Porites Coral Growth Impacted More By Ocean Temperature Change Than Acidification
By David Biello
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One of the biggest natural tragedies of recent years is the deterioration of Australia's Great Barrier Reef, a vast structure of coral off the continent's east coast that supports a profusion of wildlife. In addition to overfishing and nutrient pollution, the world's largest natural structure has suffered from rising ocean temperatures. But, perhaps less well known, Australia's west coast has some massive reefs of its own, offshore in the southeastern Indian Ocean. Massive stony corals of the genus Porites swell to the surface, and new research published February 2 in Science suggests those located in the colder waters farthest south are growing better than ever -- thanks to warming ocean temperatures.
"Where we see strong warming, there is a marked response," says marine scientist Timothy Cooper of the Australian Institute of Marine Science. "To date, it is the changes in temperature that are having the dominant impact on coral growth as evidenced by the increasing calcification rates at our southernmost locations."
The findings suggest that temperature changes play a bigger role, at least in the near term, in the fate of corals than any ocean acidification -- where rising carbon dioxide levels in the atmosphere are absorbed into seawater, rendering it more acidic.
Corals -- microscopic polyps -- build reefs by pulling calcium from the surrounding water and using it to create tiny shell-like homes for themselves, which can accumulate over centuries into massive structures. This process, known as calcification, is sensitive to temperature and acidity, both of which can hinder growth. In addition, the corals themselves are sensitive to temperature, and temperatures that are too warm can lead to so-called coral bleaching events that can kill a reef.
Cooper and his colleagues drilled into reefs of Porites at six locations spanning roughly 1,000 kilometers from north to south off Australia's west coast. They extracted 27 core samples. Each one revealed a record of the annual density of the calcium-based homes of the Porites coral. By comparing the coral's annual rate of calcification to the average density from 1900 to 2010, Cooper and his colleagues revealed when and where any unusual changes occurred. Then the researchers compared that record to the data on monthly sea-surface temperatures to try to understand whether temperature had an impact and, if so, what it was.
They found that those Porites at the southern end of Australia's west coast have been growing thicker and thicker homes in recent decades as the colder ocean water warms. Farther north, the reverse is true: Porites in already warm subtropical and tropical waters suffer as the ocean heats too much. "Where there has been little warming, there has been little change in calcification," Cooper says. "To date, it is the changes in temperature that are having the dominant impact on coral growth as evidenced by the increasing calcification rates at our southernmost locations."
The cores suggest that the increasing acidification of the ocean has had, so far, less impact on coral than temperature has had. Eventually, however, acidification is expected to take a toll on coral growth -- water temperature notwithstanding. "Ocean acidification will increasingly become another limiting factor for growth of tropical coral reefs," Cooper suggests.
But a flaw in this study may prove to be the monthly temperature records used. Corals respond to temperatures on the scale of days or weeks -- hence bleaching events in the Great Barrier Reef or elsewhere in the world—that can then "slow colony growth for years," says marine biologist John Bruno of the University of North Carolina at Chapel Hill, who also studies corals but was not involved in this research. "Unfortunately, this [temperature] information does not exist." It is such fine-grained data on local temperature changes that would be needed to better understand how temperature, acidity and coral growth interact, although it is becoming increasingly clear that impacts from warmer ocean temperatures will determine the fate of coral reefs.
It is also clear that both of the impacts of rising human CO2 emissions -- global warming and ocean acidification -- are making life harder on the reef, including rapidly declining growth for Porites coral in the Great Barrier Reef. And there is no easy cure: Bruno's work shows that even marine protected areas, essentially ocean parks, cannot protect reefs from global problems like climate change. As Cooper adds: "Changes such as this with the relatively modest amount of global warming to date -- compared with what is predicted for the next few decades -- is cause for concern."