Overgrazing of a large seagrass bed by the sea urchin Lytechinus variegatus in Outer Florida Bay

Unusually dense aggregations of the sea urchin Lytechinus variegatus overgrazed at least 0.81 km2 of seagrass habitat in Outer Florida Bay (USA) between August 1997 and May 1998. Initially, sea-urchin densities were as high as 364 sea urchins m-2, but they steadily declined to within a range of 20 to 50 sea urchins m-2 by December 1998. Prior to this event, sea-urchin densities were <1 sea urchin m-2 in this area of Outer Florida Bay. Seagrasses in Outer Florida Bay consist primarily of manatee grass Syringodium filiforme, of which 82% or 390 g dry weight m-2 of total seagrass biomass and > 95% of the short-shoot apical meristems were removed by sea-urchin grazing in our study area. Such extensive loss may severely limit recovery of this seagrass community by vegetative reproduction. Effects of the removal of seagrass biomass have already resulted in the depletion of epifaunal-infaunal mollusk assemblages and resuspension of fine-grained (<64 ??m) surface sediments - which have caused significant changes in community structure and in the physical properties of the sediments. These changes, coupled with the loss of essential fishery habitat, reductions in primary and secondary production, and degradation of water quality, may lead to additional, longer-term, indirect effects that may extend beyond the boundaries of the grazed areas and into adjacent coastal ecosystems.