Beneficial effects of diel CO cycles on reef fish metabolic performance are diminished under elevated temperature.

Elevated CO levels have been shown to affect metabolic performance in some coral reef fishes. However, all studies to date have employed stable elevated CO levels, whereas reef habitats can experience substantial diel fluctuations in pCO ranging from ±50 to 600 μatm around the mean, fluctuations that are predicted to increase in magnitude by the end of the century. Additionally, past studies have often investigated the effect of elevated CO in isolation, despite the fact that ocean temperatures will increase in tandem with CO levels. Here, we tested the effects of stable (1000 μatm) versus diel-cycling (1000 ± 500 μatm) elevated CO conditions and elevated temperature (+2 °C) on metabolic traits of juvenile spiny damselfish, Acanthochromis polyacanthus. Resting oxygen uptake rates (ṀO) were higher in fish exposed to stable elevated CO conditions when compared to fish from stable control conditions, but were restored to control levels under diel CO fluctuations. However, the benefits of diel CO fluctuations were diminished at elevated temperature. Factorial aerobic scope showed a similar pattern, but neither maximal ṀO nor absolute aerobic scope was affected by CO or temperature. Our results suggest that diel CO cycles can ameliorate the increased metabolic cost associated with elevated CO, but elevated temperature diminishes the benefits of diel CO cycles. Thus, previous studies may have misestimated the effect of ocean acidification on the metabolic performance of reef fishes by not accounting for environmental CO fluctuations. Our findings provide novel insights into the interacting effects of diel CO fluctuations and temperature on the metabolic performance of reef fishes.