Decadal Monitoring in Bermuda Shows a Widespread Loss of Seagrasses Attributable to Overgrazing by the Green Sea Turtle Chelonia mydas

Abstract

The condition of seagrass habitat and the marine environment on the Bermuda Platform, a mid-oceanic shallow water habitat in the northwest Atlantic, has been monitored since 2006. The overall oceanic climate of the Platform is subtropical; the Platform supports communities of tropical marine seagrasses, including Thalassia testudinum, Syringodium filiforme, Halodule sp., and Halophila decipiens. At the beginning of the study, the general condition of seagrass beds at 17 permanent offshore and nearshore sites indicated that 14 were healthy, complex, and thriving communities, and three represented offshore beds, which had declined precipitously prior to the initiation of study. Over the period of the study, seagrass beds declined at all 17 sites; three beds disappeared, and there was no recovery at the sites known to have declined prior to 2006. Over the same period, there was no apparent negative change in the water quality overlying the seagrass beds. Assessments of elemental content, stable isotopic composition, and leaf morphology indicated that grazing by the green turtle (Chelonia mydas) is driving the decline of the seagrasses of Bermuda. Given the feeding behavior of these turtles on the Bermuda Platform, human intervention may be required to mitigate the decline of seagrass in Bermuda.