlation between precipitation patterns and prolonged salinity changes in Florida Bay; however, further investigation is warranted.

The pattern of increasing salinity is even more apparent in Core T-24, which is located at the fringes of the Bay environment. Although a detailed chronology based on radioisotopes has not been developed for this core, the general faunal and floral patterns correspond to those at Core 6A. The relative abundance of oligohaline to mesohaline (~5-18 ppt) benthic foraminifera, molluscs, and ostracodes decreases upward in the core, and the polyhaline to euhaline (18-35 ppt) fauna increases. Benthic faunal diversity increases upward in the core. The dinocyst and pollen assemblages are consistent with the pattern of increasing salinity up-core at this site, with red mangrove and buttonwood pollen increasing and the dinocyst assemblages shifting toward more marine conditions.

Examination of selected samples from a core collected along Mud Creek near Joe Bay on the northern fringe of Florida Bay provides a long-term record of change. 14C analysis dates the base of this core (82 cm) at 2,050 BP, and 210Pb indicates that the last 150 years are included in the upper 20 cm. Preliminary analysis of molluscan fauna indicates a general trend of increasing salinity up-core. Molluscs present in the lower half of the core are limited to fresh water or terrestrial forms, but at 34-36 cm a brackish to nearly freshwater species is found. Pollen assemblages indicate relatively higher abundances of sawgrass pollen from about 600-2000 BP followed by an increasing abundance of mangrove and wax-myrtle pollen; this shift is indicative of an increase in salinity. Because of the slower sediment accumulation rate of peats, less detail is provided for the last 150 years, but cores from the fringe of Florida Bay provide a link between the Bay and the terrestrial ecosystems, and allow us to develop a broad regional and temporal picture of changes to the south Florida ecosystem.

The preliminary examination of three cores from the central and northern margins of Florida Bay indicates a Bay-wide increase in salinity over at least the last century. Examination of historical precipitation records indicate there is no strong relationship between salinity and precipitation patterns, however, other factors such as storm frequency, fresh-water flow, sea-level rise and evaporation/precipitation rates are also important. A comparison of the chronologically-placed data gathered from these cores with historical records of precipitation, streamflow and other critical parameters will facilitate understanding of the causes of salinity fluctuations in Florida Bay. In the upcoming year, paleoecologic analyses of additional cores are planned to determine if the pattern of increased salinity is upheld. Geochemical analyses of shells and sediments from these cores will be conducted to provide absolute measurements of salinity and nutrient fluctuations in the ecosystem. This information, compiled with data gathered from other ecosystem projects evaluating salinity, circulation, freshwater flow, precipitation and evaporation, will provide modelers and resource managers with information on the causes and enduring effects of salinity change.

(This abstract was taken from the Florida Bay Science Conference Proceedings, 1996)

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