Coral reefs are like underwater cities, with myriad species forming a thriving ocean metropolis. That complexity, however, can hinder a reef's survival, scientists funded by the U.S. National Science Foundation have found. 

After bleaching events, the dead coral skeletons left behind allow seaweed to outgrow new young coral, preventing reefs from recovering. The results are published in the journal Global Change Biology.

The research was conducted at the NSF Moorea Coral Reef Long-Term Ecological Research site in Tahiti, one of more than two dozen such sites funded by NSF and located in ecosystems ranging from forests to deserts and lakes to oceans.

Seaweed, or macroalgae, competes with corals for space on the reef and for light. The algae grow faster than the coral, so seaweed can overrun a reef, preventing new corals from settling and shading out colonies that do. Young coral is especially vulnerable. Once a reef turns from being covered by coral to being covered by algae, the change can be hard to reverse.

The research team, led by Russ Schmitt of the University of California, Santa Barbara, found that dead coral skeletons could help young coral that settle on a reef shortly after a bleaching event. But corals usually spawn once a year, while many algae reproduce continually, giving seaweed the advantage in colonizing newly available substrate.

"If the corals had died in a typhoon that removed both the corals and their skeletons, there’s a good chance new corals would have come in and the reef would have recovered," says Dan Thornhill, a program director in the NSF Division of Ocean Sciences. "With bleaching, however, the skeletons are left behind. This legacy of dead corals is an ideal habitat for algae to take over."