Sediment is continuously produced by living organisms and is, eventually, 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 shoreline retreat rates of some islands approaching 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 0.5 m yr^-1. Areas of seagrass mortality may expose extremely fluid mud to erosion, redepositing several centimeters of sediment per year.

Wave modeling and measurement help researchers 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. However, seagrass effectively dampens out wave energy along bank margins (Prager and Halley, in press). If seagrass is absent, significant erosion may occur in these areas. 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, in press). 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 from the accreting margins of mud banks (Holmes and others, in press). These rates are almost an order of magnitude greater than sea-level rise and indicate the mud banks can outpace sea-level rise. However, coring reveals that mud banks are 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 on the basis of studies using radiocarbon as a tracer of carbonate produced since thermonuclear atmospheric testing of the late 1950s and early 1960s, only a fraction of a percent of new carbonate sediment has been produced in the past 40 years. Although carbonate production rates are estimated in the range of 100s of gms m^-2 yr^-1 (Boscence, 1989), much of this production is redissolved (Walter and Burton, 1990). Net accumulation over the past 3,500 years 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 the summer of 1998.

Production rates of a fraction of 1 mm yr^-1, by themselves, are insufficient to keep up with sealevel rise. But the continuous redistribution of sediment, preferentially accumulating in seagrass on mud banks, will maintain the banks at approximately the annual low tide mark. The basins, on the other hand, will slowly deepen as sediment is transported by wave action and residual circulation to the margins of the mud banks. The deepening will approximate the rate of sea-level rise. The long-term evolution of Florida Bay, forced by sedimentation and sea-level change, define the context within which more recent changes such as seagrass die-off and algae blooms occur. It is important to consider recent ecological change in light of this dynamic setting.

REFERENCES

Boscence, D., 1989, Biogenic carbonate production in Florida Bay: Bulletin of Marine Science, v. 44, pt. 1, p. 419-433.

Enos, P., and Perkins, R.D., 1979, Evolution of Florida Bay from island stratigraphy: Geological Society of America Bulletin, Part I, v. 90, p. 59-83.

Holmes, C.W., Robbins, J.A., Halley, R.B., Bothner, M., tenBrink, M., and Marot, M., in press, Sedimentary dynamics of Florida Bay Mud Banks on a decadal time scale: Journal of Coastal Research.

Prager, E.J., and Halley, R.B., 1997, Bottom types of Florida Bay: U. S. Geological Survey Open-File Report 97-526.

Prager, E.J., and Halley, R.B., in press, The influence of seagrass on shell layers and Florida Bay mud banks: Journal of Coastal Research.

Scholl, D.W., 1964, Recent sedimentary record in mangrove swamps and rise in sea level over the southwestern coast of Florida, Parts I and II: Journal of Marine Geology, v. 1, p. 344-366.

Stumpf, R.P., Frayer, M.L., Durako, M.D., and Brock, J.C., in press, Variations in water clarity in Florida Bay from 1985 to 1997: Estuaries.

Walter, L.M., and Burton, E.A., 1990, Dissolution of recent platform carbonate sediments in marine pore fluids: American Journal of Science, v. 290, p. 601-643.

Wanless, H.R., and Tagett, M.G., 1989, Origin, growth, and evolution of carbonate mudbanks in Florida Bay: Bulletin of Marine Science, v. 44, no.1, p. 454-489.

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