Simulated green turtle grazing affects benthic infauna abundance and community composition but not diversity in a Thalassia testudinum seagrass meadow

Seagrass meadows host diverse invertebrate faunal communities. Infaunal organisms residing in the sediments of meadows play important roles in the functioning of these ecosystems, such as the breakdown of organic matter. Disturbance to the benthic environment through grazing by megaherbivores, such as dugongs (Dugong dugon), can reduce infauna abundance within localized areas in a meadow. However, it is not known how removal of the aboveground canopy of a meadow without severe physical disturbance to the benthic habitat, such as through grazing by green turtles (Chelonia mydas), affects seagrass meadow infaunal communities. Increasing green turtle abundance will likely lead to greater areas of grazed seagrass with implications for the invertebrate infaunal communities within meadows. We experimentally simulated green turtle grazing for 16 months in a Thalassia testudinum seagrass meadow in The Bahamas to test effects of grazing on meadow infaunal communities. Total abundance of the infaunal community was reduced by 59% within six months of simulated grazing and thereafter remained lower throughout the experiment. Six out of eleven individual infaunal groups present (e.g. nematodes) also decreased in abundance following simulated grazing, but temporal abundance dynamics varied among groups. Simulated grazing had no effect on Simpson's Diversity Index of the infaunal community inhabiting the meadow at any point during the 16-month experiment. Though diversity was not affected, relative abundance of individual groups varied over time, and simulated grazing led to a significant change in infaunal community composition. These results demonstrate how green turtle grazing may affect the infaunal communities of shallow seagrass meadows with potential implications for the ecosystem services provided by these important habitats.