THE ROLE OF SEA-URCHIN GRAZING IN REGULATING SUBTROPICAL SEAGRASS MEADOWS - EVIDENCE FROM FIELD MANIPULATIONS IN THE NORTHERN GULF OF MEXICO

Sea urchin herbivory has been demonstrated to limit kelp forest distributions but its role in influencing distribution of seagrass habitats is inadequately investigated. During two annual cycles, we manipulated sea urchins [Lytechinus variegatus (Lamarck)] at densities ranging from 0 to 80 individuals/m2, and documented their seasonal impact on turtlegrass Thalassia testudinum Banks ex Konig meadows in St. Joseph Bay, Florida. Results from these experiments indicate that the lowest densities of sea urchins required to overgraze (i.e., completely defoliate) turtlegrass occur during winter (approximately 20 individuals/m2) while higher densities (approximately 40 individuals/m2) are required for overgrazing during summer and fall. Field censuses frequently encountered sea urchin densities sufficient for overgrazing. We conclude that sea urchin herbivory is important in regulating subtropical seagrass meadow biomass and size particularly where sea urchin densities exceed 20 individuals/m2.