Simulated green turtle grazing affects structure and productivity of seagrass pastures

The seagrass Thalassia testudinum evolved under much greater grazing pressure than exists today because overexploitation by humans caused the severe decline of green turtles Chelonia mydas, the major seagrass herbivore in the Caribbean. Understanding the effects of grazing on the structure and function of seagrass ecosystems is critical to evaluating how ecosystem processes have changed since the major herbivore was essentially removed from modern Caribbean seagrass systems. We evaluated effects of grazing on the structure and productivity of T. testudinum pastures by simulating green turtle grazing in 3 x 3 m plots for 16 mo in the central Bahamas and comparing these clipped plots to adjacent control (unclipped) plots. Simulated grazing affected the physical structure of T. testudinum plots, resulting in a system with reduced structural complexity. Simulated grazing resulted in compensatory growth in T. testudinum. Clipped plots maintained levels of growth comparable to unclipped plots over the 1.6 mo clipping trial, and specific mass growth was significantly elevated in clipped plots, even without simulating the nutrient return from. green turtle feces and urine. The green turtle grazing pattern of re-cropping T. testudinum plots is sustainable for long periods-at least 16 mo. We estimated a range of carrying capacities of T. testudinum for green turtles of 1567 to 3748 kg turtle ha(-1). Our study provides a foundation for reconstruction models and restoration plans for Caribbean seagrass ecosystems.