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 I ndian Ocean, in biogeographical terms. They are remote and largely uninhabi ted, and its reefs have almost completely escaped most forms of direct huma n impact. Despite this, there has been a marked decline in their coral cove r over the last 20 years. In particular, live coral cover fell markedly fol lowing the warm sea-water episode of 1998, such that on seaward reefs of al l 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 substra te is now covered by dead coral, and another 40% by unidentifiable dead cor al 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 larg e quantities of mobile, dead coral fragments which may inhibit new recruitm ent and growth. Weather data have been recorded in Chagos since 1973. Stati stically significant trends include a 1 degrees C rise in mean air temperat ure over 25 years, and a 2 degrees C rise in the warmest 95 percentile temp erature. 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 anal ysis of temperature data shows several cycles of 2 years or longer, which w hen 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 war ming 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.