Marine Scientists Track Coral Bleaching in Real Time

Oct 23, 2019

In Oahu’s Kaneohe Bay, researchers with the Hawaii Institute of Marine Biology are engaged in a one-of-a-kind effort to document coral reefs as they undergo bleaching.

You have to take a boat just to get to the labs of the Hawaii Institute of Marine Biology. A three minute ride from Kaneohe, Coconut Island is home to a dozen labs where researchers study all aspects of ocean life, from large mammals to microorganisms. Several labs have a special focus on coral reefs.

Hawaii started breaking regional heat records back in May, a pattern that puts extreme stress on the state’s coral reefs. Abnormally high temperatures cause coral colonies to expel the microbes that supply nutrients to the coral and produce their vibrant colors. When those microbes are pushed out, the corals lose that color and turn pure white, a process known as bleaching.

Bleached corals can recover if the environmental stressors moderate, but many ultimately die. Ocean temperatures peak during fall months, making September and October the height of a potential bleaching event.

In August, researchers began extensively photographing 30 patch reefs in Kaneohe Bay, hoping to create a record of healthy reefs going through the bleaching process.

Out in the bay, 3 researchers swim in a spiral pattern, taking underwater photos with a camera tied to a PVC-pipe stake. They will capture up to 5,000 images at each site. The images will ultimately be used to create a detailed time-lapse record of the bleaching event, called a photomosaic.  

The project, launched by HIMB researcher Josh Madin and the Madin Lab, includes scientists and graduate students from several labs on Coconut Island. The collaboration represents multiple areas of research specialization that might normally be working independently.

The 30 reefs being monitored were actually selected for a different project. Mariana Rocha de Souza, a PhD candidate with the Ruth Gates Coral Lab, helped randomly select them two years ago. Their goal is to examine the impact genetic diversity in different corals colonies.

Despite the environmental stress, not all corals in Hawaii are currently bleaching. Some, like this colony in Kaneohe Bay, remain healthy.
Credit Shreya Yadav / Madin Lab/Hawaii Institute of Marine Biology

“In an event where there is high temperature or any stressor, having genetic diversity means that some of the organisms are going to respond differently and some of the organism is going to be able to survive the stressor. So it's very important to know the genetic diversity,” de Souza said while waiting for the photomosaic team to return from a survey.

The potential importance of that genetic variation is evident when the team of swimmers return to the boat. On a reef with hundreds, if not thousands of individual coral colonies, signs of bleaching are spread haphazardly throughout the reef.

Shreya Yadav, a PhD candidate with the Madin Lab and coordinator for the photomosaic project, has been documenting the changes underway on each of these reefs ever three weeks.

“All the pocillopora is beginning to die. That’s the thing that we’ve seen go from totally healthy, to beginning to bleach, to now mostly dead. But the other species, some of them look ok,” Yadav said after completing the first survey of the day.

At the next reef site, that variation is clearly visible. The team enters the water and swims about 30 yards from the boat. A mountain of brown coral rises up suddenly from the sea floor.

It’s taken millions of years of growth, death, and growth of new coral for this patch reef to reach this height. The surrounding sea floor is between 30 and 50 feet deep, but corals are now growing within one or two feet of the water’s surface. It’s like the trees in a forest growing to the edge of Earth’s atmosphere.

Hundreds of individual coral colonies make up this reef. The team reports that many appear healthy, but the telltale white of bleaching is clearly visible throughout.

Certain varieties of coral seem to be bleaching more than others. Often a bleached coral sits right beside healthy colony, with seemingly little difference between them.

That is the mystery scientists on Coconut Island are investigating. Several research teams are now using these 30 survey sites to collect a treasure trove of data, including data on coral genetics, environmental DNA for cryptic diversity, and a host of other environmental parameters like temperature, pH, and water motion.

The result may be the most comprehensive picture of any coral reef on Earth, and perhaps shed light on how these ecosystems are dealing with a changing environment.