Wednesday, June 4, 2014

Bleaching continues

Corals on Dongsha are still feeling the heat. Two weeks into the bleaching event, more corals have joined the ranks of the bleached.
A coral colony tagged on May 26 is not bleached.

The good news is that almost all of the corals are still hanging on. They are going it alone though, their symbiotic algae mostly gone. When we look closely as the surface of the bleached corals, the coral polyps are stretching their tentacles out into the water, grasping for food.

By June 1st, the same colony as above has lost most of its
color, appearing bleached. The green on top of the colony
is a layer of mucus. 
Without their photosynthesizing symbionts, the coral animal is left to feed itself. Even though most of the coral's energy is provided by the symbionts, corals are equipped to catch food themselves. Seawater is constantly flowing across the reef, and this seawater carries plankton. Corals cannot move, they live inside the large skeleton that they build, and that skeleton is cemented to the reef. Instead, corals reach their tentacles out into the water and snag food passing by. That most of the corals on Dongsha are still alive despite two weeks without their symbionts suggests these corals are catching some food.

Another possibility is that the corals are not getting enough food without their symbionts and are burning through their fat reserves. If this is the case, the clock is ticking for these corals until those fat reserves run dry. The question is whether these corals can continue to feed themselves, or live off their fat reserves, long enough to acquire new symbionts.

A mucus layer shedding off a bleached coral. Notice the
white color of the coral where the mucus has fallen away.
The thousands of tiny white bumps are the living coral
Here are some of the changes that we have observed over the past two weeks:

The coral colonies that are bleaching on Dongsha are spewing out mucus, which coats the colony. Producing lots of mucus is a general coral stress response, the mucus may be a way for the coral to shed its symbionts or harmful bacteria.

When we first observed bleaching on May 26, it was only massive, dome-shaped corals of the genus Porites. Now we are beginning to see the start of bleaching in different species of branching coral.

Bleaching is starting in several different species.
At least a few corals appear to have died from the bleaching event. The colonies are bone-white and no living coral polyps are visible. A thin layer of algae is beginning to cover the skeleton - available hard substrate is a hot commodity on reefs and algae waste little time in taking over. Hopefully the few cases of mortality that we have seen are not a sign of things to come.

Recent coral mortality. A thin layer of algae is covering the
former colony.
The cause of the bleaching event is now more clear. Data from temperature loggers deployed on Dongsha show very rapid warming, over 4°C in just two weeks. While the seasonal range of temperature on Dongsha is around 8°C, the temperature changes slowly and steadily throughout the year. It's most likely the rapid burst of warming that the corals could not handle.

We will continue to keep an eye on the bleaching happening on Dongsha, keep following our blog for more updates in the weeks to come.

- Tom DeCarlo
Joint Program in Oceanography
Woods Hole Oceanographic Institution

Temperature recorded by a logger on the Dongsha
reef flat. The sharp up and down spikes are daily
temperature changes. The 4-5°C warming trend 
beginning around May 9th likely caused the
bleaching event on Dongsha.

Partial bleaching of branching

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