Large-scale decline in offshore seagrass meadows in Bermuda

Abstract

Bermuda is an isolated 5560 ha chain of limestone islands on a 150 000 ha seamount located near 32° N, 64° W. Meadows of tropical and subtropical seagrasses, dominated by Thalassia testudinum and Syringodium filiforme, are found from inshore bays out to the inner edge of the rim reef that encircles the platform. Fine-scale computerized mapping and subsequent broad-scaled field assessment of seagrass meadows in Bermuda show that (1) meadows representing nearly one-quar- ter of the territory's total seagrass area in 1997 had declined by 2004, (2) net loss of seagrass mead- ows occurred at rim reef and lagoonal locations that are far-removed from anthropogenic distur- bances, (3) the decline appears to have been in progress as early as 1996, and (4) both T. testudinum and S. filiforme meadows declined. Nearly 2100 ha of meadows were visible in a 1997 geo- referenced mosaic of aerial photographs of the Bermuda platform. In 2004, 22 meadows that repre- sented about 475 of the 900 ha of offshore seagrass identified in 1997 and earlier were absent or in obvious decline. The size and location of inshore and nearshore meadows, which are exposed to intense anthropogenic stresses and physical damage, varied over the same 7 yr but their total area has either remained unchanged or even increased. Processes contributing to the decline in offshore meadows have yet to be determined, but may include herbivory by juvenile green turtles and parrot- fishes and below-normal productivity owing to a winter cold-water event correlated with a change in the North Atlantic Oscillation (NAO) in 1996. The potential consequences to Bermuda of the loss of nearly 500 ha of critical marine habitat are of extreme concern.