Ellen Prager
The objective of this research is to provide a better understanding of how and when sediments within Florida Bay are resuspended and deposited, to define the spatial distribution of the potential for resuspension, to delineate patterns of potential bathymetric change, and to predict the impacts of storms or seagrass die-off on bathymetry and circulation within the bay. By combining these results with the findings of other research being conducted in Florida Bay, we hope to quantify sediment export from the bay, better define the nutrient input during resuspension events, and assist in modeling circulation and water quality. Results will enable long-term sediment deposition and erosion in various regions of the bay to be integrated with data on the anticipated sea-level rise to predict future water depths and volumes. Results from this project, together with established sediment production rates, will provide the basis for a sediment budget for Florida Bay.
Devereux, M. J.
Disteche, A.
Pichon, M. Delesalle, B. Frankignoulle, M.
Suzuki, A. Saito, H.
Key, G. S.
Ginsburg, R. N. Shinn, E. A.
Ibert, E. R. Hood, D. W.
Halley, R. B.
Five of the six activities were designed to provide measures of sedimentation or erosion on mudbanks, the sixth activity documents the influence of mudbanks on water salinity. 1) Coring: Cores taken for this and other projects were x-rayed and some provided measurable sections of sediment above known (dated) horizons. These provided an average sedimentation rate based on the age of the horizon. 2) Pb-210 dating: A few cores were suitable for lead-210 dating from which an average sedimentation rate was calculated. The lead-210 method has the advantage of providing a continuous record of sedimentation rates during the last century with a resolution of a few years. However, there are only a few sites in the Bay that are suitable for analyses. 3) Sedimentation site monitoring: Fifteen local sediment survey stations were established in the bay. These were driven to bedrock and provided platforms for seasonal sediment elevation measurements accurate to a few millimeters. Five are in the eastern bay, five in the central bay, and five in the western bay. 4) Bank profiles: Each group of five survey stations is arranged in a transact across a mudbank. Repeated precision profiling across each mudbank will provide a multi-year record of sediment erosion or accretion on the bank and allow the data from individual survey stations, cores, and marker horizon sites to be placed in context of bank-wide patterns. Sedimentation rates provide basic data for determining long-term accumulation/erosion patterns and subsequent volume changes in the bay as a result of sea- level rise. 5) Salinity surveys: Salinity maps, produced semi monthly, illustrate the influence of mudbanks on circulation. The contours of salinity, constructed from bay-wide surveys, show conformity with the banks and often coincide with the banks. Turbid and algal bloom regions, monitored by other agencies, are also confined by shallow banks. 6) Productivity and calcification measurements: The measurements of the short-trem productivity and carbonate precipitaiton provided the data necessary for a comparison of current sedimentation rates with long-term sediment accumulation measures by lead-210 dating and elevation surveys from the project. Three measures of water quality (salinity, turbidity and chlorophyll) indicate that the mudbanks are a dominant control on circulation. Understanding mudbank dynamics is critical to predicting future water quality of the Bay.
Productivity measurements in Florida Bay, including calcification and net photosynthesis, were performed using geochemical techniques that have proven successful for measuring production in carbonate reef and seagrass bed ecosystems (Smith 1973, Barnes 1983, Barnes and Devereux 1984, Frankignoulle and Disteche 1984, Gattuso et al. 1993). These measurements were used to provide insight into the discrepancy between long-term sediment accumulation rates (Stockman et. al,. 1967) and short-term production measurements (Bosence, 1989). Total alkalinity, pH, calcium concentrations, salinity, irradiance, temperature, wind and current speed, and air-sea CO2 and O2 fluxes were measured along transects across carbonate mud banks in Florida Bay. Transects were located parallel to unidirectional current flow across a given bank. Sample stations along each transect were positioned at the upstream, middle, and downstream ends of each transect. Geochemical and physical parameters were measured at each station along a transect at different times (and irradiances) during the day.
Total alkalinity and pH were used to calculate calcification and net photosynthesis using the alkalinity anomaly technique of Smith and Key (1975) such that calcification (C) = half the change in total alkalinity, and net photosynthesis (P) = total carbon -calcification. Total carbon was calculated using carbonate system equations from Millero (1979). Calcium measurements provided an independent measure of calcification for comparison. Air-sea CO2 fluxes were measured directly at each station inside of a floating bell (Sugiura et al. 1963, Frankignoulle and Disteche 1984, Frankignoulle 1988, Gattuso et al. 1993, Kayanne et al. 1995) using the procedure and calculations of Frankignoulle (1988). Air-sea O2 fluxes wree determined by measuring atmospheric and water pO2 and calculating fluxes as described in Wanninkhof (1992). Differences in oxygen and carbon metabolism between upstream and downstream stations will be corrected for O2 and CO2 exchange with the atmosphere as described in (Gattuso et al., 1993).
Productivity and metabolic rates per unit area were calculated using the difference in concentration between upstream and downstream stations, the volume of water transported along a transect, and the transect area such that the change in concentration of a parameter (dC/m2/s) = C/m3 x m3/hr)/m2 (Barnes and Devereux, 1984). Productivity data from multiple transects in Florida Bay were used to estimate daily production rates for Florida Bay. Comparison of these data with previous productivity estimates and sediment accumulation rates will indicate whether discrepancies between production and accumulation rates are due to measurement and calculation techniques or to some real change in the productivity of the Bay.
U.S. Department of the Interior, U.S. Geological Survey, Center for
Coastal Geology
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