The effects of constant and fluctuating elevated pCO levels on oxygen uptake rates of coral reef fishes.

Ocean acidification, resulting from increasing atmospheric carbon dioxide (CO) emissions, can affect the physiological performance of some fishes. Most studies investigating ocean acidification have used stable pCO treatments based on open ocean predictions. However, nearshore systems can experience substantial spatial and temporal variations in pCO. Notably, coral reefs are known to experience diel fluctuations in pCO, which are expected to increase on average and in magnitude in the future. Though we know these variations exist, relatively few studies have included fluctuating treatments when examining the effects of ocean acidification conditions on coral reef species. To address this, we exposed two species of damselfishes, Amblyglyphidodon curacao and Acanthochromis polyacanthus, to ambient pCO a stable elevated pCO treatment, and two fluctuating pCO treatments (increasing and decreasing) over an 8 h period. Oxygen uptake rates were measured both while fish were swimming and resting at low-speed. These 8 h periods were followed by an exhaustive swimming test (U) and blood draw examining swimming metrics and haematological parameters contributing to oxygen transport. When A. polyacanthus were exposed to stable pCO conditions (ambient or elevated), they required more energy during the 8 h trial regardless of swimming type than fish exposed to either of the fluctuating pCO treatments (increasing or decreasing). These results were reflected in the oxygen uptake rates during the U tests, where fish exposed to fluctuating pCO treatments had a higher factorial aerobic scope than fish exposed to stable pCO treatments. By contrast, A. curacao showed no effect of pCO treatment on swimming or oxygen uptake metrics. Our results show that responses to stable versus fluctuating pCO differ between species - what is stressful for one species many not be stressful for another. Such asymmetries may have population- and community-level impacts under higher more variable pCO conditions in the future.