Topical Area: Sedimentology

Ellen Prager, U.S. Geological Survey, St. Petersburg, FL

The processes which control sediment resuspension in Florida Bay dramatically illustrate how nature is inherently multidisciplinary in form. In the Bay, the regional geology, climatic events and related oceanographic response combine with biological influences to control sediment resuspension. Consequently, none of these factors in isolation can or should be used to predict or model sediment transport in the Bay.

Using remote sensing techniques, turbidity increases and resuspension of sediments have been linked to strong wind events. However, sediment resuspension does not occur uniformly either throughout the Bay or during the year. Recent research suggests a pattern in which less resuspension occurs during the first several cold fronts of the year as compared to those which occur later in the winter season. This research was undertaken to quantify resuspension on an annual basis, and determine the factors controlling the spatial and temporal variability exhibited in resuspension events.

Analysis of over 100 surface sediment samples indicates a carbonate system dominated by mixtures of sand, mud, and shell. Samples consistently contain over 95% carbonate with variable water and organic content. Laboratory experiments were performed to assess the shear stress required to suspend the various sediment mixtures. Eighty sediment cores were collected in the Bay. Shortly after collection, the undisturbed surface of each core was subjected to an equivalent shear stress using a portable resuspension device (PRD). Results indicate that the binding effects of an algal or microbial mat on the sediment surface dominate differences due to sediment composition. The critical shear stress for an organically bound sediment surface appears to be two to three times (2 - 4 dynes/cm²) that of an unbound surface (1 - 2 dynes/cm²).

Wave modeling in the Bay suggests that if organic mats are present, only cold fronts and strong summer storms with winds of over 14 m/s generate orbital velocities and shear stresses capable of significant bay-wide sediment resuspension. However, once mats are removed winds of 10 m/s or less could cause widespread resuspension. Wave measurements within the Bay verify model results and suggest that during propagation from mid-basin to a bank’s edge, seagrass growth can cause an 80% or more reduction in wave energy.

Results of this study show that spatial variations in resuspension are due not only to differences in sediment composition, but also local basin size, orientation, seagrass and organic mat growth. Variations in sediment-derived turbidity throughout the year appear to be linked to both seasonal wind patterns and the presence of microbial or algal mats. During the calm, warm summer months algal and microbial mats flourish. Periodic squalls or tropical storms probably rip up the mats, but they grow back quickly in summer conditions. In the initial weeks of the cold front season, mats are subject to greater shear stresses at more frequent intervals. Observations during PRD experiments reveal that mats tear slowly unless subject to extremely high shear stress (>4 dynes/cm²). Consequently, limited resuspension during the initial cold fronts of the season probably occurs because mats are starting to tear, but remain somewhat intact. After several fronts, these mats may be fully removed and resuspension occurs more readily.

To better quantify resuspension of sediments in Florida Bay, further research is needed to assess the spatial and temporal extent of mat growth.