Hurricane effects on the planktonic food web of a large subtropical lake

Hurricanes Frances and Jeanne passed over Lake Okeechobee, Florida, in September 2004 and Hurricane Wilma in October 2005. The storms created large waves, strong currents, high wind seiches and uplifted over 3 million metric tons (collectively) of sediments into the water column. Suspended solid concentrations increased five-fold and there were substantial changes in the plankton. Unlike previously documented effects of hurricanes in the open ocean and estuaries, where increased nitrogen inputs stimulate primary productivity, the hurricanes resulted in substantial reductions in biomass of bacteria, phytoplankton and phototrophic nanoflagellates, both in pelagic and near-shore habitats. Increases in macro-zooplankton biomass were observed in both habitats. There were sustained large increases in dissolved inorganic nitrogen and phosphorus in the water column after the hurricanes, coincident with large declines in mean irradiance in the mixed layer. Further, results from laboratory bioassays that exposed the phytoplankton to nutrient additions and a controlled light gradient indicate that the community shifted from being frequently nitrogen limited to most commonly light limited after the storms. The results confirm that the major driver of plankton food-web dynamics in this system is light availability, and that the primary mechanism of change caused by hurricanes is an accentuation of light limitation via greatly increased sediment re-suspension. There additionally was evidence of food-web-mediated effects where the loss of submerged vegetation and increased turbidity reduced the density and efficiency of visually feeding fishes, leading to a significant increase in biomass of macro-zooplankton.