Ocean acidification (OA) is a global process that involves a reduction in the pH of the ocean. It is caused primarily by carbon dioxide from the atmosphere entering the ocean. Coastal acidification (CA) is a more localized, further reduction in pH. It is caused by high levels of respiration (typically by bacteria involved in decomposition), which releases carbon dioxide into the water. CA is often fueled by nutrients entering the water from land, stimulating outbreaks of phytoplankton that subsequently decompose on or near the seabed. CA happens in coastal waters because that is where high nutrient levels and algal blooms occur. The term ocean and coastal acidification (OCA) encompasses both the global and the local processes of acidification.
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