Heat Puts Corals at Risk

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It’s officially fall in the Northern Hemisphere, but the extreme heat we experienced this summer, not only in the U.S. but around the world, has put coral reefs at risk. The heat puts a lot of stress on the corals causing some of them to shed their color which indicates they’re going into survival mode.

Coral bleaching is an indicator of heat stress. Photo: http://flowergarden.noaa.gov/

Coral reefs are most common in the Indo-Pacific region which is where you’ll find 91.1% of all corals (i.e. Red Sea, Indian Ocean, Southeast Asia and Pacific Ocean). The Atlantic and Caribbean Seas only account for 7.6% of coral reefs. However, as sea surface temperatures remain very warm across the Caribbean (see below), it looks like corals may undergo drastic bleaching over the next few weeks as they’re very sensitive to water temperature. When corals are exposed to very warm water, they either expel or consume the colorful algae they hold, which leads to the bleached color. If the stress is prolonged and the algae populations do not recover, the coral eventually dies.

Sea surface temperature plot for Tuesday, September 28. The Caribbean has warm sea surface temperatures in the 80’s F. Image: UNISYS

In 2005, up to 90% of corals in the eastern Caribbean suffered bleaching and more than half of them died. According to scientists at the National Oceanic and Atmospheric Administrations Coral Reef Watch network, the potential for bleaching is higher this year than it was in 2005. However, the hottest year on record was in 1998 when an estimated 16% of the world’s shallow-water reefs died and according to NOAA, the first eight months of 2010 matched 1998 as the hottest period on record from January to August. In 1998, an estimated 16% of the world’s shallow-water reefs died.

A coral reef strongly hit by 1998’s El Niño. Photo: Wikipedia

Back in 1983, the eastern Pacific and Caribbean saw corals drastically dying off due to a large-scale weather event which is now identified as El Niño. 2010 has also been an El Niño year, which is characterized by unusually warm temperatures in the equatorial Pacific. Heat puts stress on the corals and causes their metabolism of algae to speed out of control and they start creating toxins. The corals begin to look white or like they’ve been bleached because without the algae, the coral polyps are mostly clear. However, if the temperatures drop, the corals’ remaining algae can reproduce and help them to recover (which make take weeks or months). But if the heat continues, the corals eventually starve to death and the only things left are their skeletons. Unfortunately, it appears that the waters are still warming across the Caribbean, especially across the southern portion, and the corals here could be at high risk.

Difference between unbleached coral (left) and bleached coral (right). Photo: Wikipedia

Over the past couple of days we’ve been tracking Tropical Disturbance 55 which was upgraded to Tropical Depression 16 on Tuesday. This could actually be beneficial to the reefs across the northern Caribbean as tropical systems tend to help cool the water. Tropical Depression 16 is expected to move just east of Florida Wednesday and merge with a cold front off the east coast later in the week.

Unfortunately, computer models continue to indicate warm surface temperatures across the Caribbean and corals may undergo drastic bleaching in the next few weeks. Close to home, a marine sanctuary off the TX-LA border, the Flower Garden Banks, has already seen serious bleaching recently with elevated sea surface temperatures throughout the Gulf of Mexico. The preferred temperature range across this area is about 68-86ºF. Most coral bleaching is the result of water temperatures going beyond the level of tolerance for too long. When sea surface temperatures stay above 86ºF for too long, the corals begin to stress.

Click here for NOAA’s Coral Reef Watch website for the Greater Caribbean. The areas of most concern are across the southern Caribbean.

The Flower Garden Banks are currently under a Bleaching Watch. Image: Coral Reef Watch

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