The Greater Everglades ecosystem stretches over 360km, from Lake Kissimee in the north to Florida Bay in the south. At its widest point it covers more than 100km from west to east. The vast flow way has been dissected by a network of canals and levees over the recent past to control flooding, provide water storage for human consumption, and protect croplands. Natural surface water flows into Everglades National Park have been seriously disrupted, in terms of quantity, quality, and timing of flow. Presently federal, state, and local agencies have established a $7.8 billion ecosystem restoration program for the Everglades, known as the Comprehensive Everglades Restoration Plan (CERP). Many science questions remain unanswered as the restoration begins. Major questions exist concerning rates of soil formation in the various wetland plant communities and about factors regulating surface elevation in these wetlands.

This report provides data from a network of Surface Elevation Tables (SETs) in the southwest coastal wetlands of Everglades National Park. At each of eight sites, three SETs were established. Three study locations were along each of the Shark and Lostmans Rivers, the two major coastal drainages of the southwest Everglades. On each river one site is in an upstream freshwater wetland, a second site is in the middle reach of the river in a brackish marsh - mangrove forest community, and the third site is downstream near the river's mouth in a pure stand of mangrove forest. The two freshwater sites are non-tidal and highly seasonal with respect to surface water flows and depths (both highest in summer and negligible in the winter dry season). Tides are measurable, but small, at the brackish waters sites and there is a noticeable variation in surface water. Tidal activity dominates the hydrological signature at the saline, downstream, mangrove forest sites. The seventh and eighth sites are located at Big Sable Creek, a marine dominated region on northwest Cape Sable. The cape was devastated by the 1935 Labor Day Hurricane and again by Hurricane Donna in 1960. At this location three SETs are in mangrove forest and three are in open, unvegetated, intertidal mudflat. Ground-water and surface-water sampling wells are present at all sites and are instrumented to record elevation and conductivity at hourly intervals. SETs have been sampled at 3-6 month intervals for more than three years (fig. 1). For each of the eight sites we calculated the rate of change in relative wetland surface elevation (only one site is currently surveyed to mean sea level) between sampling intervals. We also calculated the rate of change in daily ground and surface water elevation for the same intervals. Average surface and groundwater stage was determined for the day of SET sampling, and for the 15 and 30 days prior to sampling. Simple linear regression was used to test for differences between surface elevation change and the water level parameters and to calculate the slope of that relation for each site.

Sediment surface elevation at all sites showed apparent annual cycles that differed between sites. Surface elevation appeared to be greatest during the dry season at upstream freshwater locations (fig. 1) and lowest during the dry season at downstream saline locations. The two brackish water, middle river, sites showed little variation in sediment surface elevation. Surface-water stage, over the 15 days prior to sediment sampling, was strongly related to the change in sediment elevation between samplings (fig. 2). Most importantly, this relation was the opposite between sites. At freshwater sites, as average stage increased, sediment change decreased, whereas at saltwater sites, increasing surface-water stages led to increasing sediment elevation between sampling periods. The pattern at the two brackish sites was intermediate.

Why these two wetland systems behave in an opposite manner is unclear at this time. One hypothesis is that in the freshwater systems, a sedimentation - re-suspension process is occurring whereby, as the dry season progresses, flocculent material in the water column settles out, and as the wet season sets in and water levels (and current velocities) go up, this material becomes re-suspended and lost from the site. This hypothesis will be tested in coming months.

Figure 1. (above) Representative data showing the time series of surface & groundwater stage and sediment surface elevation at the upstream freshwater site on the Shark River (SH1). [larger image]

Figure 2. (above) Sediment elevation change versus average surface water stage for a freshwater marsh (SH1) and a salt mangrove forest (SH3). [larger image]

Contact: Smith III, Thomas J. USGS, Center for Water & Restoration Studies, 600 Fourth Street, South, St. Petersburg, FL 33701. Voice: 727-803-8747, FAX: 727-803-2030; Email: Tom_J_Smith@usgs.gov

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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