Geology of Shelf-Edge Habitats of the West Florida Shelf

Figure 1. A pilot study in the West Florida Shelf area is allowing geologists and biologists from the U.S. Geological Survey, the National Marine Fisheries Service, and Florida State University to relate observations of fish spawning and feeding behavior to the geology of the sea floor.

Fisheries scientists from the National Marine Fisheries Service and Florida State University are evaluating critical fish habitats along the West Florida Shelf edge where several species gather to spawn, feed, or reside for part of their life cycle. Because morphology and composition of the sea floor are critical factors in successful fish spawning, geologists from the U.S. Geological Survey (USGS) are cooperating with the biologists in a pilot study to map the sea floor and analyze the geologic history of the West Florida Shelf. The location of the pilot study area is given in figure 1. The sediments on the West Florida Outer Shelf are part of a band of mixed ancient (Pleistocene to early Holocene) and modern sands (fig. 2). The band of sediments slowly moves across the shelf, encompassing and partially covering rocky mounds and ridges, some of which are drowned pinnacle reefs. Although some preliminary studies of the general characteristics of the ancient shelf-edge sediments on the West Florida Shelf were published in the 1950's and 1960's, there has been no comprehensive analysis of the age, morphology, and origin of the sea-floor features or of the age, thickness, distribution, and paleoecological evolution of the ancient sediments.

Figure 2. Coarse sand from Pleistocene or early Holocene time migrates slowly across the outer edge of the West Florida Shelf, forming northeast-trending sediment waves. The waves are 1 to 4 meters high and about 100 meters from crest to crest. In the image, seams between adjacent data strips have created an east-northeast-trending artifact.

Preliminary analysis of sidescan-sonar mosaics and underwater video from the pilot study area and nearby areas south of Panama City, Fla. (fig. 1), reveals spectacular sea-floor habitats (rocky ridges, hardgrounds, pinnacles, even natural rock arches) likely to support grouper spawning aggregations. USGS geologists discovered a series of rocky ridges (fig. 3), surrounded and partially buried by an undetermined thickness of mobile sand in 60 to 80 meters of water. A subsequent acoustic survey, supplemented by underwater video, confirmed that the waters around these ridges are populated by numerous species of fish. In deeper parts of the survey area (80 to 120 meters), the presence of numerous pits, presumed to have been dug by yellow-edge grouper, suggests that the sea floor consists of finer grained, more cohesive sediments than those found in shallower waters (fig. 4).

Figure 3. This mosaic of sidescan-sonar data from the southeastern corner of the West Florida Shelf pilot study area (see fig. 1) shows 15-meter-high rocky ridges, partially buried by mobile sediments. This type of habitat is favored by some species of grouper for spawning and is attractive to other species for feeding.

Figure 4. Shallow (less than 1 meter deep) pits with diameters of several meters dot the sea floor in the parts of the study area deeper than about 80 meters. They are presumed to have been dug by yellow-edge grouper. Their use to the fish is not known, but they provide a useful clue to the geologist about the composition of the sediment. Because pits such as these would quickly collapse in sand, the sediment in the pitted areas must be of a finer grained, more cohesive texture. Sediment samples from this area confirm this supposition.

Preliminary analysis of microorganisms found in four sediment-grab samples from the pilot study area distinguished an ancient assemblage typical of shallow-water carbonate environments and a modern assemblage typical of deep-water, noncarbonate environments. The presence of these two assemblages indicates a rise in sea level in the West Florida Shelf area of approximately 50 meters during the Holocene Epoch. The pilot studies currently underway show that there is a strong link between the geologic history of the West Florida Shelf edge and the spawning and feeding habits of several species of commercial and sport fish (fig. 5). Because the West Florida Shelf is a huge area, encompassing about 140,000 square kilometers, it is imperative to understand the geologic history of the area in order to help focus limited resources on mapping areas most likely to contain critical habitats for selected fish species. Geologic mapping is helpful in establishing boundaries of reserves and in choosing monitoring sites for testing the effectiveness of reserves and other fisheries management tools. Proposals for future cooperative work are being prepared by geologists and biologists from several agencies.

Figure 5. A fine example of a large male gag grouper, one of the species that gather to spawn at the rocky shelf-edge habitats along the West Florida Shelf.

U.S. Department of the Interior U.S. Geological Survey

USGS Fact Sheet 109-99 October 1999