Within Florida Bay, circulation and outflows are inherently linked to the processes of sediment resuspension, transport, and deposition. The complex bathymetry of the Bay, a product of sediment production and transport over time, constrains flow and limits mixing. Resuspension events cause increased turbidity, the recycling of nutrients, and facilitates the export of sediment-laden waters. Understanding how sediment transport processes and the overall bathymetry of the Bay respond to processes such as sea-level rise, storm events, and changing flows will allow us to predict future responses, both natural and man-made, and provide important data for hydrodynamic and water quality modeling efforts.

Three projects at the USGS are underway to investigate 1) bathymetric change in Florida Bay, 2) the processes and rates by which sediments accumulate and erode, and 3) to understand the timing and distribution of sediment resuspension. Individually, these projects will provide information essential to understanding ecosystem dynamics, and in combination, they provide an unprecedented baseline of data to evaluate the impact of future storm events on the bathymetry and sedimentology of Florida Bay.

The bathymetry of Florida Bay has not been systematically mapped since the 1890's, and some shallow areas have never been surveyed. In the 1930's, depth measurements were documented principally in the intercoastal channel just north of Key Largo and more recently, spot soundings have been conducted to better delineate the location of cuts and navigational hazards. A modern, digital map of Florida Bay bathymetry is needed to provide numerical modelers with an accurate base map, and to aid in the assessment of long-term patterns of sediment transport and deposition. Through the comparison of historical data with a modern bathymetry data set, areas of net erosion or deposition within the Bay can be identified.

The sea-floor mapping project focuses on the collection of an updated bathymetric data set for Florida Bay, the digitization of historical and modern data for comparison, and the production of quality maps and digital grids of both historical and present-day bathymetry, as well as those changes which have occurred.

Bathymetric data collection with a GPS based hydrographic system began in the summer of 1995 in the northeast quadrant of the Bay. The area east of Russell Key/Upper Matecumbe Key will have been surveyed by the end of 1996. Completion of the bathymetric survey is anticipated in 1998. Digitization of the historical bathymetric data was initiated in 1995. It is anticipated that the majority of digitizing of both the bathymetry and shorelines will be completed in 1996. All relevant information is being archived in a ARC/Info database.

In another project, we are addressing sediment resuspension processes in Florida Bay. Sediment resuspension within the Bay is principally a function of wind-driven waves and the properties of the sediments and sea-floor. A computer simulation of wave development within the Bay is being used to understand the effects of typical wind events. Preliminary results show the importance of the Bay's bathymetry and seagrass cover in controlling wave-driven flow. Current work focuses on the incorporation of varying bottom friction within the model. For this purpose, a map of bottom type and cover is currently being produced based on over 600 sampling sites throughout the Bay. In addition, to delineate the sediment properties controlling resuspension, over 100 surface samples are being analyzed for grain size, mud, water, carbonate, and organic content. Using statistical analysis of the data, a finite number of bottom and sediment types are being identified, these include areas of hard-bottom, dense seagrass, inermediate seagrass, sparse seagrass, open mud areas, a mudbank suite, and shell ridges. We will also quantify the resuspension potential in each of the bottom and sediment types identified using a portable resuspension device.

Using the results of wave modeling, sediment and bottom type analyses, along with the measurement of resuspension potential we expect to quantify the frequency and pattern of sediment resuspension in Florida Bay. This information will aid in our understanding of sediment export events, nutrient recycling, and patterns of turbidity within Florida Bay. Calibration for wave modeling will entail the deployment of a pressure sensor array in the Bay during one or more wind events. Satellite imagery of turbidity events, discussed elsewhere in this conference, will also be used for comparative purposes.

The third study focuses on the quantification of long-term sedimentation and erosion rates through the use of geochemical analyses of cores, multi-year surveys from monitoring stations, and detailed mudbank profiling. Five high-resolution profiles (+ 2 cm) across mudbanks show that they are shaped more like small mesas or plateaus with extremely flat tops rather than the mud mounds they have been compared with in the geologic record. Along each of three of these profiles, five elevation survey stations (+ 3 mm) have been established to monitor very small accumulations or losses of sediment. Stations and profiles provide the basis for determining long-term accretion or erosion and provide a baseline for major storm events. Where appropriate, downcore analyses of lead-210, total lead, and cesium-137 are also used to establish sedimentation rates during the last 100 years. These data together with sea-level rise estimates allow prediction of which portions of the bay will deepen, shoal, or remain the same during the next century. This predicted bathymetry can then be used to evaluate future changes in circulation in the Bay.

Baseline elevation measurements and profiling have been completed. Subsequent measurements will be made at least twice annually over the next two years. Core analyses are underway and results expected in the near future.

(This abstract was taken from the Florida Bay Science Conference Proceedings, 1996)

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