MEPS 419:223-232 (2010)  -  DOI: https://doi.org/10.3354/meps08853

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