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
Although many organisms with CaCO3 shells and skeletons showed negative effects from OA, some were not affected and others even thrived in acidified water. Given the variable responses of individual types of organisms to ocean acidity, together with other stressors such as warming, deoxygenation, and pollution, it is difficult to predict how whole marine ecosystems respond to climate change and acidification. To investigate marine ecosystem response,
Since CaCO3 shells and skeletons are more soluble at lower temperatures and higher pressures, high latitude and deep water ecosystems will be more vulnerable to the stress of OA. Also, the solubility of gases, including CO2, is higher in cold water than in warm water. Thus, although OA is a global threat, cold waters in Canada may be particularly vulnerable. Ocean acidification also lowers the oceans’ capacity to absorb anthropogenic carbon directly (the solubility pump) and through changes in primary productivity and phytoplankton species composition (the biological pump), providing a negative feedback to climate change.
Ocean acidification is controlled and enhanced by various mechanisms and Canada’s three oceans have distinct drivers (Figure 2). In the Pacific, wind-driven upwelling brings the intermediate depth water, corrosive to organisms with CaCO3 shells and skeletons, to the surface.8 This upwelling is a natural phenomenon and occurs seasonally. The intermediate water of the Pacific has inherently higher CO2 than Atlantic water due to the accumulation of CO2 produced by microbial respiration. The supplementary addition of anthropogenic CO2 has, however, lowered the pH of this water to a critical level. In the Arctic, the first observations of corrosive surface ocean waters were reported.9 The Arctic Ocean receives a large amount of fresh water from rivers, seasonal ice melt, and glacial meltwater. This fresh water has little buffer capacity and effectively reduces the pH of Arctic waters. Decreasing ice cover enhances the uptake of
Ocean acidification in coastal regions exhibits much higher temporal and spatial variability than in open oceans. Seasonally variable freshwater inputs
Canada is one of the first countries in the world to experience the adverse impacts of OA. Ocean acidification is likely to alter the structure and function of its marine ecosystems. Hence, fishing industries, subsistence fisheries by Indigenous communities, and tourism will be directly influenced. The results of these changes will threaten economies, especially those directly connected with fisheries, culture, and subsistence of Indigenous peoples and eco-tourism.
Beyond Canadian waters, global food security risks caused by OA and climate change, especially for developing nations, need to be emphasized. Reducing nutrient input to the coastal area, developing new aquaculture practices such as close monitoring of intake water to the hatcheries, and cutting down the emissions of acidic gases such as NOx and SOx can help to slow down the progress of local OA and thus manage risks. However, global ocean acidification will inevitably progress further unless the global emission of CO2 resulting in the accumulation of CO2 in the atmosphere is controlled and reduced.
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