Proceedings of the U.S. Geological Survey (USGS) Sediment Workshop, February 4-7, 1997

SEDIMENTOLOGY AND FLORIDA BAY ECOLOGY: PAST, PRESENT, AND FUTURE

The South Florida Ecosystem Program, Coastal and Marine Program, and National Cooperative Geological Mapping Program provide opportunities for sedimentary geologists, geochemists, and oceanographers to apply diverse techniques to a common theme - the ecological history, current status, and future environmental prospects for Florida Bay. These programs support projects that define changes in the Bay during the past few centuries and quantify important mechanisms by which sediment directly and indirectly influences the ecosystem. Understanding sedimentary processes provides a predictive capability critical to the future management of the Bay. Complimentary approaches used in the programs have established the value of applying physics, chemistry, biology, geology, paleontology, and remote sensing of sediments to ecosystem issues in South Florida. In addition to the applied value of this work, research activities in South Florida also contribute to a better understanding of how tropical carbonate systems function.

Circulation within Florida Bay is greatly restricted by a network of interconnected mudbanks that subdivide the estuary into more than forty sub-basins. Observations of hypersalinity during the last, local, climatic dry period (1985-1990) together with subsequent environmental alteration have resulted in a salinity paradigm. The Florida Bay salinity paradigm is that Florida Bay has become more hypersaline in response to onshore water management practices. Hypersalinity has been linked to reductions in water quality, seagrass mortality, algal blooms, and increased turbidity in the region. From the 1880's to the 1960's drainage canals, dikes and pumping stations were built to recover land for urban and agricultural development and to protect it from floods. A significant portion of water that historically flowed south through central South Florida is now routed directly to the Atlantic Ocean and Gulf of Mexico bypassing the Everglades and Florida Bay. During the next two decades, state and federal agencies will attempt to "restore" the Bay to be more like it was at the turn of the century. Restoration effects have raised issues that provide opportunities for sedimentological research. Sedimentological applications focus on four general issues in the region: i) pollutants and contaminants in the sediments: ii) ecosystem history: iii) nearshore and estuarine sediment production, accumulation, erosion, transport, and turbidity: and iv) surface and subsurface water flow.

1. The geochemistry of nutrients and mercury and their uptake and release from sediments are active areas of research. Nutrients at levels normal for other regions are considered contaminants in the oligotropic environments of South Florida. Of particular interest is research concerning the removal of nutrients and metals from stormwater runoff before it flows into Florida Bay and the rates that nutrients and pollutants accumulate in freshwater marsh sediments. Accurate measurements of sediment and pollutant deposition rates is critical for calculating the efficiency of nutrient removal in stormwater treatment areas and for estimating the area and costs for required land purchases.

2. Reconstruction of ecosystem history critically depends on accurate dating of sediment cores and fossils used to interpret past environmental conditions. Sedimentological interpretations are required that demonstrate that fossils have not been transported and redeposited, but rather are representative of the time when matrix materials accumulated. Dating techniques, appropriate for decade to century time scales, use short lived radio-isotopes from the matrix in which the fossils occur, not the fossils themselves. Once dates have been verified, then geochemical (stable isotopic) and paleontological reconstruction can illustrate the response of the Bay to past anthropogenic and natural perturbations in salinity and temperature.

3. The sediments of Florida Bay, as in other marine carbonate environments, are produced by the organisms that live there. Because the bulk of Florida Bay sediment is marine carbonate, production rates can be estimated using nuclear-bomb produced carbon-14 as a tracer. Subsequent transport and accumulation influences turbidity and circulation as sediment accumulation occurs in banks and grassbeds. Turbidity is being evaluated using AVHRR satellite imagery available on almost a daily basis and posted over the INTERNET. Imagery is processed for reflectance, which is related to suspended solids and light availability, and can be analyzed to reveal seasonal, interannual, and decadal scale trends. Seafloor characterization, including sediment texture, composition, bottom morphology, and entrainment characteristics, provides an estimator of bottom friction and resuspension for wave, circulation, and sediment transport models. The satellite imagery provides means of testing and validation of the models for waves and sediment transport. Accumulation rates are measured using lead-210 dates that are verified using cesium-137 and anthropogenic lead as markers. It is anticipated that the production, transport, and accumulation studies will allow an integrated sediment budget to be established for Florida Bay.

4. The Holocene sediments of Florida Bay lie on a platform of Pleistocene marine limestone of exceptional porosity (35-45%) and permeability (>300m/d). Fluid exchange between the underlying limestone and the Bay is critical to the ecosystem because the groundwater (marine) is naturally enriched in nutrients and sulfide but depleted in oxygen. In addition, hundreds of shallow sewage disposal wells in the Florida Keys, bordering the southern and eastern Bay, introduce waste beneath the permeable islands and may be a source of excessive nutrients to the Bay. The direction and rate of flow in these rocks near the Florida Keys is a function of tidal pumping and the sedimentology of the limestones. Depositional units, separated by diagenetically altered exposure surfaces, form distinct horizontal permeability controls that are interrupted by lateral facies changes and vertical karst features. Mapping sedimentological characteristics of the limestones is an important component of understanding the geohydrologic properties of the limestone foundation for this ecosystem.

The four applications of sedimentologic principals and processes described above each play an important role in understanding the history and functioning of Florida Bay and their linkage to management issues. During the next two decades, agencies expect to invest $2 billion in changing the amount and timing of freshwater flow into the Everglades wetlands as well as the Bay. Land managers are designing large numerical models (circulation and water quality) to help predict the impact of these changes. Each of the sedimentologic studies have results that are incorporated directly into models (for example bottom friction in a circulation model), or that address conceptual models of processes that do not yet have numerical solutions (for example increased sediment resuspension after seagrass death).