The role of the sea urchin Lytechinus variegatus in the community ecology of seagrass beds

The abundance of sea urchins, Lytechinus variegatus , was manipulated in the field through the use of cages. The normally occurring sea urchin population at a field site in north Biscayne Bay did not affect the shoot density or biomass of seagrass, Thalassia testudinum . Experimentally increased sea urchin density caused a decline in these parameters, but only in the winter. During the summer, increased sea urchin grazing did not affect the seagrass. Sea urchins were introduced to a second site in central Biscayne Bay that did not previously support a sea urchin population. Grazing from these introduced sea urchins did not affect seagrass biomass, and shoot density was affected only in the fall, not spring. In the summer of 1997 a naturally occurring grazing front of L. variegatus , with up to 222 sea urchins m-2, overgrazed parts of a Syringodium filiforme bed in Florida Bay. The effects of this grazing event were monitored for two years after passage of the front. A year later regrowth of T. testudinum, but not S. filiforme, began. The site appears to be shifting from a S. filiforme-dominated to a T. testudinum-dominated community. Grazing activities of the front also caused decreases in sediment depth and percent composition of fine grains, and an increase in sediment organic content. Deposition of sea urchin feces was at least partly responsible for the observed increase in organic matter of the sediment. Laboratory experiments demonstrated that sea urchin feces can also fertilize seagrass, but only at sea urchin population densities at which negative effects from grazing would outweigh the fertilization effect