Limacina helicina, a free-swimming planktonic snail. These snails, known as pteropods, form a calcium carbonate shell and are an important food source in many marine food webs. As levels of dissolved carbon dioxide in seawater rise, skeletal growth rates of pteropods and other calcium-secreting organisms will be reduced due to the effects of dissolved carbon dioxide on ocean acidity. Image courtesy of Russ Hopcroft, UAF/NOAA.
For many years, carbon dioxide (CO2) in Earth’s atmosphere has been increasing. Regardless of the reasons for this increase, the ocean is a “sink” for CO2, so as levels of atmospheric CO2 have increased, dissolved carbon dioxide in the ocean has increased as well.
When CO2 is absorbed by seawater, chemical reactions occur that reduce seawater pH, carbonate ion concentration, and saturation states of biologically important calcium carbonate minerals. These chemical reactions are termed “ocean acidification.”
Calcium carbonate minerals are the building blocks for the skeletons and shells of many marine organisms. In areas where most life now congregates in the ocean, the seawater is supersaturated with respect to calcium carbonate minerals. This means there are abundant building blocks for calcifying organisms to generate skeletons and shells. However, continued ocean acidification is causing many parts of the ocean to become undersaturated with these minerals, which is likely to affect the ability of some organisms such as corals, shellfish, echinoderms, and many marine plankton, to produce and maintain their shells.
Research on the impacts of ocean acidification on marine organisms and ecosystems is still relatively young. Since sustained efforts to monitor ocean acidification worldwide are just beginning, it is difficult to predict exactly how ocean acidification impacts will cascade throughout the marine food chain and affect the overall structure of marine ecosystems. Impacts have been observed in many species, however, and range from interference with calcification processes to reduced resistance to other environmental stresses such as increasing temperatures and pollution.
With the pace of ocean acidification accelerating, scientists, resource managers, and policymakers recognize the urgent need to strengthen the science as a basis for sound decision making and action.