Land development and alterations of the natural system in South Florida have decreased fresh water flow and increased nutrients into Florida Bay. As a result, there has been a decrease in the water quality of the Bay which has prompted seagrass die offs and reduced fish populations. Restoration of Florida Bay will depend upon numerical circulation and sediment transport models to establish water quality targets and assess progress toward obtaining these targets. Application of these models is complicated due to the complex sea-floor topography (basin/mudbank morphology) of the Bay. The only complete topography data set of the Bay is 100 years old. Consequently, an accurate and modern sea-floor or bathymetry map of the Bay is critical for numerical modeling research.

A modern bathymetry data set will also permit a comparison to historical data in order to help assess sedimentation rates within the Bay. Previous research suggests that the mudbanks are dynamic features which migrate, accrete and erode. Less is known about the sedimentation rates in the basins. Some experts suggest the basins are filling, in association with sea-level rise, while others suggest the basins are deepening relative to sea-level.

The objective of this research is to collect new bathymetry for all of Florida Bay, digitize the historical shoreline and bathymetric data, compare previous data to modern data, and produce maps and digital grids of historical and modern bathymetry. This information will be provided to other researchers involved in the South Florida Ecosystem Restoration Program so they can better address the water quality issues of Florida Bay.

The strategy is to systematically map the bathymetry in Florida Bay to aid in the assessment of Bay sedimentation rates and to provide an accurate sea-floor surface for numerical models. The Bay is being mapped using a shallow draft boat equipped with a high-precision GPS coupled with a high-precision sounder. Data is being collected on a USGS 7.5-minute quad-by-quad basis proceeding westward from Blackwater Sound. Sounding trackline spacing vary depending upon the relief of the sea-floor, that is, closer spacing in near mud banks - wider spacing in the basins. Digital sea-floor grids are being produced from the trackline data. Historical bathymetric data (1890) have been obtained from NOS archives, digitized, and entered into a GIS. Digital comparisons will be made between the historical data and the new data. The project will produce both hardcopy and digital map products and professional papers. Historical and modern bathymetric maps will be produced as well as maps of bathymetric change. All maps will be produced at a scale of 1:24,000 and will conform to USGS I-Map standards. Map production will be performed using ARC/Info and data archiving will comply with SDTS standards.

The parts of the Bay which are accessible by a shallow draft boat have been mapped using the System for Accurate Nearshore Depth Surveying (SANDS). This system, developed by the USGS, utilizes geodetic differential Global Positioning System (GPS) receivers, a digital fathometer, a digital heave/roll/pitch sensor, and a shallow draft boat. The system is accurate to ~10 cm vertically, ~4 cm horizontally, and can collect data in water depths as shallow as 50 cm. To achieve this accuracy, the boat must not rove more than 10 km from any one GPS reference receiver. Therefore, approximately 10 new ground control points (with 1-2 cm accuracy) will be established throughout the Bay for use as reference receiver sites. For areas not accessible by boat, mud bank tops and sensitive seagrass areas, other non-intrusive (remote sensing based) methods must be employed to fully map the bathymetry of the Bay. One non-intrusive technique to map the boat inaccessible areas is the U.S. Army Corps of Engineers SHOALS helicopter LIDAR system. Another method utilizes vertical photography or imagery in combination with water stage data to determine the elevations of the mud banks. These two techniques and others are being investigated at this time. Utilizing a non-intrusive method(s) in conjunction with the SANDS system will provide basin and mud bank elevations for the entire Bay.

To date, Florida Bay has been systematically surveyed for the area from Long Sound west to Whipray Key. The data has been checked for errors, entered into a GIS, and digitally contoured. All historical shoreline and bathymetric data of Florida Bay has been digitized, projected into a modern datum, and digitally contoured. Comparison of the modern data set to the historical has been initiated. Preliminary results (for the aforementioned area) suggests that basin depths have not changed dramatically in the past 100 years. The most noticeable changes have occurred in the cuts, for example, "The Bogies", Grouper Creek, Dusenbury Creek, and Baker Cut. Many islands in this part of the Bay changed in their areal extent. We plan to quantify the amount of change and produce sea-floor change maps for this area in the near future.

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