Simulated green turtle grazing: Effects on structure and productivity in seagrass (Thalassia testudinum) beds in the central Bahamas

Seagass beds are some of the most productive habitats in the world and play an important role in estuarine and shallow marine ecosystems. Seagrasses stabilize sediment, trap detritus, and provide surfaces for epibiont attachment. Many organisms find refuge and food in seagrass pastures. Green turtles ( Chelonia mydas) are the only herbivorous sea turtles and were once the primary grazers in Caribbean seagrass (Thalassia testudinum) beds. Green turtles can maintain grazing plots for a year or longer, in which they re-crop seagrass blades near the substrate. This pattern of grazing has been thought to cause a substantial decrease in plant growth and to stress the plants. Green turtle populations today are markedly reduced when compared to historical levels. Productivity and structure of seagrass habitats are undoubtedly different today than when they were grazed by millions of green turtles. For this study, green turtle grazing was simulated by clipping T. testudinum plots from July 1999 to November 2000 at Lee Stocking Island, Exumas, Bahamas. Seagrass structure, productivity, and nutrient composition were quantified throughout the clipping experiment. Clipped seagrass was sampled again in June and October 2001 to determine the response after cessation of clipping. Simulated grazing changed seagrass structure by decreasing blade length and width, detrital layer thickness, number of blades per shoot, and above-ground biomass, indicating that turtles alter the structure of their foraging grounds. Clipped seagrass blades had higher energy content and a higher proportion of organic matter, nitrogen, phosphorus, and fiber than unclipped seagrass. Instead of dramatically declining in productivity, clipped seagrass maintained levels of growth comparable to unclipped seagrass, indicating that green turtle grazing may be more sustainable than previously believed. In addition, productivity:biomass ratios were much higher in clipped seagrass. Seagrass beds returned to an unclipped condition relatively quickly, within 11 months after clipping ended. Realistic recovery goals for green turtle populations are needed. Incorporating grazing-induced changes in T. testudinum productivity and nutrient composition based on this study improves current estimates of seagrass carrying capacity for green turtles. It is estimated that Caribbean seagrass beds could support a maximum of between 211 and 506 million 50-kg green turtles.