Kinetic bottlenecks to chemical exchange rates for deep-sea animals – Part 2: Carbon Dioxide
Increased ocean acidification from fossil fuel CO 2 invasion, from temperature-driven changes in respiration, and from possible leakage from sub-seabed geologic CO 2 disposal has aroused concern over the impacts of elevated CO 2 concentrations on marine life. Discussion of these impacts has so far focused only on changes in the oceanic bulk fluid properties (ΔpH, Δ[∑ CO 2], etc.) as the critical variable and with a major focus on carbonate shell formation. Here we describe the rate problem for animals that must export CO 2 at about the same rate at which O 2 is consumed. We analyse the basic properties controlling CO 2 export within the diffusive boundary layer around marine animals in an ocean changing in temperature ( T) and CO 2 concentration in order to compare the challenges posed by O 2 uptake under stress with the equivalent problem of CO 2 expulsion. The problem is more complex than that for a non-reactive gas, since with CO 2 the influence of the seawater carbonate acid-base system needs to be considered. These reactions significantly facilitate CO 2 efflux compared to O 2 intake at equal temperature, pressure and fluid flow rate under typical oceanic concentrations. The effect of these reactions can be described by an enhancement factor, similar to that widely used for CO 2 invasion at the sea surface. While organisms do need to actively regulate flow over their surface to thin the boundary layer to take up enough O 2, this seems to be not necessary to facilitate CO 2 efflux. Instead, the main impacts of rising oceanic CO 2 will most likely be those associated with classical ocean acidification science. Regionally, as with O 2, the combination of T, P and pH/ pCO 2 creates a zone of maximum CO 2 stress at around 1000 m depth.