Coral And The Goldilocks Effect

For as much as I love coral reefs, I've gotta admit that coral polyps are the prima donnas of the ocean. Most coral reefs are located between the tropics of Cancer and Capricorn--not much more than 30 degrees north and south of the equator. Not too far north nor too far south, but just right. Corals are also found farther from the equator in places where warm currents flow out of the tropics, such as Florida and southern Japan. Reef building corals grow best in waters with a temperature of between 21 and 29 degrees Celsius (70 and 85 degrees Fahrenheit). It is possible for soft corals to grow in hotter and colder places, but growth rates under these conditions are very slow. Again, they like it just right.

Tropical coral species are particularly fussy about temperature. As I've described before, when sea surface temperatures start to climb more than 2-4 degrees Celsius for an extended period of time, coral polyps get stressed and can expel their algal symbionts. Once the algae (zooxanthellae) are evicted, the translucent coral polyp appears white (or bleached). If temperatures remain elevated and the coral cannot reacquire new heat-tolerant algae, the coral may die.

But bleaching can result not just when conditions are too warm, but also when conditions are too cold. Some recent news from Heron Island on Australia's Great Barrier Reef describes how unusually cold weather has resulted in a winter bleaching event. Just the other day I described how the relatively rare phenomenon of sun-dried tides can cause mass coral bleaching and mortality. In a twist on that theme, if unusually low tides are coupled with chilly, windy (conditions that are just wrong for coral) a similar mass mortality can result. These images of bleaching are courtesy of Ove Hoegh-Guldberg's fantastic website Climate Shifts.