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IT鈥橲 not just warmer water that鈥檚 making life difficult for coral reefs. The
oceans are getting murkier, and that鈥檚 stunting reef growth.

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 鈥渃ompensation point鈥濃�攚here 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. 鈥淭hey are kind of caught between
the jaws of a vice,鈥� 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.

鈥淭he 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,鈥� says Yentsch. 鈥淚 think it鈥檚 had a major
别蹿蹿别肠迟.鈥�

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. 鈥淚t is safe to say that a significant proportion of the world鈥檚
reefs are not located in coastal areas affected by sedimentation,鈥� he says. But
he accepts that the murky waters might be a problem for a few coral reefs in
specific spots around the globe.