Future ocean acidification will be amplified by hypoxia in coastal habitats

Ocean acidification is elicited by anthropogenic carbon dioxide emissions and resulting oceanic uptake of excess CO2 and might constitute an abiotic stressor powerful enough to alter marine ecosystem structures. For surface waters in gas-exchange equilibrium with the atmosphere, models suggest increases in CO2 partial pressure (pCO(2)) from current values of ca. 390 mu atm to ca. 700-1,000 mu atm by the end of the century. However, in typically unequilibrated coastal hypoxic regions, much higher pCO(2) values can be expected, as heterotrophic degradation of organic material is necessarily related to the production of CO2 (i.e., dissolved inorganic carbon). Here, we provide data and estimates that, even under current conditions, maximum pCO(2) values of 1,700-3,200 mu atm can easily be reached when all oxygen is consumed at salinities between 35 and 20, respectively. Due to the nonlinear nature of the carbonate system, the approximate doubling of seawater pCO(2) in surface waters due to ocean acidification will most strongly affect coastal hypoxic zones as pCO(2) during hypoxia will increase proportionally: we calculate maximum pCO(2) values of ca. 4,500 mu atm at a salinity of 20 (T = 10 A degrees C) and ca. 3,400 mu atm at a salinity of 35 (T = 10 A degrees C) when all oxygen is consumed. Upwelling processes can bring these CO2-enriched waters in contact with shallow water ecosystems and may then affect species performance there as well. We conclude that (1) combined stressor experiments (pCO(2) and pO(2)) are largely missing at the moment and that (2) coastal ocean acidification experimental designs need to be closely adjusted to carbonate system variability within the specific habitat. In general, the worldwide spread of coastal hypoxic zones also simultaneously is a spread of CO2-enriched zones. The magnitude of expected changes in pCO(2) in these regions indicates that coastal systems may be more endangered by future global climate change than previously thought.