Green sea turtles (Chelonia mydas) consume seagrass for a large part of their diet. They exhibit a foraging strategy in which they repeatedly crop seagrass blades to clear away fibrous old growth and only consume the nitrogen-rich young growth, creating distinct grazed patches in seagrass meadows. Although the effects of grazing on seagrass dynamics have been investigated, the effects of grazing on whole primary producer communities have been less well-studied. It is critical to understand the potential ecological effects of increasing green turtle abundance to better inform conservation and management strategies for green turtle populations and seagrass ecosystems in the Caribbean. The aim of this study was to assess the effects of green turtle grazing on the primary producer community structure and composition of seagrass meadows.
We surveyed nine seagrass meadows around Little Cayman, Caymans Islands. Each meadow consisted of an area grazed by green turtles and an adjacent ungrazed area. Macroalgae and seagrass species were surveyed along a transect running through each meadow. We measured the density of each species as well as blade morphometry of the dominant seagrass species, Thalassia testudinum.
The effects of grazing were spatially variable around Little Cayman. We found that primary producer species diversity (Simpson’s Index) declined in grazed areas (p=0.034) when all surveyed meadows were pooled together. The lower species diversity in grazed areas was a result of decreased macroalgae diversity, rather than a reduction in seagrass diversity. A decline in the number of macroalgae species present following grazing could be due to a loss of protective services provided by an intact seagrass canopy, such as buffering against wave turbulence. Grazed areas also exhibited decreased seagrass shoot density for all species present (Thalassia testudinum, p=0.029; Syringodium filiforme, p=0.001; Halodule wrightii, p=0.018) as well as shorter (p<0.01) and narrower (p<0.01) T. testudinum blades compared to ungrazed areas. In light of recovering green turtle populations and declining global seagrass area, it is important to evaluate how green turtles affect seagrass meadows under natural grazing regimes and create a better understanding of how community composition and ecosystem function may be affected in the future.