Dead Zones Add Yet Another Threat to Tropical Coral Reefs

Nutrient runoff in temperate waters is widely understood to cause dead zones, but researcher are turning their focus to tropical waters where the phenomenon is less understood.

Coral reefs are dying across the world, and the biggest of them all - Australia's Great Barrier Reef - is undergoing a second major bleaching event in two years.

By far the biggest factor is climate change, which is warming waters and making them more acidic. But localized stressors, such as sedimentation and overfishing, also contribute to coral reef decline. And now, as if the prognosis for reefs wasn't bad enough, a new study has discovered an additional problem: dead zones.

Dead zones occur when an overabundance of nutrients - typically, although not exclusively, from sewage or agriculture production - encourages massive blooms of phytoplankton, which then die and sink to the ocean floor, where they are consumed by bacteria. That process consumes much of the available oxygen in the water column, causing life to flee if it is mobile, and die if it isn't.

The study, published in Proceedings of the National Academy of Sciences, sprang from a discovery in Bahia Almirante, a large, semi-enclosed bay of about 174 square miles in the Bocas del Toro region of Panama. Nancy Knowlton of the Smithsonian's National Museum of Natural History was researching reef biodiversity in the area when she discovered that the once dynamic ecosystem had transformed into "a lunar landscape."

"There was all this gray sludge growing all over the corals," Knowlton said.

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What was especially fascinating to Knowlton was the destruction began at a clear depth line. Above the line, the reefs looked relatively healthy. But below it she found not only dying coral, but dead crabs, sea urchins and sponges.

"It was really striking, so you know something really drastic was going on," Knowlton said. "So I got back from that dive and said to colleagues, 'You guys have got to go down there and measure oxygen and anything else you can think of measuring. Everything is dead below a certain depth.'"

Dead zones are well known in temperate areas where nutrient pollution is especially high. The Gulf of Mexico, for example, is hit annually due to agricultural runoff along the Mississippi River. But there are few records of tropical dead zones, which researchers attribute to observational bias.

"Honestly, if I hadn't had those settling structures at that time, you'd never know what happened, because there'd just be dead coral," Knowlton said. "You'd have absolutely no clue as to what was going on, because a dead zone can kill very quickly and then it can lift and later you can't pinpoint it very easily."

She added, "You might expect more dead ones in the tropics than in temperate zones, because the bacterial processes and the potential for things spiraling out of control vis-a-vis dead matter and bacterial activity is temperature-dependent: It happens faster in warm waters."

And that means dead zones may become even more frequent, which, in turn, exacerbates the harmful impacts of other factors like overfishing or ocean warming on reefs.

"There's no question that a lot of these things are synergistic in a very bad way," Knowlton said.

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