PALEOECOLOGY

Salinity Trends in Florida Bay During the Past 100 Years

Florida Bay Project Profile - 141

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Question: Based on evidence obtained from the geological and paleoecological record, what were the salinity levels in Florida Bay before 1900 through recent times?

Project: The results from several different geological, geochemical and paleoecological research projects were examined to compare salinity trends in the bay before 1900 through recent times. In most cases, these data were based on the analysis of fossils from sediment cores taken from the bay bottom. The cores contained shells of clams, snails, ostracods, and forams that, at one time, were alive in bay waters or sediments. Data documenting saltwater intrusion into the Biscayne aquifer (the limestone rock beneath the Everglades that holds freshwater) was also included in the comparative analysis.

Findings to Date: Multiple lines of evidence demonstrate that a shift toward more saline waters took place after 1910 and continued until about 1945. The fossils found in cores indicate the bay was more often brackish before 1910. Although salinity levels of Florida Bay have always varied widely, at that time, the bay was generally less saline than it is today. The cores, dating between 1910 through about 1945, contained species more common at higher salinity levels. Salinity levels in the bay climbed gradually during this period. At the same time, the fresh-saltwater boundary was pushed inland as seawater seeped into the porous limestone aquifer. The saltwater intrusion and the shift toward higher salinity levels in the bay probably occurred for two reasons. The wetlands of the Everglades, that fed into Florida Bay, were drained by an extensive system of canals and drinking-water wells were dug into the aquifer. This lowered the water table allowing seawater to seep in from nearby areas. Since the 1940’s, data from core studies show no long-term trends toward increasing or decreasing salinity levels. In fact, salinity continued to be highly variable during the past fifty years, changing the bay from a hypersaline lagoon in the late 1980’s to a brackish estuary in the mid 1990’s. Regional saltwater intrusion has also halted since the 1940’s.

Status: Completed in 1998.

Restoration Impacts: Determining the salinity levels that characterized the bay during the last 100 years is important for ecosystem managers charged with restoring the Everglades ecosystem to one which resembles pre-drainage times.

Funding Source: United States Geological Survey, South Florida Water Management District

Related Profiles:

• FBPP - 140 • FBPP - 143

• FBPP - 147