Widespread coral bleaching and El Niño

One effect of the shifts in ocean conditions produced by the El Niño/Southern Oscillation, possibly assisted by global warming, is the bleaching of corals and other animals on tropical coral reefs. Corals usually live near the upper limit of their temperature tolerance, and even a small increase in temperatures above their normal temperature limit can stress them and lead to the expulsion of their symbiotic algae, which give them their colors as well as providing nourishment. Corals can often recover from short bleaching events, but extended or repeated bleaching can be fatal.

During the major El Niño in 1997-98, bleaching occurred on coral reefs ranging from Kenya to French Polynesia and Baja California in the Indo-Pacific, and from the Florida Keys to the Yucatan coast in the Caribbean. For instance, in the Maldives, a massive bleaching event was reported in May 1998. More than 90% of animals with algal symbionts, including corals, giant clams, anemones and soft corals showed heavy bleaching to depths of 20 metres at 8 locations on North Mahe atoll. Swimming up the reef was described as like going up the snowcapped Alps. Conditions at the time in the Maldives included surface water temperatures of 32 degrees C extending down to 15 metres, very low wind with no surface mixing, and a low tidal range of less than half a metre.

Bleaching was also reported early in 1998 as starting in the southernmost part of the Australian Great Barrier Reef and moving northward, reaching New Caledonia. Corals in the Galapagos Islands bleached at temperatures of 29 degrees C, a degree and a half warmer than the critical temperature for bleaching at that site. Sea surface temperatures in the major El Niño events of the 1980s were not quite this warm.

In the Caribbean on the Belize barrier reef, the largest in the Northern Hemisphere, sea surface temperatures, which rarely exceed 29 degrees C, reached 31.5 degrees and were greater than 30 degrees for months in 1998. This killed the most abundant coral on the reefs, Agaricia tenuifolia, causing the first recorded complete collapse of a coral population from bleaching in the Caribbean, and damaging other species, as demonstrated by surveys in 1999 and 2000. Coral cores from the reef showed that no similar bleaching had happened for more than 3,000 years.

This widespread bleaching on coral reefs, and other aspects of the state of coral reefs, are reviewed every two years in the Status of Coral Reefs of the World . This information is being collected and updated by the Global Coral Reef Monitoring Network.

A new episode of bleaching was reported in the mid-Pacific from mid-February to late April 2000, associated with La Niña, the reverse of El Niño. Hot spots ranging from the Solomon Islands to Easter Island measured 30-31.5 degrees C caused bleaching in 50 to 90 percent of the corals. For instance, in Fiji, up to 90 percent of corals bleached down to a depth of 30 metres. All coral species were affected, although some shallow water populations showed selective recovery.

A near-real-time chart of unusually warm sea surface temperatures associated with coral bleaching is available from US NOAA at http://psbsgi1.nesdis.noaa.gov:8080/PSB/EPS/SST/climohot.html.

Source: UNEP Coral Reef Unit