Predictions: The Future of Hawaiian Coral Reef Ecosystems

Original Diagram (photo: S. Alexander). Research has shown calcification rates to halt or reverse at 480 ppm (cited in Hoegh-Guldberg et. al), and coral bleaching to occur with increases in temperature of 1-2 degrees Celsius (Middlebrook, Hoegh-Guldberg, and Leggat).

The Future of the Hawaiian Coral Reef Ecosystems depends on a myriad of factors. The two that this blog has focused on are ocean acidification and warming, and this page will go on to discuss the future of coral reefs in Hawaii with respect to these. Some other ways in which humans affect coral reefs are introducing invasive species, over-fishing, sedimentation, and damage from boats (HCRI).

It should be made clear that there are thousands of species of coral and many more species of their symbiotic zooxanthellae. As Shamberger et. al found, some species' abilities to calcify under increased carbon dioxide concentrations declines more so than others. The same differences are to be expected for the trait of persistence through temperature anomalies.

What we know is that ocean acidification will continue to occur. Geo-engineering plans to use aerosols to stop global warming, should they be implemented, will not alter carbon dioxide levels and thus will not have an effect on ocean acidification. Researchers have found that calcification rates are halted at atmospheric carbon concentrations of 480 ppm (cited in Hoegh-Guldberg et. al). Also, in the absence of such attempts to curb climate change oceanic warming will continue to occur as well. What we don't know, and a current focus of research, is how coral reef building animals may be able to acclimate or possibly even evolve.

A study by Middlebrook, Hoegh-Guldberg, and Leggat (2008) exposed Acropora aspera to 31 degrees C water for 2 days, waited several days, and then raised the temperature to 34 degrees C. The researchers, measuring the density of zooxanthellae, found that coral that were stressed prior to crossing their thermal max did not have a decrease in density of their symbiont. On the other hand, the density of zooxanthellae in coral that were not previously stressed was reduced by 40%. While this finding gives hope that coral may be able to acclimate, predictions remain grim. The authors conclude the abstract: "Clearly increases in sea temperatures that extend beyond 1-2 degrees C will exhaust the extent to which acclimation can modify the thermal threshold of corals."