Climatic Threat to Coral Reefs I: Ocean Acidification

Many are aware of the surge of carbon dioxide in the atmosphere, but less aware of the effects that increased carbon dioxide has on the ocean. The ocean absorbs about a quarter of atmospheric carbon dioxide, so any increase in atmospheric carbon dioxide means an increase in oceanic carbon as well (PMEL). Further, carbon dioxide has the effect of decreasing pH, which in turn decreases the abilities of organisms like coral to create calcium carbonate shells.

The specific chemistry, diagrammed below, is that carbonate reacts with H+ to form bicarbonate. An increase in carbon dioxide affects the first reaction shown, in which carbon dioxide reacts with water and creates the H+ ion as a product, resulting in a decreased pH. Because pH is logarithmic, a small decrease in pH can have a huge effect. Since the Industrial Revolution pH has decreased by about 0.1, yet acidity has increased 30%. This change is more rapid than any other shift in the pH that has been recorded for the past 55 million years (Ocean Acidification The Facts). Below there is an image showing projected aragonite (calcium carbonate in its crystalline form) concentrations (mmols/kg) across the oceans under increasing carbon dioxide concentrations (ppm). Today's carbon dioxide concentrations are around 390 ppm, the 2007 IPCC Report predicts this number to rise to 700 ppm by the year 2100 following a business-as-usual scenario.

Hoegh-Goldberg et. al

Below is a stark prediction of what may face the coral reefs as carbon dioxide pollution continues to spike. The Omega aragonite symbol may be interpreted as how easy it is for calcifying organisms to form aragonite shells, with the blue being the easiest. Coral dissolves towards the red. The numbers at the top left of each map are world carbon dioxide concentrations in parts per million (ppm). Today (2012) we are hovering around 390 ppm. However, while making sense of researchers' results, it is important to recognize that different experiments will suggest different things. It is only once enough research has been done that a clear picture may emerge. Keep this in mind while reading on about Jokiel's research.

Hoegh-Guldberg et. al

In a mesocosm experiment (pictured below) by P.L. Jokiel and K.S. Rodgers (2008) on Montipora capitata, a coral species common in Hawaii, researchers increased carbon dioxide levels to the concentrations they are expected to reach by 2100 (following a business-as-usual model). The researchers found that crustose crystalline algae (CCA) coverage of the tanks was decreased by 86% under experimental conditions, while for M. capitata calcification rates decreased by 15% in one trial, and 20% in the next. CCA are ecologically important as algae that help to cement coral reef. A plausible reason for CCA being affected more than M. capitata is that magnesium calcite, a component of CCA's skeleton, is more water soluble than aragonite. The study may be interpreted as a warning that CCA, an essential part of Hawaii's coral reef ecosystems, may break down before coral polyps.

Mesocosm facility at the Hawaii Institute of Marine Biology, Coconut Island