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Marine Ecology Progress Series

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MEPS 190:211-222 (1999)  -  doi:10.3354/meps190211

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

C. D. Rose1,*, W. C. Sharp2, W. J. Kenworthy3, J. H. Hunt2, W. G. Lyons4, E. J. Prager5, J. F. Valentine6, M. O. Hall4, P. E. Whitfield3, J. W. Fourqurean1

1Department of Biology and the Southeast Environmental Research Center, Florida International University, University Park, Miami, Florida 33199, USA
2Florida Department of Environmental Protection, Florida Marine Research Institute, 2796 Overseas Hwy, Suite 119, Marathon, Florida 33050, USA
3National Marine Fisheries Service, Beaufort Laboratory, 101 Pivers Island, Beaufort, North Carolina 28516, USA
4Florida Department of Environmental Protection, Florida Marine Research Institute, 100 8th Ave. SE, St. Petersburg, Florida 33701, USA
5U.S. Geological Survey, MS953 National Center, Reston, Virginia 20192, USA
6Dauphin Island Sea Lab and Department of Marine Sciences, University of South Alabama, 101 Bienville Blvd, PO Box 369-370, Mobile, Alabama 36528, USA

ABSTRACT: 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 µm) 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.

KEY WORDS: Seagrass · Sea urchin · Herbivory · Syringodium filiforme · Lytechinus variegatus · Florida Bay · Disturbance

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