Temperature and O2, but not CO2, interact to affect aerobic performance of European sea bass (Dicentrarchus labrax)

Climate change causes warming, decreased O₂, and increased CO₂ in marine systems and responses of organisms will depend on interactive effects between these factors. We provide the first experimental assessment of the interactive effects of warming (14 to 22°C), reduced O₂ (∼3 – 21 kPa O₂), and increased CO₂ (∼400 or ∼1000 µatm ambient CO₂) on four indicators of aerobic performance (standard metabolic rate, SMR, maximum metabolic rate, MMR, aerobic scope, and hypoxia tolerance, O_2crit), blood chemistry, and O₂ transport (P₅₀) of a marine fish, the European sea bass (Dicentrarchus labrax). Warming increased SMR and O_2crit (i.e. reduced hypoxia tolerance) as well as MMR in normoxia but there was an interactive effect with O₂ so that hypoxia caused larger reductions in MMR and aerobic scope at higher temperatures. Increasing CO₂ had minimal effects on SMR, MMR and O_2crit and did not show interactive effects with temperature or O₂ for any measured variables. Aerobic performance was not linked to changes in blood chemistry or P₅₀. Despite lack of effects of CO₂ on aerobic performance, increased CO₂ induced 30% mortality of fish exercised in low O₂ at 22°C indicating important threshold effects independent of aerobic performance. Overall, our results show temperature and O₂, but not CO₂, interact to affect aerobic performance of sea bass, disagreeing with predictions of the oxygen- and capacity-limited thermal tolerance hypothesis.