Topical area: Paleoecology

Robert B. Halley, USGS, St. Petersburg, FL; Thomas M. Cronin, G. Lynn Wingard, and Scott E. Ishman, USGS, Reston, VA

Multiple lines of paleoecological and geochemical evidence indicate that Florida Bay became more saline during the early part of this century. Species distributions of mollusks, foraminifers, and ostracodes from sediments accumulated during the past two centuries show a shift toward more marine and hypersaline conditions beginning about 1910 and ending during World War II. Stable isotopic analyses of the mussel Brachiodontes exustus support the timing and interpretation of the salinity shift. Previously published results of fluorescence patterns from annually banded coral skeletons link the salinity shift to coeval decreases in fresh-water flow into the Bay (Smith et al., 1989). All data sets show there has been no long-term trend during the past 50 years, although salinity has been highly variable since the 1940’s. Interannual salinity variation, both before and after the shift, is primarily driven by climate.

Saltwater intrusion along the southeastern boundary of the Biscayne Aquifer has been documented since 1904. In this century, movement of the salt front exhibits timing similar to that of the salinity increase in Florida Bay. Parker et al. (1955) illustrate substantial intrusion between the turn of the century and the 1940s. Since W. W. II, intrusion has been regionally arrested, although local problems persist (Leach et al., 1972; Klein and Waller, 1985; Fitterman, 1996; Sonenshein and Koszalka, 1996) associated with canal dredging and aquifer pumping. In some instances the salt front has even moved seaward since the 1940s, for example, southeast of Florida City.

We propose that during the early part of this century both saltwater intrusion of the Biscayne Aquifer and increased salinity of Florida Bay resulted from the same process, drainage of the peripheral Everglades. Decreased water tables had the effect of decreasing surface flow to Florida Bay as well as disturbing the salt water/freshwater balance at the edge of the aquifer. Although the major constructions associated with drainage from 1900 to 1940 are well documented, their effects on surface and ground water went largely undocumented. Photographic and anecdotal information are available, but flow and groundwater monitoring in South Florida were not well-established prior to W. W. II (McIvor et al., 1994). A few recording stations appear to capture the last decade of uncontrolled drainage, for example the 1930s portion of the water table record from South Dade well 196 and 196A. Generally, however, knowledge of the detailed effects of hydrologic changes during the early part of this century will have to be inferred from models and tested with paleoecological and geochemical reconstructions.

REFERENCES:

Fitterman, D. V., 1996, Geophysical mapping of the freshwater/saltwater interface in Everglades National Park, U. S. Geological Survey Fact Sheet FS-96-173.

Klein, Howard and Waller, B. G., 1985, Synopsis of saltwater intrusion in Dade County, Florida through 1984; U. S. Geological Survey Water Resources Investigations Report 85-4101, 1 sheet.

Leach, S. D., Klein, Howard, and Hampton, E. R., 1972, Hydrologic effects of water control and management of southeastern Florida; Florida Bureau of Geology, 114p.

McIvor, C. C., Ley, J. A., and Bjork, R. D., 1994, Changes in freshwater inflow from the Everglades to Florida Bay including effects on biota and biotic processes: a review; in: Davis, S. M., and Ogden, J. C., Everglades, the Ecosystem and its Restoration, St. Lucie Press, Delray Beach, FL, pp. 117-146

Parker, G. G., Ferguson, G. E., and Love, S. K., Water Resources of Southeastern Florida, with special reference to the geology and groundwater of the Miami area; U. S. Geological Survey Water-Supply paper 1255, 965p.

Sonenshein, R. S., and Koszalka, E. J, 1996, Trends in water-table altitude (1983-93 and Saltwater intrusion (1974-93) in the Biscayne Aquifer, Dade County, Florida; U. S. Geological Survey Open-File Report 95-705, 2 sheets.

Smith, T. J. III, J. H. Hudson, M. B. Robblee, G. V. N. Powell, and P. J. Isdale, 1989, Freshwater flow from the Everglades to Florida Bay: a historical reconstruction based on fluorescent banding in the coral bands in the coral Solenastrea bournoni ; Bulletin Marine Science, v. 44, pp. 274-282.