SOFIA - Sedimentation, Sea-Level Rise, and Circulation in Florida Bay

During the next century the physiography of Florida Bay will change due to a complex interplay between sediment production, transport, and accumulation and the local, relative rate of sea-level rise. Some estimates have predicted that the Bay will eventually fill with sediments and become part of the Everglades (Enos and Perkins, 1979). Others have suggested erosion and removal of sediment will result in the expansion of the open Gulf of Mexico throughout the Bay (Wanless and Tagett, 1989; Parkinson and Meeder, 1991). These predictions, while geologically correct, incorporate time-scales of centuries to millennia, beyond those applicable to ecosystem restoration planning. Recent process studies, including estimates of sediment production (Bosence, 1989; Halley and others, 1999; Yates and Halley, 1999), transport (Prager and Halley, 1999; Stumpf and others., 1999), and accumulation (Robbins and others., in press), indicate neither of these scenarios is correct for the next hundred years. Rather, subtle and complex changes that have characterized the physiographic evolution of the Bay during the 1900s will continue. Process studies provide more accurate forecasting for this century because they may be inferred on time-scales of decades to centuries.

About 60 percent of Florida Bay sediment has accumulated in mud banks during that last 4,000 years (Scholl, 1964). The remaining sediment (38 percent) is in flanking deposits and spread over lake floors in the central and western parts of the Bay. Islands, tidal channel deltas, and filled solution holes account for only a few percent of the sediment in the Bay. Geochemical and constituent sediment analyses demonstrate that the sediment has formed from the biota living in the bay. About 90-95 percent of the sediment is mineral carbonate skeletal debris and 5-10 percent is organic debris, mostly mangrove and seagrass. There is a small detrital fraction, generally less than a few percent, consisting of quartz, clays and dolomite (Prager and Halley, 1997). Surface sediments are about three-fifths sand and two-fifths mud by weight, but analyses of cores indicate subsurface sediments contain more mud with a sand/mud ratio of about 1:5. The sand-enriched surface sediments result from winnowing processes that preferentially transport fine sediment to mud banks leaving a sand veneer over large areas of the Bay floor.

Sediment is continuously produced by living organisms and is remobilized by erosion. Erosion occurs along unprotected shorelines, exposed mud banks, and areas recently denuded by seagrass mortality. Eroding islands show the most rapid erosion rates, with extreme shoreline retreat rates that approach 1 m yr^-1 . The presence or absence of seagrass is a first-order control of subtidal erosion and deposition. Some mud banks that erode on exposed margins are accreting on protected margins, causing a net migration on the order of a few decimeters yr^-1 . Areas of seagrass mortality may expose extremely fluid mud to transport, resulting in redeposition of several centimeters of sediment per year.

Wave modeling and direct measurement help to understand the complexities of erosion and deposition and the importance of seagrass. Waves form in the basins and propagate most effectively across basins with long axes parallel to the wind. Refraction effectively turns waves parallel to the banks. Seagrass diminishes wave energy along bank margins (Prager and Halley, 1999). If seagrass is absent, significant erosion may occur on bank margins. Although summer thunderstorms account for some erosion and turbidity, remote sensing studies indicate that winter cold fronts account for most of the turbidity and sediment transport in the Bay. Turbidity patterns also reveal a seasonally consistent west-central clear zone, indicating that little sediment from the central and eastern Bay escapes the estuary (Stumpf and others, 1999). Only hurricanes are energetic enough to redeposit sediment on islands and along shorelines.

Sedimentation rates between 0.5 and 2 cm yr^-1 have been measured margins of mud banks (Holmes and others., in press). These rates are almost an order of magnitude

Figure 1. Profiles of Florida Bay mudbanks. Mudbanks have extremely level and uniform tops resulting from the interplay of sediment deposition and erosion influenced by seagrass presence and absence. The slopes of the banks, together with vertical accretion and erosion rates, may be used to estimate lateral migration rates. These profiles were surveyed with an electronic level and stadia rod, accurate to +/-3cm. Click for larger image.

greater than sea-level rise and indicate that mud banks can outpace sea-level rise. However, coring reveals that mud banks are just keeping up to sea level, not catching up or becoming islands. It is hypothesized that seagrass dynamics prevent the banks from growing above sea level. Periodic episodes of seagrass mortality, probably caused by excessive aerial exposure, limit the ability of the banks to accrete above the annual low tide.

Newly formed sediment is a minor contribution to the banks. Radiocarbon can be used as a tracer of carbonate produced since thermonuclear atmospheric testing of the late 1950s and early 1960s. It can be shown that only a fraction of a percent of new carbonate sediment has been produced in the past 40 years (Halley at al., in preparation). Although carbonate production rates are estimated in the range of 100s gms m^-2 yr^-1 (Bosence, 1989), much of this production is redissolved (Walter and Burton, 1990). Net accumulation over the past 3500 yrs has been approximately 0.2 mm yr^-1 (2.2 gms m^-2 yr^-1). These production rates are similar to carbonate productivity estimates calculated from alkalinity and pH measurements made during 1998 and 1999.

Production rates of a fraction of a mm yr^-1 , by themselves, are insufficient to keep up with sea-level rise. But the continuous redistribution of sediment, preferentially accumulating in seagrass on mud banks, will maintain the banks close to the annual low tide (fig. 1). The basins, on the other hand, will slowly deepen because sediment produced in the basins is removed and transported to the mudbanks. Basin deepening will result directly from sea-level rise and can be approximated using the methods of Titus and Narayanan (1995). The long-term evolution of Florida Bay, forced by sedimentation and sea-level change, defines the context within which long-term restoration activities will occur.

Sea-Level Rise and the Future of Florida Bay in the Next Century