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

Abstract

In previous work, in St. Joseph Bay, Florida (30° N, 85.5° W), in the northeastern Gulf of Mexico, we found that sea urchin Lytechinus variegatus grazing usually controlled seasonal changes in turtlegrass Thalassia 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 (∼25° N, 85°W). In the second experiment, we examined the effects of chronic low levels of grazing on seagrass growth and biomass in a deeper-water seagrass habitat (6 to 7 m) in Hawk Channel (∼25° N, 80° W). These new studies suggest that the impacts of sea urchin grazing are highly variable in the Florida Keys. Depending on the season, urchin grazing (at densities of 20 ind. m -2 ) had both negative and positive effects on seagrass biomass in Big Pine Key. If grazing occurred during spring, turtlegrass biomass was significantly reduced by short bouts of sea urchin herbivory. The impacts of this spring grazing extended into early summer. If grazing occurred in summer, sea urchins reduced turtlegrass biomass for only short periods of time, after which urchin grazing stimulated turtlegrass production. These findings are similar to those of previous grazing experiments conducted 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 production or biomass in the deeper waters of Hawk Channel. The differences we found in grazer impacts on turtlegrass growth and abundance, in a well-lit shallow water habitat and deeper-water habitat with less light, suggest that there is a critical need not only to repeat experiments in environments with differing physical conditions but also to develop a more complete understanding of the mechanisms by which seagrasses can compensate for losses of tissues to marine herbivores under varying environmental conditions.