Benthic microalgal biomass and irradiance at the sea floor on the continental shelf of the South Atlantic Bight: Spatial and temporal variability and storm effects

Variables relating to the potential for benthic microalgal primary production on the continental shelf of the South Atlantic Bight (SAB) were characterized over a depth range of 14–40 m off Georgia and 20–40 m off northeastern Florida. The areal concentration of chlorophyll a in the surface 0.5 cm of the sand sediments generally exceeded the depth-integrated chlorophyll in the entire overlying water column, often by a factor of 4–6. Visible irradiance at the sediment surface was often 4–8% of surface levels over much of the region, and exceeded 10% of surface levels on occasion (mid-summer at 14–30 m depth). HPLC pigment analyses and qualitative microscopy indicate that sediment chlorophyll is associated with a distinct benthic microflora dominated by diatoms. Relatively high concentrations of diatom pigments extend to at least 2–3 cm in the sediments, apparently associated with intact cells. At small spatial scales, variability in benthic microalgal biomass may be related to bioturbation and grazing by benthic fauna. Intense storms can strongly affect the potential for benthic primary production on the shelf by physical disturbance of sediments and by resuspension of fine particles, which limits light penetration to the sediment surface.