Florida Bay Bottom Type Map
Topical Area: Sedimentology
Ellen Prager and Robert Halley, U.S. Geological Survey, St. Petersburg, FL
As part of a wave modeling effort, a map of bottom types within Florida Bay was produced to assess bottom friction associated with sediment types and benthic communities. The map represents the results of site surveys during the summer of 1996 and January of 1997, boat transects and comparison with aerial photographs (Dec. 1994 - Jan. 1995) and SPOT satellite imagery (1987). A small, flat-bottomed boat, high resolution GPS, and snorkeling gear were used to locate and examine over 650 sites within the Bay. In areas where depth and water clarity permitted, observations from the boat along transects between survey sites were made to identify the extent of bottom types. Over 100 sediment samples were also collected and analyzed for grain size, water, carbonate, and organic content.
For the purposes of map development, two descriptors were found to be particularly important, density of seagrass cover and sediment texture. Seagrass cover, as used here, is a visual estimate of both the number of plants and leaf length. Therefore, seagrass cover may be greater in areas with long leaves than in areas with short blades, even though the number of shoots may be the same. Seagrass cover is a different measure than density. It is used here to reflect hydrodynamic influence rather than the standing crop of seagrass. The map and associated descriptions are meant for general research purposes, and are not meant to assess ecologic communities or detail sedimentological facies, and small-scale changes in bottom type (e.g. small seagrass patches) are not delineated.
Eight bottom types were identified and mapped throughout the Bay; hardbottom, sparse, intermediate and dense seagrass cover, open mud, open sand, a bank top suite and a mixed bottom suite. Hardbottom refers to areas having little to no seagrass cover and only up to 5 cm of sediment overlying the Pleistocene bedrock. Fauna indicative of this bottom type are gorgonians, loggerhead or vase sponges. Other benthos which occur, but are not considered indicative, include other sponges, the algal genera Batophora and Penicillus, and various coral species including the genera Siderastrea and Solenastrea. Hardbottom sediments tend to be either slightly muddy-carbonate sand or sand, contain relatively little organic matter, and have a low water content. Sparse, intermediate and dense grass patches occur in sediment-filled karst solution depressions in the bedrock. These depressions can be as much as 2 meters deep and up to a few hundred square meters in area. A slightly higher mud content, up to 15 cm of sediment, and Calinassia burrows can also occur on a limited extent.
Sparse seagrass cover describes areas in which greater than 50% of the bottom is exposed. Penicillus, Batophora and scattered sponges often occur in this bottom type. Sediments are predominantly slightly muddy-carbonate sand, sandy- mud, or sand and vary from approximately 3 cm to over 2 m in thickness. The organic and water content in these sediments ranges from 0.5 13% and from 36 - 77%, respectively. Although sparse seagrass areas are generally muddy, in some areas the coarse fraction may be as much as 20%. Intermediate seagrass describes a bottom type that has greater than 50% seagrass cover with open areas of exposed sediment on the bottom (less than 50%). In areas near the Gulf and Atlantic, Calinassia burrows are common. Sponge and Penicillus are also frequently found in this bottom type. Sediments are dominated by sandy and shelly- (gravely) carbonate mud. Sediment thickness tends to be greater than 0.33 m and may reach up to over 2 m. Organic and water content vary from 1 - 11 % and 41 - 76%, respectively. Seagrass cover is considered dense where the bottom is completely obscured from view by grass growth. Sponges and blooms of the pearl oyster are common. Sediments, generally thicker than 0.33 m, are deposited amidst a tangle of seagrass rhizomes just below the surface. These sediments are muddy carbonate sand, with a relatively high organic (6 - 8%) and/or shell (10 - 53%) content.
Open mud occurs where no significant seagrass occurs and other benthic fauna, with the exception of algal mats, are not apparent. Sediments within these areas are predominantly carbonate mud with a relatively high organic (up to 15%) and water (up to 83%) content. Some of the open mud areas contain a relatively high shell content (17 - 35%) and rarely, exposed seagrass rhizomes at the sediment surface. It is suggested that open mud areas having a high shell content are sites of seagrass mortality which previously supported a live mollusc community. As a bottom type, open mud areas are not distinguished from seagrass die-off areas. Areas of recent seagrass mortality are recognized by extensive exposure of rhizomes or the dead stubble of shoots protruding from the sediment.
Bank-top suite is used to describe bottom types within the Bay commonly called mud banks and occur in depths typically less than 0.6 m. These areas are spatially complex environments that are dominated by mud, sand or in the western portion of the Bay, gravel. Bank-top sedimentary facies include a variety of seagrass densities and open muddy or shelly areas. Due to periodic exposure, seagrass shoot length typically decreases on the top of mud banks. Banks are commonly flanked by zones of intermediate to dense seagrass growth. In addition, north and east facing margins may be topped with a distinctly shelly sand layer or an emergent shell ridge. In the southwestern Bay and the Gulf transition zone, bank tops are often characterized by sand and gravel, along with live coral and Halimeda growth.
In the west-central region of the Bay an area exhibiting a highly variable bottom type occurs. Within several meters, variations in seagrass density are extreme and open mud areas occur on what seems to be an irregular basis. Consequently, this region has been termed a mixed bottom type. During January of 1997, large (m2) patches of seagrass, with rhizomes attached, were observed floating in the area. The patches appeared to be eroded at their edges, and portions of the seagrass mat were peeling off the sediment surface.
Areas of open sandy bottom occur in the Gulf transition zone of the Bay. These areas are distinct from hardbottoms in that no significant benthic life is present and the sediments are coarse shelly-carbonate sands. One large area of east - west running open sand occurs just south of Cape Sable and is most likely an erosional product of strong tidal flow within the area.