Any budding marine biologist, or even an avid diver has heard of the term ‘coral bleaching’ when discussing or studying coral reefs.

According to Project Aware “coral bleaching occurs when corals change colour, generally from dark brown to a lighter shade of brown or white. 

The colour change is associated with a loss of symbiotic dinoflagellates (algae) from the coral’s tissue.  Symbionts provide the brown colour in corals, and bleached tissue contains fewer symbionts than healthier tissue. 

Bleaching can be caused by a variety of environmental factors including increased or decreased water temperature, exposure to ultraviolet light, changes in salinity and exposure to chemicals.”

It has been a hot topic of late on Koh Tao and no doubt in other reef environments around the equator. A quick look at NOAA’s Coral Reef Watch website clearly identifies many areas across the globe in a ‘coral bleaching ‘watch’ or ‘alert’ zone.

In the last month or so on Koh Tao these changes have definitely become apparent and we have received a number of emails from different interested parties – Reef Check, Project Aware, Marine Conservation Koh Tao – to increase our monitoring and data collection of this unusual and historically rare event. With global warming and associated stressors from an ever increasing global population these bleaching events will no doubt be less and less rare.

What we have seen over the past month has been an interesting tale of events and has unfolded on an almost daily basis. In mid to late April certain species of coral started to bleach. Some large (what we would call ‘submassive’) corals were heavily affected as well as most of the solitary/mushroom corals. As time has gone on more and more species seem to be effected as the problem spreads. Any experienced diver/snorkeler would be hard pressed to not notice the changes.

Temperature would appear to be the major culprit with sea temperatures remaining steady around 32 degrees Celsius for the last few weeks with 33 degrees and above in the shallow waters surrounding the island not uncommon for late afternoon. This is VERY warm.

These two pictures show how the solar radiation is having a detrimental effect on many of the corals. The picture on the left is the top (exposed) side of the colony. The picture to the right has been shaded and as a result it still retains its symbionts and consequently some of its colour.

Bleaching is interesting how it effects different species. One colony may be heavily effect while a neighbouring colony remains virtually unchanged. This illustrates the different resilience levels of the different species. It may also offer clues to marine resource managers when choosing species for reef restoration projects.

But temperature, as mentioned, is not the only blame. Solar radiation is also having an effect as minimal wind and clear skies make the surface waters very flat and calm. This leads to a clear passage for solar rays through the surface waters to the corals below.

Another interesting element to note is how more stressed the corals are that are subjected to more direct sunlight illustrating the effect solar radiation has on corals. A bit like people and sunburn, we have seen numerous examples of corals that on one side have bleached completely and on the underside are still quite dark and healthy looking.

But all is not lost. Far from it. These remaining ‘brown’ areas of the coral colonies indicate that symbiotic algae remain providing recovery potential for all of the bleached corals we have seen. Weather patterns have slowly been changing also. Small rain showers, windy mornings and more regular cloud cover are both helping to mitigate the effects of bleaching. If we can see an increase in these conditions then we are confident in a return to health of the corals on Koh Tao.

We have increased our monitoring efforts using a number of methodologies at our disposal – Reef Check’s Eco Monitoring, Project Aware’s Coralwatch and we’ve also started some of our own tagging, numbering and photographing surveys. It is important to collect as much data as we can during this unusual phenomenon to establish the resilience of our local ecosystem.

Equally important will be gathering information and tracking changes as the ecosystem recovers and returns to its vibrant self in the near future.

For more information please contact Nathan Cook

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PADI IDC Staff Instructor #479720
Reef Check Trainer
Manager Environmental Awareness and Action
Eco Koh Tao
E: nath@ecokohtao.com, www.ecokohtao.com