Florida Bay, home to several endangered species, is a valuable breeding ground for marine life and an important recreational and sport fishing area. Florida Bay (fig. 1) encompasses about 850 square miles in total area with an average depth of less than 3.5 feet. It is bordered by the mainland portion of Everglades National Park to the north, the Florida Keys to the east and south, and is open to the Gulf of Mexico to the west. During the last decade, Florida Bay has experienced algal blooms and seagrass die-offs which are signals of ecological deterioration that has been attributed to an increase in salinity and nutrient content of bay water. Salinity and nutrient content are directly related to the amount and quality of freshwater that enters the bay and to flow patterns within the bay. Restoration of the Florida Bay ecosystem requires a better understanding of the linkage between the amount of water and nutrients flowing into the bay and the salinity and quality of the bay environment.

As part of the South Florida Ecosystem Program, the U.S. Geological Survey, in collaboration with Everglades National Park, the U.S. Army Corps of Engineers, and the South Florida Water Management District, is conducting a study to measure flows into east Florida Bay. Information from this study will be used in conjunction with data from other studies to help determine the effects of changes in water deliveries to Everglades National Park on the Florida Bay ecosystem. Flow into Florida Bay is closely related to sediment transport, salinity, and chemical characteristics of the bay, which in turn, determine and interact with biological characteristics. Additionally, freshwater-inflow data will be used as input to hydrodynamic models of Florida Bay, for calibration of hydrologic models of the mainland, and for water-budget determinations for south Florida--all of which are essential elements for resource management and the ecosystem restoration.

Prior to the development of currently available acoustic instruments, it was very difficult to gage flows in streams discharging into Florida Bay. Standard methods for field data collection and flow computations are impractical and inaccurate because of the low velocities, flow reversal, and bi-directional flow in which high-salinity water flows inland under freshwater flowing out to the bay (fig. 2). With today's state-of-the art acoustic instrumentation, such as the Acoustic Velocity Meter (AVM) and the Acoustic Doppler Current Profiler (ADCP), it is possible to accurately gage flows in this environment because of the ability of these instruments to quickly measure low or rapidly changing water velocities, even during stratified or bi-directional flow. AVM systems have proven to be accurate instruments in the measurement of water velocities along a horizontal plane across stream and can be permanently installed to collect continuous velocity data that, along with water-level data, are used to produce continuous records of discharge.

ADCP instruments are used to measure water velocities in three dimensions. These measurements are then used to calculate the total flow through a stream section at a given time. The ADCP uses the Doppler shift from four acoustic beams sent downward in set angles to measure the velocity of water, depth, and distance traveled across the stream transect. Field measurements made with the ADCP's are used to develop relations between AVM velocities and discharge at gaged sites.

With the assistance of Everglades National Park, discharge measurements were made with ADCP's near the mouths of the major streams flowing into Florida Bay. Results of these measurements verified the applicability of ADCP's for discharge measurements under these environmental conditions, provided data on high flows into the bay, and helped in the understanding of flow patterns for each of the measured streams.

Project plans are to instrument selected streams flowing into Florida Bay with AVM's and temperature and specific conductance sensors in order to measure most of the total freshwater flow from the mainland into the bay. Sites are located along the mainland coast of east Florida Bay and represent most of the freshwater flowing south into the bay from Taylor Slough and the C-111 Canal basins. Three of these sites (Trout Creek Canal station and two C-111 Canal stations) are instrumented and maintained by Everglades National Park (fig. 1). Monthly ADCP discharge measurements are planned for rating AVM systems, and monthly collection of water samples are planned for total nutrient analysis. This work will be coordinated with activities from other agencies and institutions who need simultaneous flow data during biological or chemical samplings.

(This abstract was taken from the Florida Bay Science Conference Proceedings, 1995)

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