Multiple Threats of Ocean Acidification to a Life and Death Balance on Coral Reefs

Ocean acidification is changing the carbonate chemistry of surface ocean waters about 30 times faster than at any other time in the past 56 million years. This means that organisms are being exposed to conditions they have never had to endure during their long evolutionary past. Even though organisms living near the surface, and particularly those living in shallow coastal water, regularly experience large swings in pH, experiments have shown that an overall reduction in the average pH on a daily, seasonal or annual basis does have consequences. For example, Many shell- and skeleton-building organisms grow more slowly. Some species have been shown to reproduce less successfully or develop more slowly through early life history stages. Organisms as diverse as corals and fish have been found to miss vital settlement cues resulting in reduced recruitment. In an extreme example, one species of fish was found to lose the ability to smell and avoid its natural predator. However, not all outcomes are negative.  Some species of phytoplankton grow faster under low pH conditions and seagrasses are a clear winner. How will these physiological effects translate into life and death outcomes for individual species? One would expect that adversely impacted species would become less abundant or disappear completely to be replaced by better-adapted species.  This expectation is consistent with observations made at three cool, shallow volcanic CO₂ seeps in Papua New Guinea.  Along a pH gradient from 8.1 to 7.8, we observed reductions in coral diversity, recruitment and abundance of structurally complex corals. However, within this zone coral cover did not decline because of the persistence of the massive Porites corals.  However, below pH 7.7 reef development ceased completely.  We predict that ocean acidification, along with increased temperature stress, will lead to reduced diversity, structural complexity and resiliency of coral reefs in this century.