to a reduction in the pH of the ocean over an extended period, typically decades or longer, which is caused primarily by uptake of carbon dioxide from the atmosphere, but can also be caused by other chemical additions or subtractions from the ocean Anthropogenic ocean acidification refers to the component of pH reduction that is caused by human activity.”

Ocean Acidification and Changes in the CO2 and Carbonate System: Atmospheric carbon dioxide is absorbed by the ocean, where it reacts with seawater to form carbonic acid, which then dissociates to form bicarbonate ions (HCO3-) and hydrogen ions (H+). The increase in hydrogen ion activity (decrease in pH) is buffered by the carbonate system: some of the added hydrogen ions react with carbonate ions (CO32-) to form more bicarbonate, which makes CO32- less abundant. If atmospheric carbon dioxide rises slowly, ocean pH and carbonate ion levels will remain relatively stable due to dissolution of existing calcium carbonate deposits in the ocean (1,000s+ of years), weathering of terrestrial rock (100,000s+ of years), and tectonic processes (millions of years). However, the current rapid rise in atmospheric CO2 is faster than the time required for natural processes to buffer changes in the ocean carbonate system and avoid large changes in pH or ocean carbonate levels. Increased nutrient input from runoff can result in larger than usual algal blooms (i.e., eutrophication) that produce organic matter, which contributes to increases in CO2 when respired.

Effects on Biological Processes: The increase in CO2 and HCO3- availability has the potential to increase photosynthesis by some but not all photosynthesizers in the ocean. The decreased availability of CO32- at calcification sites makes it more difficult for many types of calcifying organisms, including some phytoplankton, corals and bivalves (clams and mussels) to build their calcareous shells or skeletons. Lastly, a decrease in pH may cause important physiological changes, many of which are associated with negative impacts such as increased energetic costs for regulating internal H+ concentrations.

Simultaneous Changes Impacting Biological Processes: Global increase in ocean temperature and decrease in dissolved oxygen are stressors for many marine organisms that will likely add to or amplify the impacts of ocean acidification, resulting in changes in the composition, abundance, and production of biological communities. In addition, regional human impacts—such as overfishing, eutrophication, pollution, or oil spills to name a few—also affect biological processes.

acidification is generally unknown to the public and has been the subject of substantially less scientific research than have the effects of CO2 on climate. However, the relative lack of attention given to ocean acidification belies its potential importance as a threat to marine organisms, ecosystems, and socioeconomic activities dependent on a healthy ocean (e.g., IPCC, 2011, Caldeira and Wickett, 2003; Gattuso and Hansson, 2011). A few key background data and the interactions illustrated in Box 1.1 help



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