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Kelps, perennial brown seaweeds of the order Laminariales, are foundation species in Arctic coastal ecosystems. Presently, their ability to persist under polar night conditions might be significantly affected by increasing winter temperatures. We assessed physiological parameters (photosynthesis, pigment content, respiration, carbohydrate storage) in 2 species of Arctic kelp, the boreal-temperate Saccharina latissima and the Arctic-endemic Laminaria solidungula, during the polar night 2016/17. Algae were sampled from Kongsfjorden, Svalbard, shortly before the onset of the dark period in October, and at the end of the polar night in early February. Analyses were conducted for different tissue sections along the phylloid (meristem, centre region, distal region). Data suggest that kelp maintain their photosynthetic competence throughout the entire winter period, as indicated by photosynthesis vs. energy (PE) curve parameters and photosynthetic pigment contents. Overall laminarin content was reduced by 96% in S. latissima and by 90% in L. solidungula during winter, indicating that this storage glucan fuelled metabolic function during the polar night. Marked differences in laminarin content between the phylloid regions and across species indicated specific adaptive mechanisms between boreal-temperate and Arctic-endemic kelp. We suggest that laminarin turnover represents a sensitive parameter for assessing kelp physiology under a changing temperature regime.