The high Arctic Kongsfjorden (79°N) is mainly influenced 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 regularly found in Kongsfjorden – in addition to the previously prevailing arcto-boreal species Thysanoessa inermis and T. raschii (Buchholz et al. 2010). Krill occupy a central trophic position in the pelagic food-web. 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. In our study we aim at investigating the thermal limits of metabolic adaptability and the allocation of energy reserves in order to predict each species’ potential to persist in this challenging environment, in which temperature and food supply are among the most important factors determining survival. Total lipid content and lipid class composition show remarkable differences between species, reflecting the specific adaptations to the environments of origin. Furthermore, Thysanoessa spp. appear more stenotherm than the boreal and the subtropical krill species: the upper level of respiration is reached at temperatures < 12°C (Fig. 1). The other 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 (e.g. Zhukova et al. 2009). Accordingly, at least one of the latter species may profit from the increasing “Atlantification” of the Kongsfjord ecosystem. In turn, due to the differences in biochemical composition, a change in species composition may result in significant changes in the marine food-web of Kongsfjorden - especially for higher trophic levels.