Effects of green turtle grazing on seagrass and macroalgae diversity vary spatially among seagrass meadows

Seagrass meadows are often comprised of diverse assemblages of seagrasses and algae. Green turtles (Chelonia mydas) are a prominent megaherbivore in seagrass meadows-capable of consuming large amounts of aboveground seagrass biomass and driving shifts in seagrass species dominance. Previous green turtle grazing studies have focused on diversity and dynamics of seagrasses, but there has been little focus thus far on diversity of other primary producers within meadow communities. We investigated the effects of grazing on seagrasses and benthic macroalgae composition by surveying a series of seagrass meadows across two separate bays in Little Cayman, Cayman Islands, in which green turtles had established foraging areas. Effects of grazing on diversity were spatially variable among meadows. Diversity of the seagrass and macroalgae community (Simpson's Index) was not significantly different between areas grazed by turtles compared to ungrazed areas in one surveyed bay but was significantly lower among grazed areas in the other bay. Lower diversity was the result of lower densities of all seagrass and macroalgae species other than Thalassia testudinum among grazed areas in one bay. Diversity was also positively correlated with densities of total seagrass and total macroalgae across all meadows. Density of the dominant seagrass, T. testudinum, did not differ between the two bays or between grazed and ungrazed areas. As green turtle abundance increases and more seagrass meadows return to a naturally grazed state, it is important to understand the effects grazing has in these ecosystems and how effects may vary among locations.