Climatic Threat to Coral Reefs II: Warming and Coral Bleaching

As Hawaii is tropical, the coral species that live in Hawaiian waters are suited to the tropics. In ecology it is commonly known that tropical species have a narrow range of temperatures in which they can survive and thrive. This is a consequence of temperatures only fluctuating small amounts between seasons in the tropics, so organisms have physiologies that maximize the benefits of thermal stability. As the graph from Deutsch et. al shows, tropical species (labeled B) live close to their temperature threshold (critical thermal max), and are thus predicted to decline in fitness due to a warming environment (graph C). As coral reef builders like coral polyps are tropical organisms, we can predict that they too will decline in fitness as a result of breaching their critical thermal max. Indeed, temperature anomalies of 1-2 degrees C (depending on the species) have shown to be fatal (Coles et. al, cited in Jokiel and Brown).

Deutsch et. al

The principle physiological process that will increase coral mortality with climate change is coral bleaching. Recall that the dinoflagellate algae of the genus Symbiodinium (zooxanthellae) live in coral's tissue in a mutualistic relationship. Bleaching is the expulsion of around 70-90% of the zooxanthellae from the tissue of the coral. The zooxanthellae are what give coral their color. Without them all that is to be seen through the translucent tissue of the coral is the white calcium carbonate (Douglas).

The molecular and evolutionary biology behind bleaching is only partially understood. What is known is that Photosystem II is damaged and photosynthesis is inhibited in the zooxanthellae, and the coral then expel the algae. Bleaching is often fatal to the coral, so why do they do it? One hypothesis is that coral have a chance of being recolonized by another species of zooxanthellae that have a higher thermal limit (Douglas).

There were, as of 2004, two well documented cases of coral bleaching in Hawaii discussed by Jokiel and Brown (2004), one in 1996 and the other in 2002. Both were in the summertime and included temperature anomalies of 1-2 degrees C from the mean summer maximum of 27-28 degrees C, but interestingly one was in the Main Hawaiian Islands, and the other was in the North Western Hawaiian Islands. The reason for the waters reaching high enough temperatures to cause bleaching in one area and not the other is that many factors affect local temperatures, including cloud coverage, wind speed, and the presence of eddies. The coral mortality for the 1996 event was less than 2%. Though this data seems innocuous, the worry is that climate change will cause future bleaching events to be more harmful than those already experienced.

Climate change will affect coral bleaching through warming the world's oceans and cause more El Nino years. Jokiel and Brown (2004) found that Hawaiian oceanic temperatures were increasing an average of 0.15 degrees C per a decade. As the temperatures creep up towards the coral's thermal maximum (29-30 degrees C in one part of Hawaii, where the average summer maximum is 27-28 degrees C), there remains uncertainty about how coral will respond (note: oceanic temperatures vary within the oceans of Hawaii) .

Trend in Warming Near O'ahu (Jokiel and Brown 2004)