Abstract
Microplastics (MP) are a relevant stressor in Arctic marine ecosystems. Their small size and ubiquity make them readily ingestible by zooplankton, placing copepods at the entry point of MP into high-latitude food webs. Calanus hyperboreus is a key Arctic species characterised by exceptionally large lipid reserves that fuel overwinter survival, support higher trophic levels as energy-rich prey, and facilitate carbon sequestration via the lipid pump. Successful lipid accumulation is therefore critical for individual fitness and ecosystem functioning, and MP-driven disruptions may have cascading effects on food-web dynamics and regional carbon sequestration. To address this, we exposed wild-caught C. hyperboreus copepodite stage V (CV) from southeastern Greenland to pristine and biofouled MP under feeding and food-deprived conditions during late summer (July–August 2024) aboard RRS Sir David Attenborough. Across experiments, control copepods exhibited seasonal shifts consistent with increasing lipid reserves and changes in fatty-acid composition over time. Responses to MP exposure were strongly context dependent. Pristine MP produced modest and variable changes in lipid content and fatty-acid profiles, whereas the most pronounced shifts were observed during late-summer exposure to biofouled MP under food-replete conditions, including reduced lipid mass and altered fatty-acid composition characterised by lower long-chain monounsaturated fatty acids and higher relative contributions of docosahexaenoic acid. Together, these patterns suggest that MP exposure during periods of active lipid accumulation may interfere with normal energy storage and fatty-acid allocation. These findings also identify a potentially crucial late-summer exposure context in which effects of MP on copepod lipid metabolism are most evident, rather than isolating biofouling as a sole causal factor. Overall, this study highlights the importance of seasonal physiological state and particle conditioning in shaping MP impacts on Arctic zooplankton, with potential implications for food-web dynamics and lipid-driven carbon export.