Hoping for a hurricane? Coral reefs are

From a post by Stephanie Wear on The Nature Conservancy's Cool Green Science blog:

It may surprise you to know that, given the warming trends in the ocean and the fact that El Niño seems to be setting up for this winter, a hurricane is just what coral reefs need to avoid a mass bleaching event.

Don’t get me wrong: Big hurricanes can cause serious damage to coral reefs. But generally, storms are something they have adapted to and as long as they are in good health, will be able to recover from.

But why are hurricanes good for coral reefs?
The combination of still hot water and radiation stress from cloud-free summer days is a deadly duo for corals. But with hurricanes, you get lots and lots of wind, and the ocean gets all stirred up. The clouds come in and darken the sky and cool things off with lots of rainfall. This is just what a reef needs to keep from bleaching when they have been cooking in the sun, getting stressed from the heat.

Here’s why: When corals are stressed, they expel the tiny algae cells that live in their tissues, turning the corals white. This bleaching (the appearance of “whitened” coral where there was once-colorful coral) is a symptom of stress in corals and other reef animals with symbiotic algae. These tiny algae are known as zooxanthellae and are present in most healthy reef-building corals. Zooxanthellae provide nutrients and oxygen to the coral through photosynthetic activities, allowing their host to direct more energy toward growth and constructing its calcium carbonate skeleton.

The host coral polyp in return provides zooxanthellae with a protected environment and a constant supply of carbon dioxide needed for photosynthesis. When sea temperatures become too warm (above 28 C), the photosythetic system of the zooxanthellae can not effectively process incoming light. This results in production of “superoxides,” such as hydrogen peroxide, toxic by-products of this process. These toxins contribute to coral stress reactions, which lead to bleaching. In extreme cases of bleaching, corals die.