West Everglades Winter Freshets During the Current Wet Period and Seasonal Phase Shifts in Salinity Cycles Across Florida Bay

Topical area: Water Circulation and Currents

Robert B. Halley, USGS, St. Petersburg, FL

Since late 1994, South Florida has experienced above average precipitation. Also during this period, annual cycles of salinity in the east and west have been out of phase. Salinity measurements in Florida Bay indicate that although values increase from east to west, seasonal minima in the west lag behind minima in the east by about 3 months. This suggests that predictions of salinity without benefit of a well-tested, hydrodynamic circulation model are difficult if not impossible.

During the past three years, salinity values have been lower in the eastern Bay than in the western Bay. This trend results because the eastern Bay is more heavily influenced by rainfall and freshwater runoff from the Everglades than the western Bay and the Gulf Transition Zone. Gulf Transition Zone values are buffered by mixing with the Gulf of Mexico and the Atlantic Ocean. From 1995 to 1998, salinity ranged from about 15 to 25 ppt in the eastern Bay and from 27 to 37 ppt in the Gulf Transition Zone.

Lowest salinity values in the eastern Bay occurred in mid summer, lagging about 4 weeks behind the onset of the subtropical rainy season. In contrast, annual salinity minima in the western Bay occurred during the winter, with lowest values attained during January/February. Low salinity in the west is the result of surface runoff from the western Everglades, particularly through Shark River Slough. These flows reached maximum recorded volumes in 1995 when fresh water, equivalent to more than twice the volume of Florida Bay, entered the Shark River Slough system.

Processes causing the winter west coast freshets are not well understood, but McCallum and Stockman (1964) suggested that the flow of freshwater is controlled, in part, by the seasonal variation of Gulf of Mexico tides. A 10-20 cm decrease of monthly-averaged sea level occurs throughout November, December, and January, lowering base level and increasing the hydraulic gradient from the Everglades to the Gulf. This annual tidal cycle may account for a lag of about 12 weeks between the rainy season and the observed low salinity in western Florida Bay and the Gulf Transition Zone during winter months.

Records of freshwater flow into Shark River Slough indicate that the past three years have been atypical and that, on average since 1940, flow has been diminished by a factor of 2 to 4 from that of recent years. It is uncertain if more typical low-flow conditions can sustain the winter salinity minima that have occurred in the west during the past 3 years. The current phase relations of salinity cycles across the Bay may be a function of the present wet period. Bay circulation models will have to hindcast effects of low flow conditions on the west coast until sufficient data become available to verify models during low flow, dry period, conditions.

McCallum, J. S., and Stockman, K. W., 1964, Florida Bay Water Circulation, reprinted in Ginsburg, R. N., 1972, editor, South Florida Carbonate Sediments, Sedimenta II, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami Florida, 72p.