Fjords are physically isolated areas characterised by distinct oceanic conditions, making them perfect case- studies to investigate the consequences of local adaptation and in situ long-term acclimatisation in marine organisms. In these environments, global change drivers such as ocean acidification (OA) pose a significant threat to marine ecosystems, including keystone zooplankton species such as the copepod Calanus glacialis. Both local adaptation and long-term acclimatisation may lead to differential responses of C. glacialis to OA along its range of geographical distribution. Consequently, our study aimed to explore how these phenomena can influence the ability of various populations of C. glacialis to cope with future OA conditions. To do so, copepods were collected from four fjords spanning from southern Norway to Svalbard, and exposed in the laboratory under controlled conditions to seven different increasing concentrations of pCO 2 (300 to 30,000 ppm) over time to explore the threshold at which each population shows signs of sensitivity to OA. Targeted metabolomes for each treatment were profiled using high-performance LC-MS/MS, to quantify 49 metabolites involved in metabolism. Our analyses will help identifying signs of local acclimatisation, which would be visible if copepods show population-specific metabolome profiles in response to altered ocean chemistry. Moreover, breakpoint analyses will be used to characterise potential abrupt changes in the relationships between the pCO 2 stress gradient and copepods’ metabolomics response. Our work will provide crucial insights to help predicting the long-term impacts of OA on C. glacialis populations.