Coral Can’t Take the Heat Either

Corals at a southern site at Lisianski Island were previously bleached in 2014, but showed signs of recovery in August 2015. Photo by Courtney Couch

In keeping with the temperature theme of my last column, I am sorry to report our ocean life is also being greatly impacted by this summer’s heat. As predicted by the National Oceanic and Atmospheric Administration’s Coral Reef Watch, due to record high ocean temperatures caused by this summer’s El Niño event, coral reefs across Hawai‘i are already starting to see signs of bleaching. This bleaching is expected to peak in October.

Coral bleaching occurs when zooxanthellae living within the coral’s tissue is lost. This symbiotic algae gives coral its energy and color, so when it dies the coral bleaches. It results in a loss of energy, causing the coral to become extra vulnerable to other environmental stressors, such as pollution.

This is the second season in a row for bleaching in Hawai‘i, which is alarming because it greatly diminishes coral’s ability to recover from the previous event. Last year, severe bleaching occurred in several locations, including Kane‘ohe Bay on O‘ahu, the Northwest Hawaiian Islands and Northeast Kaua‘i. Of the sites on Kaua‘i surveyed by the State Division of Aquatic Resources, Anini, Lepeuli and Anahola, all experienced bleaching at various levels, some of it severe.

Once bleached coral colonies are identified, action should be taken to reduce other stresses on the reef, such as land inputs (e.g. pollution), avoiding damaging or disturbing corals while in the water (e.g. not throwing boat anchors on the reef), according to Brian Neilson, Aquatic Biologist with DAR, as stated in an informational video on dlnr.hawaii.gov/reefresponse.

It is important to know bleached coral are not dead, and they have a chance to recover, but only if conditions are just right and there are no additional stressors. Follow-up surveys are necessary to assess mortality rates after a bleaching event.

Lisianski Island, the site of a severe coral bleaching last year. The purple rice coral Montipora dilatata used to cover 70 percent of the reef at this site, but since the bleaching was documented 11 months, 99 percent of this coral species died. The picture at right shows the dead corals overgrown with algae, though they may provide habitat for new corals to grow. Photo by Courtney Couch

Courtney Couch, PhD, a researcher with the Hawai‘i Institute of Marine Biology, studies coral health in the Papahanaumokuakea Marine National Monument in NWHI. In August 2014, she documented a mass bleaching event on the east side of Lisianski Island. When they returned this July, they found almost 100 percent mortality, where the coral cover went from 70 percent to 1 percent. The most sensitive taxa was rare purple rice coral, Montipora dilatata. The more hearty corals that remained, Porites species and Montipora capitata, were beginning to show signs of recovery, however it is unknown whether this recovery will continue.

One potential silver lining for Lisianski, at least, is that the source and directional movement of the warmer waters is different, according to Couch. Last year, the warm water came from the Pacific Northwest and went to the topside of the NWHI. This year, it is coming from the equator, and is associated with the El Niño event. “If the predictions that NOAA released are true, the lower half of NWHI will see the brunt of the thermal stress this year.”

Kaneohe Bay on O‘ahu is already experiencing bleaching, and the public is starting to observe it elsewhere, including Kaua‘i, according to a press release from DLNR. Anyone can go to the Eyes of the Reef website to report bleaching observations (eorhawaii.org).

At press time for this article, EOR was organizing surveys for Oct. 3 at all the northeast Kaua‘i sites, and I was gearing up to go to the training session. I will be sure to report back on the results, and to highlight more research from UH HIMB. This research is really important because scientists are starting to get a better sense of the effects on corals from the overall ecosystem level down to the physiological scale. From this work, they can perhaps get a better sense of what might make reefs more resilient to climate changes.

• Ruby Pap is a Coastal Land Use Extension Agent at University of Hawai‘i Sea Grant College Program. She can be reached at rpap@hawaii.edu.