Stressed Overwintering Bottleneck Hypothesis: Ocean Warming and Acidification Synergistically Disrupt Arctic Zooplankton Overwintering

Ocean warming (OW), associated with increasing influence of warm Atlantic water masses at depth, and acidification (OA) are reshaping environmental conditions in Arctic overwintering habitats. However, their potential synergistic effects are poorly understood, especially during the Polar Night when marine species are particularly vulnerable to stressors. Here, we tested our Stressed Overwintering Bottleneck Hypothesis (SOBH) which predicts that warming disrupts the overwintering of the dominant pan‐Arctic copepod Calanus glacialis , a pivotal secondary producer, by impairing fitness‐related traits underpinning survival and reproduction. We exposed C. glacialis to current and projected future OW levels (0°C and 4°C) and OA levels (pH 8.0 and 7.4–7.3) for 53 days during the Arctic Polar Night. We assessed survival, development, and physiological (oxygen consumption, lipid depletion) and molecular mechanisms (DNA damage and genes related to oxidative stress and damage repair). OW alone did not affect C. glacialis mortality, while OA increased copepod survival at 0°C. Notably, their combined effects (OWA) synergistically doubled mortality, as predicted by SOBH. Warming also accelerated moulting from copepodite stage V to adulthood in December, and increased respiration, resulting in lipid reserves being exhausted by early March, approximately 1–4 months prior to the spring algal bloom, further supporting SOBH. DNA damage and gene expression patterns indicated low investment in maintenance and damage repair. Collectively, these findings reveal mechanisms by which OW and OA synergistically threaten overwintering Calanus copepods by increasing mortality, accelerating moulting, raising metabolic rates, and causing early lipid depletion. These effects generate cross‐seasonal phenological mismatches among overwintering survival, energy reserves, reproduction, and primary production. Such stressed overwintering bottlenecks in dominant secondary producers like Calanus copepods provide explanations for how OW and OA can constrict Arctic marine food webs. From a broader perspective, SOBH provides a conceptual framework for understanding how multiple stressors that disrupt overwintering in key food web components could reshape biodiversity, productivity and trophic transfer across ecosystems.