Biennium horribile: very high mortality in the reef coral Acropora millepora on the Great Barrier Reef in 2009 and 2010

Coral cover has declined markedly in the recent past in many regions of the world, including the Great Barrier Reef (GBR), Australia. The major causes of this decline are generally considered to be mortality associated with large-scale severe disturbances (i.e. catastrophic mortality), such as Acanthaster planci outbreaks, cyclones and bleaching. However, background rates of mortality (i.e. not associated with catastrophic disturbance), are rarely quantified, but without these it is difficult to assess the relative importance of these 2 types of mortality (catastrophic and background). We quantified spatial and temporal variation in catastrophic and background whole-colony mortality of the common reef coral Acropora millepora over 24 mo at 2 sites in 3 regions separated by 700 km along the GBR. The study period included 2 cyclones and a flood. Overall mortality rates were exceptionally high. Of 180 colonies tagged in April 2009, only 36 (20%) were alive in April 2011, and 68% of this mortality occurred in intervals following the 3 large disturbances. Background mortality rates were also high in the Palm Islands, where they approached 40% yr(-1) compared to <5% in the Whitsunday and Keppel Islands. These results support the hypothesis that catastrophic mortality has been the major cause of coral loss in recent years on the GBR and also suggest that background rates of mortality are increasing at some locations. Projected increases in the agents of catastrophic mortality, such as cyclones and bleaching, as a result of global warming are likely to threaten the persistence of many coral species.