Description: The kelp Alaria esculenta represents a key species in high Arctic marine fjord ecosystems. However, the European Arctic is currently experiencing extensive environmental change. Glacial fjord systems, such as Kongsfjorden (Spitsbergen, Svalbard), are subjected to rising temperature, increased freshwater inflow from glaciers and melting snow and a changing ultraviolet (UV) radiation regime related to stratospheric ozone depletion. Thus, in addition to natural seasonality, sessile organisms require acclimation in order to adapt to an environment in transition. We examined the physiological and ultrastructural responses of A. esculenta to the combined exposure to hyposalinity and UV radiation. Photosynthetic quantum yield slightly decreased during a low-salinity treatment of 7 d. Exposure to UV radiation also lowered quantum yield, but specimens previously treated with hyposalinity were significantly less susceptible to UV than nontreated individuals. Concomitant with a loss of chlorophyll during the hyposaline treatment, phlorotannin and antioxidant contents were maintained, and samples treated with low salinities exhibited higher UV-screening characteristics as demonstrated by significantly higher absorption ratios at 300/680 nm. Ultrastructural analyses revealed a treatment-dependent swelling of cell walls and accumulations of phlorotannin-containing vesicles. Our findings point to a strategy by which kelps apply a fast and cost-efficient redistribution of phlorotannins rather than increased synthesis as a general stress response to different environmental drivers in contrast to stress-specific responses. The notion that acclimation to one stressor (low salinity) reflects increased tolerance towards a second stressor (UV radiation) supports the concept of 'cross-acclimation' as established for higher plants but not yet for seaweeds.