Green Pages

Coral Bleaching

Bad news and (a little) good news for Turks & Caicos reefs.

By Brian Riggs, Curator, National Environmental Centre

Photos By Tatum Fisher, Science Officer, DECR

Many observant scuba divers have noticed and reported to the Department of Environment & Coastal Resources (DECR) an apparent increase in the amount of coral bleaching on Turks & Caicos reefs. These reports reached their peak last October and correlate well with information from throughout the Caribbean and the rest of the world in late 2005.

monararewWhat is bleaching?

Coral bleaching is caused by a variety of stresses, most notably increased sea surface temperatures. This causes the corals to expel the symbiotic algaes that live within their tissues. These tiny algae provide coral polyps with much of their food and also determine the greenish and brownish colors that we associate with living reefs. Losing their algae leaves coral tissues devoid of color, thus appearing to be bleached.

Prolonged coral bleaching (over a month) can lead to the death of the polyps and the subsequent loss of coral reef habitats for a range of marine life. As the water cools, though, corals can re-acquire their symbiotic algae and recover from the bleaching as long as they are not further stressed by other factors like pollution or disease. High temperatures for a longer period will kill the corals outright. Recovery may take decades, but only if conditions remain favorable.

Pan Caribbean event

This is the second major worldwide coral bleaching event in the past decade. The high ocean temperatures that accompanied the 1997-98 El Ni–o period bleached coral reefs in more than 50 tropical countries. The 2005 ocean warming event affected even more. Last Fall’s ocean temperatures throughout the Caribbean were the highest recorded since satellite monitoring began over 20 years ago. And the temperatures stayed higher for longer than any previous temperature spike. As much as 90% of some corals were bleached in parts of the Lesser Antilles, most notably the Virgin Islands. And as temperatures remained high for weeks, the weakened polyps were attacked by diseases, leading to massive die-offs. As of March 2006, 1/3 of corals at monitoring stations in the U.S. Virgin Islands had died.

Almost yearly bleaching events have occurred for the past several decades, but have generally been limited to certain coral species in shallow water. In 2005, bleaching struck far more of the Caribbean region and affected corals of many different species in all depths. But the Caribbean is much better off than areas of the Pacific and Indian Oceans where mortality rates have been in the 90% range for the past several years.

In the Turks & Caicos Islands, we missed the worst of the 2005 bleaching event, but still DECR on-site monitoring has reported that as much as 70% of our reef areas have been affected with partial bleaching of several important reef building species. No major die-offs have been reported or found and it has been noted that our reefs have generally been able to recover over the cooler winter months.

Australia, in the southern hemisphere, where the hottest months are January through April, has recently reported massive bleaching comparable to last year’s.

Global warming and other culprits

Most marine scientists agree that the main agent at work in these ever-more-serious bleaching events is global warming and that the evidence is overwhelming. “This is probably a harbinger of things to come,” says John Rollino, chief scientist for the Bahamian Reef Survey Project. “Coral bleaching is more a symptom of disease – widespread global environmental degradation . . .”

bleaching-surveyBut global warming is not the only culprit in the decline of corals worldwide. Researchers at the University of Florida, after surveying archeological and historical records that span the past 500 years, have postulated that reef degradation actually began with human exploitation of tropical coastal areas. Overfishing, especially of the large and easy to catch herbivores like green turtles and parrotfish, and predators like sharks and groupers, seriously depleted fish stocks long before the first European colonists arrived. Grand Turk’s early pre-Columbian site, GT-2, is littered with the bones of large turtles and predator fish. Other sites in the TCI, Haiti and the Bahamas show a decline in the sizes of faunal remains and number of species over their periods of occupation.

Worldwide, these declines have occurred over differing lengths of time and are more advanced in some places than others. In regions where the process is most advanced, such as Jamaica, corals are dead or dying, the remaining fish are small, few other organisms like shellfish or urchins exist and reef structures are coated with algae. What had been a vibrant and exuberant reef ecosystem is gone, probably forever.

There may be other, less obvious, causes for concern, as well. While we recognize that carbon dioxide, a greenhouse gas found in auto and factory emissions, is probably a factor in the process of global warming, it is also causing our ocean waters to become more acidic.

If you remember your high school chemistry lessons, carbon dioxide mixed with water forms a weak acid called carbonic acid. And acids, as we know, dissolve base (or alkaline) substances. Ocean waters are slightly alkaline, about 8.2 on the pH scale, due to the vast amounts of minerals dissolved in them. Ocean creatures, especially crustaceans (like lobsters, shrimps and crabs), mollusks (like conch and all other shellfish) and corals (both hard and soft), extract these minerals from the seawater to build their skeletons and their homes.

But the huge amounts of carbon dioxide being released into the atmosphere, and subsequently being dissolved in the sea, in recent decades is making seawater less and less alkaline over time. When seawater falls to a pH level below about 7.2, sea creatures will not be able to build up or repair their shells. Some hardier animals might be able to maintain their shells but would be unable to reproduce. Not only would this affect shellfish populations, but it would also be bad news for the many larger animals that feed on these shelled creatures. Populations of many carnivorous creatures like salmon, mackerel, cod, most rays and even baleen whales feed on shellfish and small crustaceans.

Added to these newly recognized concerns is, of course, the ongoing problem of pollution. Many localities have been able to make remarkable progress in diminishing and eradicating pollution sources, but water-borne pollution, whether from agricultural runoff, sewage disposal or shipping mishaps, is still a threat to reefs in areas with large coastal populations. Not only do these influxes of toxins and organic chemicals kill reef denizens outright, they also weaken the few that can survive. Already stressed corals and reef animals will have an even harder task trying to survive the yearly bleaching events and predicted seawater acidification in the upcoming decades.

bleaching-staghorn-copyGood news

It may be a bit of a stretch, but there are a few small rays of hope for coral reefs among all the disappointing bulletins. Researchers from the U.S. Environmental Protection Agency and the Nature Conservancy have found a few mitigating factors that help some corals to survive even the most serious bleaching events.

Two geographical features that apparently help to mitigate reef damage from the higher sea surface temperatures that cause bleaching are at work right here in the Turks & Caicos Islands. For instance, shallow water corals that are located near areas of cold water upwellings are more likely to survive when water temperatures on the reef reach critically high temperatures. Colder water from the depths, like those that surround our banks, can regularly and rapidly replace the sun- warmed surface waters, lowering temperatures quickly.

In addition, the researchers found that corals that are regularly exposed at low tides may also be more tolerant of heat stress than those that are constantly submerged in shallow water. Areas in the TCI like the shallow reefs around Gibbs and Round Cay and the tidally exposed reef flats of the Southwest Reef at West Caicos suffered minimal bleaching and have, by early spring, fully recovered.

And of course, higher sea surface temperatures worldwide may actually be a boon to coral reefs that are growing in chillier waters like Bermuda and northern Florida, allowing reefs there to expand and possibly colonize new areas.

Working for the future

Starting this summer, the Department of Environment & Coastal Resources will begin formulating a management and mitigation plan to come to the aid of our own important coral reefs. Utilizing the coral monitoring data from the past two decades and research from other organizations, the DECR will develop a strategy for coral reef restoration as a national priority, examining those technologies that allow corals to grow faster and to increase their survival rate in high temperatures (artificial reefs, transplantation and the new electrical/mineral accretion process). Along with our ongoing work in pollution abatement and sustainable fishing techniques, our glorious reefs may have a fighting chance after all.



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Liliane
Mar 13, 2012 21:22

Just went snorkeling off the coast of Grand Turk and Gibbs Cay early March 2012…terribly bleached coral reef!

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