Ocean Acidification in Canadian Waters

About one quarter of the carbon dioxide (CO2) produced by human activities since the start of the Industrial Revolution (anthropogenic CO2, mostly from fossil fuel burning with much smaller contributions from cement production and land use change) has been taken up by the oceans.1 The oceans provide a great service to the planet by slowing down the accumulation of CO2 in the atmosphere, which is the major cause of global warming. However, this additional CO2 is changing the fundamental chemistry of the oceans. CO2 dissolves in the surface water to form carbonic acid, which upon dissociation results in a decrease in pH and the concentration of the carbonate ion, a building block of calcium carbonate (CaCO3) shells and skeletons. Ocean acidification (OA) refers to the decrease in pH and carbonate ion concentration due to the increasing anthropogenic CO2 in the ocean (Figure 1). The upper ocean pH has decreased by 0.1 pH unit (approximately 30 percent increase in acidity) over the past 200 years and is expected to fall an additional 0.3 pH unit by 2100 (approximately 150 percent increase in acidity).2 Oceans have not experienced such a rapid pH change for at least the last 66 million years, and possibly the last 300 million years. This raises serious concerns about the ability of marine organisms to adapt. During some of the acidification events in the Earth’s history, selective extinction and slow recovery of some species have occurred.3 Organisms that form CaCO3 shells and skeletons will experience direct impacts because acidity increases the solubility of CaCO3. Both ecologically and economically important organisms in a variety of tropic levels have CaCO3 structures. Some examples of ecologically important organisms are coccolithophores, which are the basis of some marine food chains, pteropods, which are a food source for a variety of northern fish, and warm and cold water corals which provide important habitats for other organisms. Economically