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Tech

Coral reefs take a beating from El Niño

By Environmental News Network staff

Web posted Thursday, July 2, 1998
at 12:34 AM ET


NOAA's HotSpots satellite tracking system shows that the water temperature in the Indian Ocean is slowly cooling, but not before causing coral bleaching.
El Niño just keeps on comin'. The latest victims of the rampaging weather phenom are the coral reefs in the Indian Ocean, which have suffered considerable bleaching during the first six months of 1998.

Scientists with the National Oceanic and Atmospheric Administration have been measuring sea surface temperatures with satellites, and say the migration of extremely high ocean temperatures from south to north throughout the Indian Ocean has caused the bleaching and raised concerns for the reefs' fragile ecosystems.

In 1987 El Niño caused similarly high temperatures in the Indian Ocean; however, in 1988 the water cooled enough that the reefs in the Indian Ocean north of the equator were spared heavy bleaching.

In 1998, this has not been the case. Bleaching has been reported in the Indian Ocean reefs of Seychelles; Kenya; Reunion; Mauritius; Somalia; Madagascar; Maldives; Indonesia; Sri Lanka; Gulf of Thailand [Siam]; Andaman Islands; Malaysia; Oman; India and Cambodia.

Bleaching doesn't necessarily kill coral, but it's certainly not good for it. Coral reefs -- the "rainforests of the oceans" -- form the structural and ecological foundation of the reef system. Every coral species, as well as numerous other reef inhabitants, maintains a symbiotic relationship with a microscopic organism (algae) called zooxanthallae.

The zooxanthallae provide their hosts with oxygen and some organic compounds they produce through photosynthesis. When stressed, many reef inhabitants have been observed to expel their zooxanthallae en masse -- think of it as a giant exhaling. The polyps of the coral are left without any pigmentation and appear nearly transparent on the animal's white skeleton. This is what scientists mean when they talk about coral bleaching.

Corals can recover from bleaching unless high ocean temperatures persist for too long a period or become too warm to permit recovery. The coral's ability to feed itself in the absence of zooxanthallae seems to be a key to survival, and reestablishing the symbiotic relationship with zooxanthallae could take from two months to a year. When the level of environmental stress is high and sustained however, death of the coral may result. This will go on to effect the rest of the ecosystem in a highly negative way.

The bleaching events reported prior to the 1980s were generally attributed to localized phenomena such as major storm events, severe tidal exposures, sedimentation, rapid salinity changes, pollution, or thermal shock. The events since 1980 have not been so easily explained. Numerous laboratory studies have shown a direct relationship between bleaching and water temperature stress.

NOAA's Multi-Channel Sea Surface Temperature satellite imagery, 1982 to present, is allowing investigation of the phenomena on a global scale.

In the meantime, the waters in the Indian Ocean appear to be cooling and the area covered by "HotSpots" is now only in the northernmost fringes of the Indian Ocean. However, during June the Philippines and the Florida Keys regions have been seeing sea surface temperatures high enough that bleaching has been reported and biologists are concerned for reefs there.

Copyright 1998, Environmental News Network, All Rights Reserved


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