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-quarter of the territory's total seagrass area in 1997 had declined by 2004, (2) net loss of seagrass meadows occurred at rim reef and lagoonal locations that are far-removed from anthropogenic disturbances, (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 georeferenced mosaic of aerial photographs of the Bermuda platform. In 2004, 22 meadows that represented 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 parrotfishes 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.

Effects of excluding sea turtle herbivores from a seagrass bed: Overgrazing may have led to loss of seagrass meadows in Bermuda

“…Seagrass beds have declined precipitously on the Bermuda Platform, especially in offshore sites distant from obvious human impact (Murdoch et al 2007) where turtles were abundant (BTP unpubl. data).…”

“…32.3°N, 64.8°W), has nearshore benthic marine communities dominated by coral reefs and tropical seagrass beds (Murdoch et al 2007). Bermuda has a juvenile green sea turtle population that preferentially feeds in seagrass beds (Godley et al 2004).…”

“…The soluble carbohydrates stored in rhizomes are an important photosynthate reservoir that supports growth and maintenance of other tissues during periods of low photosynthetic production (Dawes & Lawrence 1980, Pirc 1985. Indeed, the belowground biomass was depressed in grazed plots, a pattern observed in the offshore decline of seagrass beds in the 1990s (Murdoch et al 2007). Rhizome soluble carbohydrate reserves are critical for survival and regrowth during extended periods of light reduction or in response to defoliation (Dawes & Lawrence 1979, Drew 1983, Dawes & Guiry 1992).…”

“…In Bermuda, the distribution of tropical seagrass meadows is restricted in terms of area compared to similar coastal environments in more tropical locations. The rapid loss of the seagrass resources documented in the late 1990s (Murdoch et al 2007), whether caused by overgrazing by the sea turtle population or due to some other as yet unidentified driver, could have dire consequences for the juvenile turtle population of the island, since the nearest tropical seagrass beds are over 1000 km away.Acknowledgements. This work was funded by the Department of Conservation Services, Government of Bermuda and B. Dilke, a private benefactor.…”

“…data). While analyses of the trajectories of green sea turtle populations in Bermuda are incomplete and equivocal, anecdotal evidence suggests that the populations increased slightly between 1999 and 2004, but that the numbers were lower than 40 yr ago (Godley et al 2004).Seagrass beds have declined precipitously on the Bermuda Platform, especially in offshore sites distant from obvious human impact (Murdoch et al 2007) where turtles were abundant (BTP unpubl. data).…”