Coral decline and weather patterns over 20 years in the Chagos Archipelago, central Indian Ocean

The atolls of the Chagos Archipelago occupy a key location in the central Indian Ocean, in biogeographical terms. They are remote and largely uninhabited, and its reefs have almost completely escaped most forms of direct human impact. Despite this, there has been a marked decline in their coral cover over the last 20 years. In particular, live coral cover fell markedly following the warm sea-water episode of 1998, such that on seaward reefs of all six Chagos atolls, only 12% of the substrate is now living coral compared with 50-75% before the warming event. On seaward reefs, 40% of the substrate is now covered by dead coral, and another 40% by unidentifiable dead coral and bare substrate. Lagoonal reefs fared better than seaward reefs, but still lost half of their corals over the last year. All reefs now have large quantities of mobile, dead coral fragments which may inhibit new recruitment and growth. Weather data have been recorded in Chagos since 1973. Statistically significant trends include a 1 degrees C rise in mean air temperature over 25 years, and a 2 degrees C rise in the warmest 95 percentile temperature. At the same time there has been a fall in mean annual pressure, a reduction in cloud cover, and winds have become more variable. Fourier analysis of temperature data shows several cycles of 2 years or longer, which when combined indicate a greater climate variability today compared with 25 years ago. Periods of higher temperatures coincide with several previous El Nino events and other climatic records of warming. Although the latest warming of 1998 is responsible for the recent mass coral mortality, it is seen to be a severe continuation of a longer trend, which if continued leads to a poor prognosis for rapid recovery.
Sheppard Charles .
Scleractnia (hard corals)