An increased energetic cost of acid-base regulation in a high-CO2 environment has been predicted to affect key metabolic traits and performance in fish. This chapter examines the experimental evidence for the effect of elevated CO2 and low pH on minimum (resting) oxygen consumption rate (ṀO2min), critical oxygen level (O2crit), maximum oxygen consumption rate (ṀO2max), aerobic scope (AS; here defined as the absolute aerobic scope, i.e., the difference between ṀO2max and ṀO2min), and critical swimming speed (Ucrit), and discusses potential drivers of variability in the responses. Extreme increases in CO2 or pH tends to reduce ṀO2min, ṀO2max and Ucrit. The increases in CO2 expected from global climate change are minor from a physiological viewpoint, and many studies find no effect, while others find an increase in ṀO2min, and ṀO2max and AS can be unchanged or change in either direction. Routine oxygen consumption rate (ṀO2rout; the average oxygen consumption rate) is either unaffected or decreases, possibly influenced by changes in spontaneous activity, although changes in ṀO2min may drive the response as well. Because it is difficult to tease apart the causes of a change in ṀO2rout, unless activity and ṀO2min are both measured, ṀO2rout is not an appropriate measure for determining if there are added energetic costs of exposure to elevated CO2. There is a stronger relationship between ṀO2max and AS, than between AS and ṀO2min, and it cannot be assumed that AS will decrease if ṀO2min increases. The ecological implications of these variable and complex effects on traits related to aerobic metabolic rates are challenging to interpret. Since AS is unchanged in many cases, additional performance measures (e.g., behavior, growth, reproduction, survival) may be important, through non-oxygen limited mechanisms, when assessing the sensitivity of a given species to climate change relevant increases in CO2.
Lefevre S., in press. Effects of high CO2 on oxygen consumption rates, aerobic scope and swimming performance. Fish Physiology. Article (subscription required).
