In 2005, corals in the large reef off the coast of Florida were saved by four hurricanes. Tropical storms seem to be unlikely heroes for any living thing. Indeed, coral reefs directly in the way of a hurricane, or even up to 90km from its centre, suffer serious physical damage. But Derek Manzello from the National Oceanic and Atmospheric Administation has found that corals just outside the storm’s path reap an unexpected benefit.
Hurricanes can significantly cool large stretches of ocean as they pass overhead, by drawing up cooler water from the sea floor. And this cooling effect, sometimes as much as 5°C, provides corals with valuable respite from the effects of climate change.
As the globe warms, the temperature of its oceans rises and that causes serious problems for corals. Their wellbeing depends on a group of algae called zooxanthellae that live among their limestone homes and provide them with energy from photosynthesis. At high temperatures, the corals eject the majority of these algae, leaving them colourless and starving.
These ‘bleached’ corals are living on borrowed time. If conditions don’t improve, they fail to recover their algae and eventually die. But if the water starts to cool again, they bounce back, and Manzello found that hurricanes can help them to do this.
It’s not a good time for corals. Last year, a third of coral species went straight into the endangered lists after being assessed for the first time, and it looks like 2009 isn’t going to bring any reprieves to the doom and gloom. In particular, a new study provides hard evidence that the mightiest of coral super-colonies – the Great Barrier Reef – is in trouble.
Like reefs across the world, the Great Barrier Reef faces many threats, including pollution, physical destruction, predatory starfish and perhaps most importantly, the many effects of climate change. Glenn De’ath and colleagues from the Australian Institute of Marine Science have found that the corals among this greatest of reefs are starting to yield under these multiple assaults, adding new material to their limestone skeletons at ever-declining rates. The Reef’s growth is slowing to a worrying degree, the likes of which are unprecedented in at least the last 400 years.
De’ath’s group focused on one group of corals called Porites. They are a widespread and important group, and like most of their kin, they build reefs by laying down external skeletons of aragonite, a version of calcium carbonate or limestone. Like trees, they have annual growth rings that reveal how quickly they expand. And because coral growth depends on a variety of environmental conditions, the skeletons of the Porites provided a potted history of environmental changes, recorded in unchanging limestone.