Synergistic effects of temperature extremes, hypoxia, and increases in CO 2 on marine animals: From Earth history to global change

Currently rising CO 2 levels in atmosphere and marine surface waters as well as projected scenarios of CO 2 disposal in the ocean emphasize that CO 2 sensitivities need to be investigated in aquatic organisms, especially in animals which may well be the most sensitive. Moreover, to understand causes and effects, we need to identify the physiological processes that are sensitive to CO 2 beyond the current emphasis on calcification. Few animals may be acutely sensitive to moderate CO 2 increases, but subtle changes due to long‐term exposure may already have started to be felt in a wide range of species. CO 2 effects identified in invertebrate fauna from habitats characterized by oscillating CO 2 levels include depressed metabolic rates and reduced ion exchange and protein synthesis rates. These result in shifts in metabolic equilibria and slowed growth. Long‐term moderate hypercapnia has been observed to produce enhanced mortality with as yet unidentified cause and effect relationships. During future climate change, simultaneous shifts in temperature, CO 2 , and hypoxia levels will enhance sensitivity to environmental extremes relative to a change in just one of these variables. Some interactions between these variables result from joint effects on the same physiological mechanisms. Such interactions need to be considered in terms of future increases in atmospheric CO 2 and its uptake by the ocean as well as in terms of currently proposed mitigation scenarios. These include purposeful injection of CO 2 in the deep ocean or Fe fertilization of the surface ocean, which reduces subsurface O 2 levels. The resulting ecosystem shifts could develop progressively, rather than beyond specific thresholds, such that effects parallel CO 2 oscillations. It is unsure to what extent and how quickly species may adapt to permanently elevated CO 2 levels by microevolutionary compensatory processes.