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U.S. Geological Survey Open-File Report 02–5

Multibeam Mapping of the West Florida Shelf, Gulf of Mexico

By James V. Gardner1, Peter Dartnell2, and Kenneth J. Sulak3

1University of New Hampshire, Durham, N.H.
2U.S. Geological Survey, Santa Cruz, Calif.
3U.S. Geological Survey, Gainesville, Fla.

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CRUISE REPORT

 [perspective view of West Florida Shelf]

Perspective view of Madison-Swanson Reserve on the West Florida Shelf, Gulf of Mexico. The distance across the bottom of the image is about 13km with a vertical exaggeration of 20x.


Introduction

A zone of deep-water reefs is thought to extend from the mid and outer shelf south of Mississippi and Alabama to at least the northwestern Florida shelf off Panama City, Florida (Figure 1). The reefs off Mississippi and Alabama are found in water depths of 60 to 120 m (Ludwick and Walton, 1957; Gardner et al., 2001, in press) and were the focus of a multibeam echosounder (MBES) mapping survey by the U.S. Geological Survey (USGS) in 2000 (Gardner et al., 2000, Gardner et al., 2001, in press). If this deep-water-reef trend does exist along the northwestern Florida shelf, then it is critical to determine the accurate geomorphology and reef type that occur because of their importance as benthic habitats for fisheries.

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 [ Gulf of Mexico location map showing the 2000 multibeam sonar survey off Mississippi and Alabama and the 2001 multibeam sonar survey off the west coast of Florida ]

Figure 1. Location map showing northwest Florida shelf area mapped in 2001 (red rectangles) and area mapped in 2000 on Mississippi-Alabama shelf and slope (green rectangle).

Georeferenced high-resolution mapping of bathymetry is a fundamental first step in the study of areas suspected to be critical habitats. Morphology is thought to be critical to defining the distribution of dominant demersal plankton/planktivores communities. Fish faunas of shallow hermatypic reefs have been well studied, but those of deep ahermatypic reefs have been relatively ignored. The ecology of deep-water ahermatypic reefs is fundamentally different from hermatypic reefs because autochthonous intracellular symbiotic zooxanthellae (the carbon source for hermatypic corals) do not form the base of the trophic web in ahermatypic reefs. Instead, exogenous plankton, transported to the reef by currents, serves as the primary carbon source. Thus, one of the principle uses of the morphology data will be to identify whether any reefs found are hermatypic or ahermatypic in origin.

Objective
Our objective was to map the region between the 50 to 150-m isobaths south from the eastern edge of De Soto Canyon as far as Steamboat Lumps using a state-of-the-art multibeam mapping system (MBES)(red polygons labeled 2001 on Figure 1). The cruise used a Kongsberg Simrad EM1002 MBES, the latest generation of high-resolution mapping systems. The EM1002 produces both geodetically accurate georeferenced bathymetry and coregistered, calibrated, acoustic backscatter. Acoustic backscatter is the intensity of an acoustic pulse that is backscattered off the seafloor back to the transducer. The signal can give an indication of the type of material exposed on the ocean floor (i.e. rock vs. mud). These data should prove extremely useful in relating dominant species groups (which display highly specific biotope affinities) to the geomorphology (e.g., reef flattop, forereef crest, reef wall, reef base, circum-reef talus zone, circum-reef, high-reflectivity sediment apron, etc.).

For more information, see the West Florida Shelf Multibeam Cruise Report, or visit our Pacific Seafloor Mapping website.

This Open-File Report publishes the multibeam bathymetry and acoustic backscatter data, along with images and a GIS of the data.

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References

Ludwig, J.C and Walton, W.R., 1957, Shelf-edge calcareous brominences in northeastern Gulf of Mexico. Bull. Amer. Assoc. Petroleum Geologists, v. 41, p. 2054-2101.

Gardner, James V., Peter Dartnell, Kenneth J. Sulak, Brian Calder, and Laurent Hellequin. 2001. Physiography and late Quaternary-Holocene processes of northeastern Gulf of Mexico outer continental shelf off Mississippi and Alabama. Gulf of Mexico Science. in press

Gardner, James V., Kenneth J. Sulak, Peter Dartnell, Laurent Hellequin, Brian Calder, and Larry A. Mayer. 2000. Cruise Report RV Ocean Surveyor Cruise O-1-00-GM, The Bathymetry and Acoustic Backscatter of the Pinnacles Area, Northern Gulf of Mexico, USGS Open File Report 00-350.



FGDC Compliant Metadata

The multibeam bathymetry and acoustic backscatter data are available on this Open-File Report in two formats; ASCII XYZ and ArcInfo GRID. The northern, central, and southern regions are available at 8-m resolution, while the Steamboat Lumps region is available at 4-m resolution. FGDC metadata is provided for each format and and is available in three different forms, FAQ (Frequently Anticipated Questions), HTML, and text.

ASCII XYZ
FAQ
HTML
TEXT

ArcInfo GRID
FAQ
HTML
TEXT


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For additional information:
Contact Information, Pacific Coastal and Marine Science Center
U.S. Geological Survey
Pacific Science Center
400 Natural Bridges Drive
Santa Cruz, CA 95060
http://walrus.wr.usgs.gov/


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