This year’s extreme heat is putting the world’s coral reefs under such severe stress that scientists fear widespread die-offs, endangering not only the ocean’s richest ecosystems, but also fisheries that feed millions of people. From Thailand to Texas, corals are reacting to the heat stress by bleaching, or shedding their colour and going into survival mode.
Many have already died, and more are expected to do so in the coming months.
Computer forecasts of water temperature suggest that corals in the Caribbean may undergo drastic bleaching in the next few weeks. What is unfolding this year is only the second known global bleaching of coral reefs. Scientists are holding out hope that this year will not be as bad, overall, as 1998, the hottest year in the historical record, when an estimated 16 per cent of the world’s shallow-water reefs died. But in some places, including Thailand, the situation is looking worse than in 1998. Scientists say the trouble with the reefs is linked to climate change.
For years they have warned that corals, highly sensitive to excess heat, would serve as an early indicator of the ecological distress on the planet caused by the buildup of greenhouse gases. “I am significantly depressed by the whole situation,” said Clive Wilkinson, director of the Global Coral Reef Monitoring Network, an organisation in Australia that is tracking this year’s disaster. According to the National Oceanic and Atmospheric Administration, the first eight months of 2010 matched 1998 as the hottest January to August period on record. High ocean temperatures are taxing the organisms most sensitive to them, the shallow-water corals that create some of the world’s most vibrant and colourful seascapes.
Home to a quarter of marine species
Coral reefs occupy a tiny fraction of the ocean, but harbour perhaps a quarter of all marine species, including a profusion of fish. Often called the rain forests of the sea, they are the foundation not only of important fishing industries but also of tourist economies worth billions. Drastic die-offs of coral were seen for the first time in 1983 in the eastern Pacific and the Caribbean, during a large-scale weather event known as El Nino. During an El Nino, warm waters normally confined to the western Pacific flow to the east; 2010 is also an El Nino year. Serious regional bleaching has occurred intermittently since the 1983 disaster. It is clear that natural weather variability plays a role in overheating the reefs, but scientists say it cannot, by itself, explain what has become a recurring phenomenon.
“It is a lot easier for oceans to heat up above the corals’ thresholds for bleaching when climate change is warming the baseline temperatures,” said C Mark Eakin, who runs a programme called Coral Reef Watch for the National Oceanic and Atmospheric Administration. “If you get an event like El Nino or you just get a hot summer, it’s going to be on top of the warmest temperatures we’ve ever seen.”
Symbiosis
Coral reefs are made up of millions of tiny animals, called polyps, that form symbiotic relationships with algae. The polyps essentially act as farmers, supplying the algae with nutrients and a place to live. The algae in turn capture sunlight and carbon dioxide to make sugars that feed the coral polyps. The captive algae give reefs their brilliant colours. Many reef fish sport fantastical colours and patterns themselves, as though dressing to match their surroundings. Coral bleaching occurs when high heat and bright sunshine cause the metabolism of the algae to speed out of control, and they start creating toxins. The polyps essentially recoil. “The algae are spat out,” Wilkinson said.
The corals look white afterward, as though they have been bleached. If temperatures drop, the corals’ few remaining algae can reproduce and help the polyps recover. But corals are vulnerable to disease in their denuded condition, and if the heat stress continues, the corals starve to death. Even on dead reefs, new coral polyps will often take hold, though the overall ecology of the reef may be permanently altered. The worst case is that a reef dies and never recovers. In dozens of small island nations and in some coastal areas of Indonesia and the Philippines, people are heavily dependent on reef fish as a source of protein.
The death of corals is not immediately lethal to the fish, but if the coral polyps do not recover, scientists say the reef can eventually collapse, and the associated fishery will become far less productive. Research shows that is already happening in parts of the Caribbean, though people there are not as dependent on fishing as those living on Pacific islands. It will be months before this year’s toll is known for sure. But scientists tracking the fate of corals say they have already seen widespread bleaching in Southeast Asia and the western Pacific, with corals in Thailand, parts of Indonesia and some smaller island nations being hit especially hard earlier this year. Temperatures have since cooled in the western Pacific, and the immediate crisis has passed there, even as it accelerates in places like the Caribbean.
Serious bleaching has been seen recently in the Flower Garden Banks, a marine sanctuary off the Texas-Louisiana border. In Thailand, “there are some signs of recovery in places,” said James True, a biologist at Prince of Songkla University. But in other spots, he said, corals were hit so hard that it was not clear young polyps would be available from nearby areas to repopulate dead reefs.
Varying temperatures
The water temperatures in the Florida Keys are only slightly above normal this year, and the beloved reefs of that region have so far escaped serious harm. Parts of the northern Caribbean, including the US Virgin Islands, saw incipient bleaching this summer, but the tropical storms and hurricanes moving through the Atlantic have cooled the water there and may have saved some corals. Farther south, though, temperatures are still remarkably high, putting many Caribbean reefs at risk. Summer is only just beginning in the Southern Hemisphere, but water temperatures off Australia are also above normal, and some scientists are worried about the single most impressive reef on earth. The best hope now, Wilkinson said, is for mild tropical storms that would help to cool Australian waters.“If we get a poor monsoon season,” he said, “I think we’re in for a serious bleaching on the Great Barrier Reef.”