J. Bruno et al./University of North Carolina

Losing some battles. Some, but not all, of the world's coral are being overtaken by seaweed (inset).

Coral Hold Their Own Against Seaweed

Since the 1980s, marine biologists have been bracing for "total seaweed domination." They believed that the algae were smothering coral, and that they would devastate reefs around the globe. Although reef experts remain highly concerned about the health of the world's corals, a new survey suggests that seaweed is considerably less of a threat than expected. The study "completely overturns one of our most cherished concepts about coral reefs," says Richard Aronson, a coral-reef ecologist at the Florida Institute of Technology in Melbourne.

Coral reefs thrive in shallow, tropical waters. Individual organisms called polyps build the reefs by growing on the calcium-carbonate skeletons of their dead ancestors. Despite their sturdy appearance, reefs are vulnerable to environmental threats such as agricultural runoff, ocean acidification, and predators such as the crown–of-thorns starfish, all of which interact in complex ways.

Marine biologist John Bruno of the University of North Carolina, Chapel Hill, and colleagues became curious about one threat in particular. Seaweed was first noticed overrunning coral reefs in the late 1980s. It often doesn't kill the coral directly; rather, it competes with it for space and light, moving into places where the older coral has died. In the process, the seaweed chokes off the ability of new polyps to grow, thereby reducing the ability of coral populations to recover from environmental threats. Eventually, the seaweed dominates the ecosystem, an example of a phenomenon biologists call a phase shift.

Such a shift occurred in the Caribbean Sea off Jamaica in the 1990s. Live coral cover plummeted from about 70% to 10%, and reef scientists worried that similar collapses were happening around the world. "All the leaders in the field told us this was the case," Bruno says, but no one had taken a comprehensive look.

So Bruno and colleagues examined more than 3500 surveys of the health of about 1800 reefs worldwide from 1996 to 2006. In this month's issue of Ecology, the team reports that only 4% of reefs worldwide were dominated by seaweed, and no area had grown appreciably worse. "There is more seaweed [on the reefs], but not nearly as much as we assumed," Bruno says.

Aronson says the study shows that seaweed is opportunistic and not the main threat. "The critical problem was and remains what is killing the corals in the first place," he says.

Meanwhile, marine ecologist Laurence McCook of Australia's Great Barrier Reef Marine Park Authority in Townsville cautions that reef algae include many other forms whose impact has not been similarly examined. It's a "concern that shouldn't be lost in the excitement about seaweeds being less dominant," he says.

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