Marine Research News Cambodia

During April and May 2010 Marine Conservation Cambodia (MCC) research divers observed a significant amount of bleaching affecting the coral reef systems around Koh Rong Samloem and Koh Koun. More than 50% of the coral coverage was suffering from bleaching on the surrounding reefs. Bleaching affects corals and also certain invertebrates. Bleaching is actually the loss of a specific algae (Zooxanthellae) due to stress. This algae has a symbiotic relationship with its host, supplying colour and providing the majority of energy needed by the host organism.

Luminescence of organisms is an indicator of stress before the final bleached stage. Luminescence was observed in a large majority of hard and soft corals, giant clams and anemones. The process of luminescence is a reaction of the zooxanthellae to increased sun exposure and acts as a sunscreen. The algae pigment cells react to the additional sun energy by breaking up the sunlight into the colour spectrum and expelling the more damaging energies, giving the coral a fluorescent colour. Near the end of this process, if the health of the coral is not improving, the algae leave the zooxanthellae and the coral is considered bleached.

One factor which is known to have an effect on the reef’s organisms and cause them to come under stress is water temperature. The proposed explanation for the current bleaching incident is due to an increase in oceanic temperature. Tropical reefs normally exist within a temperature range of 27⁰ Celsius to 32⁰ Celsius. When the temperature rises above this range, the organisms within the reef show bleaching. This has occurred in other coral reef systems around Jamaica in the Caribbean Sea and also the Great Barrier Reef in Australia. The water temperature in Cambodia around Koh Rong Samloem and Koh Koun rose from an average of 30oCelcuis to a peak temperature recording of 35oC.

Bleached coral may recover if zooxanthellae return to the coral. It has been documented that corals that recover zooxanthellae within four to six months after a bleaching event are more likely to return to a healthy state (Diaz-Pulido and McCook 2002). The fate of the bleached coral is dependent on the ability of the coral to out-compete the algae. Oceanic temperatures have decreased dramatically with the arrival of the wet season. Temperatures are once again averaging 30oCelcius and dropping frequently to 29oCelcius. Hopefully with this reduction in temperature the majority of the coral reef system will successfully recover.

In the past two months it appears that some of the corals are starting to recover and the majority of clams are also appear much healthier. MCC research staff and volunteers will continue to monitor the extent of bleaching and rates of recovery.

We have so far produced to documents on the bleaching event here in Cambodia the surveys conducted focus on four main tourist dive sites, the first document is on the initial extent of the bleaching during the months of April and May the second was conducted during July and August to monitor for signs of recovery or continued bleaching. These documents are available on request.