Ocean acidification refers to the ongoing change in the chemistry of the ocean caused primarily by the ocean’s absorption of carbon dioxide from the atmosphere. For the last 250 years, the burning of fossil fuels — coal, oil, natural gas — for energy, cement production, and deforestation has been pumping carbon dioxide or CO2, into the atmosphere. The ocean absorbs about 1/4 of this excess CO2 released into the atmosphere every year. This addition of CO2 into the ocean is changing the chemistry of seawater by increasing the acidity and lowering the seawater’s naturally occurring carbonate ion. CO2, when combined with water, forms a weak acid, which increases the hydrogen ion concentration in the ocean, lowering the pH and making the oceans less alkaline or more acidic. As the ocean becomes less alkaline there is a reduction in the amount of carbonate ions and calcium carbonate minerals, biologically important building blocks for many marine organisms.
Calcifiers—organisms with shells or skeletons made from calcium carbonate—are among the most abundant forms of marine life. Ranging from tiny plankton species such as pteropods and foraminifera that form the basis of marine food chains, to the vast coral reefs that provide habitat for many ocean animals, calcifiers are an essential part of many marine ecosystems. As ocean acidification decreases the availability of carbonate ions, these organisms must work harder to produce shells. As a result, they have less energy left to find food, to reproduce, or to defend against disease or predators. As the ocean becomes more acidic, populations of some species could decline, and others may even go extinct. This threat is real!
Many species known to be vulnerable to ocean acidification play critical roles in ecosystems, such as the corals and seagrasses that form habitat for diverse communities of organisms. When ocean acidification triggers even a relatively subtle change in an organism’s performance, it can scale up to a change in the size of the population (e.g. through changes in survival, growth, or reproductive success), altering the composition of an ecological community, and potentially the entire marine ecosystem.