MEPS 419:223-232 (2010)  -  doi:10.3354/meps08853

Effects of excluding sea turtle herbivores from a seagrass bed: Overgrazing may have led to loss of seagrass meadows in Bermuda

James W. Fourqurean1*, Sarah Manuel2, Kathryn A. Coates2, W. Judson Kenworthy3, Struan R. Smith4

1Marine Science Program, Department of Biology, and Southeast Environmental Research Center, Florida International University, North Miami, Florida 33181, USA
2Applied Ecology Section, Department of Conservation Services, Bermuda Ministry of Environment and Sports, Smiths, FL 04, Bermuda
3Center for Coastal Fisheries and Habitat Research, NCCOS, NOAA, Beaufort, North Carolina 28516, USA
4Bermuda Aquarium, Museum and Zoo, Department of Conservation Services, Bermuda Ministry of Environment and Sports, Smiths, FL 04, Bermuda

ABSTRACT: Protecting a Thalassia testudinum-dominated seagrass meadow from grazing by sea turtles for 1 yr caused an increase in the biomass of seagrasses and an increase in the structural complexity of the seagrass canopy, as the length and width of the seagrass blades increased in comparison to grazed plots. Plots from which turtles were excluded had higher rates of primary production on a per-shoot or areal basis, but the relative growth rate was not affected. The leaves of seagrasses protected from grazing had lower concentrations of nitrogen and phosphorus than grazed blades, but the storage of soluble carbohydrates in the rhizomes increased markedly in the protected plots, suggesting that reduced carbon fixation caused by the removal of photosynthetic leaves is the mechanism for seagrass decline in heavily grazed meadows, not nutrient limitation as has been suggested in the literature. The continued grazing of sea turtles in our plots did not lead to significant changes in seagrass shoot density or nutrient content over the 1 yr duration of our experiments. The decreased canopy cover and the shorter, thinner seagrass leaves induced by sea turtle grazing in our experimental plots suggest that the progressive narrowing and thinning of seagrasses observed before the collapse of 2 offshore seagrass beds in Bermuda during the 1990s may have been in response to repeated and intense grazing of those seagrass beds.


KEY WORDS: Caribbean Coastal Marine Productivity Program · CARICOMP · Thalassia testudinum · Nutrient content · Soluble carbohydrates · Productivity


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Cite this article as: Fourqurean JW, Manuel S, Coates KA, Kenworthy WJ, Smith SR (2010) Effects of excluding sea turtle herbivores from a seagrass bed: Overgrazing may have led to loss of seagrass meadows in Bermuda. Mar Ecol Prog Ser 419:223-232

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