To estimate the potential effects of climate change on polar marine macroalgae, studies on interactive stress effects of multiple climate-related parameters are essential. Interactions of temperature, radiation and salinity on two different life history stages of Alaria esculenta (L.) Greville from the Kongsfjord (Spitsbergen) were investigated for the first time within this study. Adult macroscopic sporophytes of A. esculenta were exposed to different temperatures between 4 and 21 degrees C combined with artificial irradiation conditions [photosynthetically active radiation, ultraviolet (UV) radiation: UV-A/UV-B, first experiment] and with different salinities [34, 28, 20 practical salinity units (p.s.u.) second experiment]. Effects of photosynthetic activity were determined by measuring variable chlorophyll fluorescence of photosystem II. Germination success of young microscopic zoospores of A. esculenta was studied under multifactorial stress. Zoospore suspensions were exposed to the three different salinities and irradiation conditions at four temperatures between 2 and 16 degrees C. Overall, A. esculenta exhibited a highly stage-specific susceptibility towards the experimental treatments. In both experiments using sporophytes, photosynthetic activity showed significant temperature effects and only very few significant radiation and salinity effects. Microscopic stages of A. esculenta were shown to be more sensitive than the adult macroscopic stages, since germination capacity of zoospores was significantly affected by temperature and salinity changes, and interactions of both. These results suggest that multiple stress factors interact synergistically. Temperature seems to be a predominant environmental parameter for the kelp A. esculenta. Overall, A. esculenta proved to be relatively tolerant and adaptable to increasing temperature and UV radiation, as well as to diluted salinities, but only up to a specific limit.