Increasing the alkalinity of seawater flowing over a coral reef, and, hence, decreasing the seawater’s acidity, increases coral reef growth, reports a paper published in Nature this week. Although declines in coral reef growth have been shown previously, this study is the first controlled field experiment in a natural coral reef community to isolate the effects of ocean acidification from other contributing factors, such as temperature, coastal pollution and overfishing.
Approximately one-quarter of annual global anthropogenic carbon dioxide emissions are absorbed by the oceans, leading to oceans becoming more acidic. Ocean acidification is one of several factors that are projected to threaten coral reef ecosystems, but separating its effects from other factors is difficult.
Rebecca Albright and colleagues conducted their study on One Tree Reef in the southern Great Barrier Reef, where, at low tide, there are lagoons that become effectively cut off from the ocean. They added sodium hydroxide to the seawater of one lagoon to increase its alkalinity, and then measured the change in alkalinity as the seawater flowed across the reef into another lagoon. The authors were able to isolate the alkalinity change that results solely from an increase in net calcification for the reef community (a measure of growth), and find that calcification increases when ocean chemistry is restored closer to pre-industrial conditions (i.e. when the ocean is less acidic and more alkaline).
In an accompanying News & Views, Janice Lough writes: “By turning back time, they [Albright and colleagues] demonstrate that, all else being equal, net coral-reef calcification would have been around 7% higher than present, suggesting that ocean acidification may already be diminishing coral reef growth.”