The Heat Is Online

Mass Coral Die-Off Due to Record Surface Temperatures

ONCE MORE INTO THE BLEACH

The highest sea temperatures ever recorded in the Caribbean have caused the first mass die-off of coral in Belize in 3,000 years, and scientists suspect that global warming and the El Nino weather phenomenon are to blame. The high temperatures in 1998 lasted for several months and caused much of the Belizean coral reef to bleach and die, according to a study by American scientists that has been published in the latest issue of the journal Nature. "There is growing concern that global climate change is degrading coral reef ecosystems, with coral mortality increasing as a result of bleaching and emergent diseases. Our results from Belize appear to justify this concern," said researcher Richard Aronson. Serious coral bleaching has been previously reported in the Pacific and Indian oceans. http://www.gristmagazine.com/grist/heatbeat/thisjustin042000.stm

Ecosystems: Coral bleach-out in Belize

Nature May4, 2000

RICHARD B. ARONSON*, WILLIAM F. PRECHT†, IAN G. MACINTYRE‡ & THADDEUS J. T. MURDOCH* * Dauphin Island Sea Lab, 101 Bienville Boulevard, Dauphin Island, Alabama 36528, USA
† PBS& J, 2001 Northwest 107th Avenue, Miami, Florida 33172, USA
‡ Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington DC 20560, USA

The highest sea surface temperatures ever recorded, related both to the 1997–98 El Niño/Southern Oscillation and to global warming1, caused severe bleaching of corals worldwide in 1998 (ref. 2). This thermal anomaly induced mass mortality of scleractinian corals on lagoonal reefs in Belize, the first time that a coral population in the Caribbean has collapsed completely from bleaching. Cores extracted from the Belizean reefs showed that these events were unprecedented over at least the past 3,000 years.

Coral bleaching occurs when coral colonies under physiological stress expel their symbiotic algae (zooxanthellae). Usually a response to high temperature and/or high incident solar radiation, bleaching has become more frequent on coral reefs worldwide over the past two decades3, 4. Bleaching-related mass coral mortalities, involving population collapses that can lead to local extirpation, have occurred several times recently in the Indo-Pacific5, 6. In contrast, bleaching in the Caribbean has until now been followed by substantial recovery of the affected coral populations7.

There are no records of mass bleaching along the Belizean barrier reef before an episode in 1995, from which most coral colonies recovered8. In 1998, sea temperatures in the central sector of the barrier reef, which rarely exceed 29 °C, were greater than 30 °C (maximum, 31.5 °C) at depths of 2 and 10 m from 10 August to 14 October (K. H. Koltes, personal communication). Storm waves and rain associated with hurricane Mitch (27–31 October) reduced temperatures by 1 °C or less, but temperatures did not drop below 29 °C until 6 November. Mass bleaching was evident in both fore-reef and lagoonal environments during the summer and autumn (T. Bright, personal communication). Some coral colonies on the fore reef experienced partial mortality, but most colonies recovered their coloration in the months following hurricane Mitch9. In contrast, levels of coral mortality were catastrophic in the lagoon.

The lettuce coral Agaricia tenuifolia was the most abundant coral on reefs of the central Belizean lagoon in 1998 (Fig. 1). Surveys at two sites on 22–23 October showed that virtually all living coral colonies were bleached white. Some of the bleached Ag. tenuifolia had already died by late October (Fig. 1b); the dead skeletons were standing in their growth position, indicative of recent mortality.

Complete bleaching was also evident in almost all large, plate- and head-forming species down to the lagoon floor at 21 m. Surveys in 1999 and 2000 revealed that Ag. tenuifolia had undergone almost total mortality at all depths. Heavy mortality occurred in the other species as well (Fig. 1a). Similar patterns were seen throughout a 375-km 2 area of the central lagoon. High water turbidity and the broad depth range of these effects suggest that temperature, rather than solar radiation, was primarily responsible for the bleaching and subsequent coral mortality.

Ag. tenuifolia had been the dominant space-occupier on lagoonal reefs since the late 1980s. At that time, the previous dominant, Acropora cervicornis (staghorn coral), was almost eliminated by white-band disease (a presumed bacterial infection unrelated to bleaching), permitting Ag. tenuifolia to increase opportunistically10. To determine whether similar events occurred in the past, we extracted 12 cores by hand from nine reefs in the central lagoon.

These push-cores penetrated several metres into the uncemented reef frameworks. Radiocarbon analysis yielded a maximum uncorrected 14C date of more than 3,000 yr. The cores showed continuous domination of lagoonal assemblages by Ac. cervicornis, with no widespread Acropora mortality, and no replacement by and subsequent disappearance of Ag. tenuifolia , in the three millennia before the late 1980s. The loss of Ac. cervicornis to disease, its replacement by Ag. tenuifolia, and the loss of Agaricia to bleaching a decade later were novel events on a timescale of millennia, and they occurred following an extended period of stable sea level (a sea-level rise of about 1 m in the past 3,000 yr)10, 11.

There is growing concern that global climate change is degrading coral reef ecosystems12, 13, with coral mortality increasing as a result of bleaching and emergent diseases4, 12, 14: our results from Belize appear to justify this concern.

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