Role of sea urchin Lytechinus variegatus grazing in regulating subtropicalturtlegrass Thalassia testudinum meadows in the Florida Keys (USA)

In previous work, in St. Joseph Bay, Florida (30 degrees N, 85.5 degrees W) , in the northeastern Gulf of Mexico, we found that sea urchin Lytechinus v ariegatus grazing usually controlled seasonal changes in turtlegrass Thalas sia testudinum abundance. In this study, we tested the generality of those conclusions by conducting new grazing studies in 2 turtlegrass habitats in the subtropical Florida Keys (USA). In the first experiment, we varied the duration of sea urchin grazing bouts in order to understand the impacts of temporally varying herbivory on seasonal changes in turtlegrass biomass in shallow water (< 2 m) near Big Pine Key (similar to 25 degrees N, 85 degree s W). In the second experiment, we examined the effects of chronic low leve ls of grazing on seagrass growth and biomass in a deeper-water seagrass hab itat (6 to 7 m) in Hawk Channel (similar to 25 degrees N, 80 degrees W). Th ese new studies suggest that the impacts of sea urchin grazing are highly v ariable in the Florida Keys. Depending on the season, urchin grazing (at de nsities of 20 ind. m(-2)) had both negative and positive effects on seagras s biomass in Big Pine Key. If grazing occurred during spring, turtlegrass b iomass was significantly reduced by short bouts of sea urchin herbivory. Th e impacts of this spring grazing extended into early summer. If grazing occ urred in summer, sea urchins reduced turtlegrass biomass for only short per iods of time, after which urchin grazing stimulated turtlegrass production. These findings are similar to those of previous grazing experiments conduc ted in St. Joseph Bay. In contrast, we found little evidence that ambient ( 0 to 8 ind, m(2)) densities of sea urchins could control either seagrass pr oduction or biomass in the deeper waters of Hawk Channel. The differences w e found in grazer impacts on turtlegrass growth and abundance, in a well-li t shallow water habitat and deeper-water habitat with less light, suggest t hat there is a critical need not only to repeat experiments in environments with differing physical conditions but also to develop a more complete und erstanding of the mechanisms by which seagrasses can compensate for losses of tissues to marine herbivores under varying environmental conditions.