Improved predictions of coral bleaching using seasonal baselines and higher spatial resolution

Coral bleaching spread across the southern Great Barrier Reef in January 2006, after sea temperatures reached climatological summer maxima 2 months before normal. Current satellite-derived warning systems were unable to detect severe bleaching conditions in the region because of their use of a constant thermal threshold (summer maximum monthly mean) and low spatial resolution (50 km). Here it is shown that such problems can be ameliorated if the thermal threshold is adjusted for seasonal variation and a 4-km spatial resolution is used. We develop a seasonally and spatially improved thermal threshold for coral bleaching on the basis of a weekly climatology of sea surface temperatures extending from austral spring to late summer, and apply the method to two case-study sites. At both sites, and in particular at the nearshore site that was undetected by the 50-km satellite product, the seasonally adjusted thermal threshold produced a greatly improved consistency between accumulated heating and bleaching severity. The application of thermal stress algorithms that reflect the longterm mean pattern in seasonal variation allows coral bleaching to be forecast with higher precision. Coral bleaching occurs when the mutualistic relationship between the coral host and symbiotic dinoflagellates is destabilized, and symbionts are lost from the coral tissues. This has occurred over vast areas of the world’s oceans in response to periods of warmer-than-normal sea temperatures (Hoegh-Guldberg 1999). Under mild conditions, corals will recover their symbionts once the stress has been removed. If the warm conditions are severe and extended, however, corals may die (Anthony et al. 2007). Under projected global warming scenarios (Meehl et al. 2007), coral reef ecosystems are threatened on a worldwide scale (Hughes et al. 2003; Hoegh-Guldberg 2004). All species of reef-building coral have temperature thresholds above which bleaching occurs. Thermal thresholds vary geographically, reflecting adaptation by corals and their symbionts to local water temperatures (over hundreds if not thousands of years). These thresholds underpin the highly successful coral bleaching prediction program at the National Oceanic and Atmospheric Administration (NOAA), which assumes a constant bleaching threshold 1uC above the long-term mean temperature of the warmest summer month, or ‘‘maximum monthly mean’’ (Strong et al. 2006). In most cases, exceeding this threshold for prolonged periods leads to mass coral bleaching and potential mortality. Most organisms have the ability to acclimate to changes in temperature (Wilmer et al. 2000). In the case of reefbuilding corals, thermal thresholds have been shown to vary seasonally, decreasing in the winter by 1–2uC relative 1 Corresponding author ([email protected]).