Ocean Acidification and the Battle Between Coral and Seaweed

Ocean acidification creates plenty of potential problems for life in the oceans, but corals might have it the worst

The Great Barrier Reef
The Great Barrier Reef Photo courtesy of flickr user Barbara Rich

As we pump more and more carbon dioxide into the atmosphere, the ocean absorbs some of it. And as CO2 dissolves, it makes the oceans’ water more and more acidic. This acidification creates plenty of potential problems for life in the oceans, but corals might have it the worst. If the ocean becomes too acidic they won’t be able to create their calcified skeletons; the chemical reaction they rely on slows down under lower pH levels . But scientists in Australia say that the situation is more dire than expected. In their study, published in Ecology Letters, they show that higher CO2 levels may be give seaweed an advantage in a competition with coral.

Corals compete with seaweeds for space on the reef. When corals are healthy, the coral–seaweed competition reaches a balance. But if the corals aren’t doing so well because of something like eutrophication, then seaweed can take over.

In this new study, the researchers studied the coral-seaweed battle in miniature, setting up bits of each (Acropora intermedia, the most common hard coral in the Great Barrier Reef, and Lobophora papenfussii, an abundant reef seaweed) in tanks in the lab. Each tank had one of four CO2 levels in the air above it, resulting in four different pH levels: 300 parts per million (equivalent to pre-industrial CO2 and pH levels), 400 ppm (present-day), 560 ppm (mid-21st-century estimate) and 1140 ppm (late-21st-century estimate).

When there was no seaweed, the corals survived. But with its competitor present, the corals declined under each scenario. However, the decline was worse under higher CO2 levels, to the point where under the late-21st-century scenario, there was no living coral left after a mere three weeks.

“Our results suggest that coral (Acropora) reefs may become increasingly susceptible to seaweed proliferation under ocean acidification,” the researchers write. This area of research is still in the early stages and this experiment was a simplification of the coral–seaweed dynamic (there were only two species tested, for example, and plant-eating fish were left out of the equation), but it may provide even more reason to worry about the future of the coral reefs.

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