|Session VII: Marine Ecology
||Abstract #: 97703
SEAGRASS FACIES AND PHASES RECORDED IN THE SEDIMENTS OF FLORIDA BAY
Halley, R.B., Holmes, C.W., Prager, E.J., and Shinn, E.A
US Geological Survey, Coastal Center, St. Petersburg, Fl 33701
Widespread seagrass mortality in Florida Bay during the past decade is unique to human memory and has raised concerns about ecosystem health. However, the sedimentary record of Florida Bay indicates great variation of seagrass cover in the more distant past. In east-central Florida Bay, sediments clearly record the presence and absence of sea grass in sediment facies deposited during the last two centuries. Sea grass and grass-free (mud) facies are defined by the texture, composition, and structure of sediment. Seagrass facies have a chaotic structure, a significant coarse fraction, and contain fossil species of carbonate-producing epibionts. In contrast, mud facies (grass-free) are characterized by sediments that have a laminated structure, less shelly fauna, and are typically finer grained than grassy areas. Seagrass and mud facies accumulate in mudbanks as deposits of locally allochthonous (transported) and autochthonous (produced in situ) carbonate sediment originating in the Bay. Mudbanks slowly migrate in response to wave-induced suspension and traction sediment transport, eroding on exposed margins and accumulating on protected sides. Mudbanks vary greatly in the amount of seagrass cover but for the past 30 to 50 years have been dominated by seagrass facies.
Several researchers have applied facies analyses to mudbank cores in order to document changes in seagrass cover in the past. Newly applied dating techniques and studies of recent erosion and accumulation indicate that in the early 1800s seagrass cover was much like that of today. However, during the mid-1800s, the influence of sea grass greatly declined, and a phase of physical sedimentation became prevalent. This physical phase dominated until the 1920s and 1930s when seagrass facies returned to the area. The phase change from physical to seagrass-dominated sedimentation during the early part of this century may be associated with decreased circulation and freshwater inflow during that time. Although the causes of seagrass loss in the mid 1800s are subject to speculation, it is clear from the sedimentary record that areally extensive changes in seagrass cover have occurred in the past.