The impacts of climate change are already being felt, especially in the most fragile and marginal ecosystems around the world. In particular, coral reefs, which are extremely sensitive to changes in the temperature and acidity of the water in which they form, are being destabilized by a changing ocean environment. Several scientific studies have demonstrated that many of the world's coral reefs are precariously close to total failure. A new study, while confirming this conclusion, does offer some hope.
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Corals are marine animals that live in symbiosis with single-celled algae called zooxanthellae. Inside each coral polyp (a tiny jellyfish-like sac) lives one of these photosynthetic algae. The coral provide shelter for the algae, which in turn provide food, oxygen, and nutrients for the coral. Zooxanthellae also give corals their bright coloration.
Since zooxanthellae require light for photosynthesis, corals form and thrive in clear waters near the surface, generally in depths less than 150 feet. Corals also require warm water temperatures – between 68°F and 82°F – and thus generally live near continental land masses, where warm water flows and where wave action brings in nutrients and oxygen.
Coral reefs are often referred to as "rainforests of the sea." Home to more than 25 percent of all marine species, reefs are highly biologically diverse. They are highly productive, providing a habitat and breeding ground for commercially important species of fish, shrimp, and lobsters. Reefs also act as natural buffers, protecting vulnerable coastal areas from bearing the full brunt of storms.
Corals are also often thought of as the ocean's "canaries in a coal mine." Since reefs are highly sensitive to changes in water temperature, clarity, and acidity, they are early indicators of the impacts of climate change and human activities. They are vulnerable to coastal development, sediment runoff, water pollution, and many fishing practices.
Coral reefs currently face two major – and growing – threats:
Coral bleaching. Increased environmental stress often causes corals to expel their symbiotic zooxanthellae, which removes their color and results in "bleaching." Bleached corals are weaker and more prone to disease. The combination of increased sea surface temperatures and ocean acidification has led to mass bleachings, even with temperature increases as slight as 1°C; with increases of 2°C-3°C, the corals may die.
A new study, recently published in the journal Science, highlights the dire state of the world’s coral reefs. The results come from the Global Marine Species Assessment (GMSA), a joint effort of the International Union for the Conservation of Nature (IUCN) and Conservation International. Data on coral health are lacking: of the 40,000 species monitored, only 1,400 are marine. The report's 39 co-authors categorized hundreds of coral species using IUCN’s Red List Categories and Criteria, previously used to catalog the health of many terrestrial species.
The results are disturbing: nearly one-third of the 704 species studied are listed as "Critically Endangered," "Endangered," or "Vulnerable." Certain types of corals, like staghorn coral or elkhorn coral (see images below), are critically endangered. The high-biodiversity "Coral Triangle" between Malaysia, Indonesia, and the Philippines, is at greater risk, mainly due to the large number of people living near the coast.
Staghorn coral (left) and elkhorn coral (right) are critically endangered.
The global deterioration of coral reefs is happening rapidly: just ten years ago, only 13 species would have been placed in one of the three threatened categories, and most (671) would have been listed as "of least concern." However, a mass bleaching event in 1998 wiped out an estimated 16 percent of all reefs (see map below). And as sea surface temperatures and ocean acidities have continued to rise, the problem has been exacerbated.
Exposure for one month at temperatures 1-2°C higher than the mean averages at the warmest time of year is sufficient to cause corals to bleach. This map shows the distribution of sea surface temperature anomalies and coral bleaching events, during one of the largest El Niño events of this century, from late 1997 until mid-1998.
Source: EarthTrends
(Click image for larger version.)
Yet the news isn't all bad. The health of coral reefs differs significantly between geographic regions. The IUCN study shows that while only a quarter of Caribbean reefs are in good or excellent condition, nearly 70 percent of reefs in the Pacific are healthy. And there are indications that many Pacific reefs are thriving in warmer water. Other studies have also indicated that some coral may be able to adapt by exchanging their symbiotic algae for species that can survive in warmer, more acidic waters. While this may not be a long-term adaptation – especially as the climate continues to change – it could "buy time" for certain coral reefs. The only real hope for corals in the long term, however, is a quick and sustained reduction in greenhouse gas emissions.
Top photo from Flickr users Sam and Ian
Staghorn coral photo from Flick user fishgirl17
Elkhorn coral photo from Flick user August Rode
RELATED LINKS:
NOAA's Coral Reef Conservation Program
EARTHTRENDS DATA:
Map: Coral Bleaching Events and Sea Surface Temperature Anomaly Hot Spots, 1997 - 1998
