The Risk of Ocean Acidification to Ocean Ecosystems

Ocean acidification is a process that refers to major changes to the ocean’s carbonate chemistry, mainly caused by ocean uptake of anthropogenic emissions of carbon dioxide. This process involves a decrease in ocean pH (important for regulation of the internal acid balance and physiological health of many organisms) carbonate ions and calcium carbonate minerals such as aragonite and calcite (important for shell and skeleton builders) and an increase in bicarbonate ions (important for algal photosynthesis). To understand what marine ecosystems may look like in the future if carbon emissions continue unabated, it is necessary to know the severity of the perturbation that different ecosystems will be exposed to and their ability to adapt within the time-scales of change. The severity and speed of ocean acidification, the exposure and vulnerability of the component organisms of an ecosystem to ocean acidification and their role in an ecosystem contribute to the risk of impacts to ecosystem structure and function. Although there are great uncertainties moving from impacts on individual organisms to impacts on complex marine ecosystems, these basic changes to marine chemistry pose a substantial risk to marine ecosystem structure and function through the impacts on the growth, physiology, behaviour, predator-prey interactions, competitiveness and population dynamics of individual species and how these may cascade through the rest of the ecosystem. Some organisms are able to adapt to ocean acidification, especially if food resources are high, by trading-off energy from one physiological function to another, although this may impact their long-term survival and ecosystem function. Foodwebs where vulnerable organisms provide key trophic links, especially those exposed to undersaturated waters in polar, sub-polar and upwelling regions where severity will be greatest, will be at high risk of impact from ocean acidification. However, ecosystems formed by the aragonitic skeletons of deep-sea or tropical corals are also at high risk of impact from ocean acidification, either due to high severity, exposure or vulnerability or a combination of all three. Risk increases further when ocean acidification acts in concert with other global and/or local ocean stressors. Predicting impacts of changing biodiversity or community dynamics on ecosystem structure and function also requires expanding the scope of current experimental research to examine multi-stress impacts in multi-level foodwebs and complex ecosystems. Key Messages • Basic changes to marine chemistry pose a substantial risk to marine ecosystem structure and function through the impacts on the growth, physiology, behaviour, predator-prey interactions, competitiveness and population dynamics of individual species and how these cascade through the rest of the ecosystem; • Though some organisms may be able to adapt to ocean acidification by trading-off energy from one physiological function to another, this may impact their long-term survival and ecosystem function; • Risk of impacts will depend on the severity (of which strength and speed are components) of ocean acidification, and the exposure and vulnerability of organisms to ocean acidification especially those playing key roles in ecosystems. The higher the severity, exposure and vulnerability the greater the risk of impact to numbers of organisms and therefore to foodwebs and ecosystems; and • Foodwebs where vulnerable organisms provide key trophic links, especially those exposed to undersaturated waters in polar, sub-polar, deep sea and upwelling regions where severity will be greatest, will be at high risk of impact from ocean acidification. However, as ocean acidification progresses with increasing anthropogenic CO2 emissions, ecosystems across the whole ocean will be at risk.