The oceans have absorbed almost 50 % of the CO2 humans released into the atmosphere, which has driven CO2 in the oceans to rise, causing - because of the effect of increasing CO2 in producing carbonic acid - a decline in ocean pH, termed ocean acidification. Ocean acidification has been argued to threaten calcifying organisms, such as corals and planktonic calcifiers, as coccolhitophores and pteropods.
However, CO2 does not only affect pH, but also affects the efficiency of aquatic aerobic respiration, which depends on the relative partial pressures of oxygen and CO2 in the water with which the organisms are to exchange their gases. In addition, reduced pH reduces the binding affinity for oxygen in blood. As a result, increased partial pressure of CO2 reduces the efficiency of aerobic respiration of aquatic organisms and, most importantly, raises the thresholds of oxygen required to support respiration.
Although not as widely discussed as ocean acidification, the role of increased CO2 in raising the oxygen levels required to support aerobic respiration in the ocean is most important. Oxygen concentrations are declining in the ocean as CO2 levels increase, particularly in coastal waters but also in the open ocean. Ocean deoxygenation is an emerging problem, that is already expressed in mass mortality events in hypoxic coastal waters, which are growing worldwide.
In a recent paper, my co-workers and I demonstrated how elevated CO2 acts as a hinge, connecting two otherwise independent threats to marine life, acidification and hypoxia. In particular, we demonstrated how elevated CO2 in Pacific waters off the upwelling region along the Chilean coast (Mayol et al. 2012).
Our results showed that a significant fraction of the water column along the Chilean sector of the Humboldt Current System suffers from CO2–driven compromises to biota, including corrosive waters to calcifying organisms, respiratory stress to organisms or both. Ocean acidification affects most waters below 150 m depth, while respiratory compromises due the combined effect of reduced O2 and increased CO2 are located within the 200 to 400 m layer. Only those waters shallower than 100 m present conditions free of stress to aerobic organisms.
Increased CO2 in the future may increase the thickness of the water column where ocean acidification and the relative concentrations of CO2 and O2 compromise marine life, therefore, compressing the vertical extent of the stress-free habitat. Whereas these impacts will affect vulnerable habitats first, such as the Humboldt Current System off Perú and Chile, these impacts will eventually extend across the world’s ocean.
Warm tropical waters are also of concern, as respiratory demands are enhanced at high temperature to the extent that oxygen concentrations are already near critical levels at the saturation concentrations, which are reduced, due to reduced solubility in warm waters. Increasing CO2 in warm tropical waters may bring aerobic organisms close to their respiratory limits even when the waters are saturated in oxygen. As increased CO2 is associated with further warming, breathing stresses are likely to compromise tropical marine life in the future.
Ocean warming, deoxigenation and acidification are, thus, connected pressures that increasingly compromise marine life.
The good news is that corals may be less vulnerable to ocean acidification, are more resistant than previously thought (e.g. Pandolfi et al. 2011). My colleagues at the UWA Oceans Institute have published today a paper unveiling the mechanism allowing most corals to withstand lower pH values than hitherto believed. Malcolm McCulloch and co-workers (2012) showed that corals up-regulate pH at their site of calcification such that internal changes in pH are approximately one-half of those in ambient seawater, and calculated that warming may counteract the effects of lowered pH on coral calcification (cf. https://theconversation.com/some-corals-could-survive-a-more-acidic-ocean-6203). Hence, the future of coral reefs in a high-CO2 world may not be as grim as we thought.
Mayol, E., S. Ruiz-Halpern, C. M. Duarte, J. C. Castilla, and J. L. Pelegrí. 2012. Coupled CO2 and O2-driven compromises to marine life in summer along the Chilean sector of the Humboldt Current System. Biogeosciences 9: 1183-1194.
McCulloch, M., J. Falter, J. Trotter and Paolo Montagna. 2012. Coral resilience to ocean acidification and global warming through pH up-regulation. Nature Climate Change, doi:10.1038/nclimate1473
Pandolfi, J.M., S.r. Connolly, D.J. Marshall, and A.L. Cohen. 2011. Projecting Coral Reef Futures Under Global Warming and Ocean Acidification. Science 333: 418-422