Description: The high Arctic Kongsfjorden (79°N) is influenced mainly by cold Arctic but also warmer Atlantic water masses. In recent years, the proportion of Atlantic inflow increased, attributed to climate change. Concurrently, two boreal and one subtropical krill species are now being found regularly in Kongsfjorden – in addition to the previously prevailing arcto-boreal species Thysanoessa inermis and T. raschii. Generally, krill occupy a central trophic position as secondary producers linking primary production to higher trophic levels. Many adult fish, seabirds and marine mammals rely on them as a food source. Although a change in a krill population may have a significant impact on the ecosystem, knowledge on the physiological performance of the species inhabiting Arctic waters is still scarce. We aim at determining the ecophysiology of this key group within the challenging Arctic ecosystem. Using non-invasive optical oxygen sensors, respiration measurements helped to characterize the species’ metabolic reaction to temperature variation, i.e. to reveal the thermal limits of metabolic adaptability. Thysanoessa spp. appear more stenotherm than the boreal and the subtropical krill species: the upper level of respiration is reached at temperatures 〈 12°C while mortality increases and the animals are less tolerant to ambient oxygen concentrations. The species’ thermal limit of adaptive capacity found may explain its current arcto-boreal distribution. In contrast, the other boreal and a subtropical krill species show a higher tolerance to temperature changes, which may support the species’ success in northward expansion as reported through increasing abundances at lower latitudes. Accordingly, at least one of the latter species may profit from the increasing “Atlantification” of the Kongsfjord ecosystem due to its superior physiological plasticity.