Abstract
High‐latitude ecosystems are simultaneously warming and acidifying under ongoing climate change. Arctic cod ( Boreogadus saida ) are a key species in the Arctic Ocean and have demonstrated sensitivity to ocean warming and acidification as adults and embryos, but their larval sensitivity to the combined stressors is unknown. In a laboratory multi‐stressor experiment, larval Arctic cod were exposed to a combination of three temperatures (1.8, 5 and 7.3°C) and two carbon dioxide (pCO 2 ) levels (ambient: 330 μatm, high: 1470 μatm) from hatching to 6‐weeks of growth. Mortality rates were highest at 7.3°C (5% day −1 ); however, both growth and morphometric‐based condition were also highest at this temperature. When these metrics were assessed via a mortality: growth (M:G) ratio, 5°C appeared to be an optimal temperature for net population biomass, as faster growth at 7.3°C did not fully compensate for higher mortality. In contrast, although morphometric‐based condition was lowest at 1.8°C, lipid‐based condition was highest, which may reflect prioritization of lipid storage at cold temperatures. The capacity of larval Arctic cod to acclimate to a range of temperatures was exhibited by two lipid‐based indicators of membrane fluidity, including a ratio of unsaturated to saturated fatty acids and a ratio of polar lipids to sterols. The effects of elevated pCO 2 were subtle, as well as temperature‐ and metric dependent. When exposed to elevated pCO 2 levels, Arctic cod at 1.8°C exhibited signs of lipid dysregulation, suggesting potential interference with membrane acclimation; larvae at 5°C were in lower morphometric‐based condition; and larvae at 7.3°C had higher activity eicosanoid substrates, indicating possible physiological stress. Overall, Arctic cod physiological response to temperature variation was more pronounced than their response to elevated pCO 2 . Future projections of pCO 2 effects on Arctic cod health in a warming ecosystem will need to consider the complexity of temperature‐dependence and the specificity of multiple physiological responses.