Kinetic bottlenecks to chemical exchange rates for deep-sea animals – Part 2: Carbon Dioxide

Abstract. Increased ocean acidification from fossil fuel CO₂ invasion, from temperature-driven changes in respiration, and from possible leakage from sub-seabed geologic CO₂ disposal has aroused concern over the impacts of elevated CO₂ concentrations on marine life. Discussion of these impacts has so far focused only on changes in the oceanic bulk fluid properties (ΔpH, Δ[∑ CO₂], 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 CO₂ at about the same rate at which O₂ is consumed. We analyse the basic properties controlling CO₂ export within the diffusive boundary layer around marine animals in an ocean changing in temperature (T) and CO₂ concentration in order to compare the challenges posed by O₂ uptake under stress with the equivalent problem of CO₂ expulsion. The problem is more complex than that for a non-reactive gas, since with CO₂ the influence of the seawater carbonate acid-base system needs to be considered. These reactions significantly facilitate CO₂ efflux compared to O₂ 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 CO₂ 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 O₂, this seems to be not necessary to facilitate CO₂ efflux. Instead, the main impacts of rising oceanic CO₂ will most likely be those associated with classical ocean acidification science. Regionally, as with O₂, the combination of T, P and pH/pCO₂ creates a zone of maximum CO₂ stress at around 1000 m depth.