Sunlight is essential for coral reefs. Symbiotic algae called zooxanthellae living in the corals photosynthesise to produce carbohydrates and oxygen that the corals use to make reef-building calcium carbonate. If waters become less transparent this process is harmed, says Charles Yentsch of the Bigelow Laboratory for Ocean Sciences in West Boothbay Harbor, Maine. He and his colleagues found that some reefs in the Florida Keys are getting barely enough sunlight to sustain themselves. The reefs are functioning close to the "compensation point"-where the coral and the zooxanthellae use up all the products of photosynthesis just to survive, leaving little or nothing to build onto the reef. Yentsch says that coral reefs in the Bahamas are similarly endangered. As the water gets murkier the corals are also forced to move towards shallower water, where waves can damage them. "They are kind of caught between the jaws of a vice," says Yentsch.
The development of coastal areas and the erosion of beaches are partly to blame for the increasingly murky waters. Loss of coastal mangroves and seagrass beds from lagoon floors and reef flats adds to the problem, as they would normally trap sediment before it reaches the coral reefs. In addition, an increase of fertiliser run-off into the ocean, from sugar-cane farms in Jamaica, for instance, is encouraging algal blooms.
"The transparency has changed significantly in the past 10 or 20 years, so that the amount of light reaching the reef corals in some areas is really too low to sustain dynamic growth," says Yentsch. "I think it's had a major effect."
However, the two biggest causes of coral death are still overfishing and climate change, says Gregor Hodgson, director of the Reef Check Foundation in Los Angeles. "It is safe to say that a significant proportion of the world
Contact: Claire Bowles