The key species of many temperate and Arctic kelp forests are strongly constricted in their vertical and latitudinal distribution by their tolerance to UV radiation and by their temperature demands. The stages in the life history of seaweeds most sucseptible to environmental perturbaties are their unicellular propagation units. Therefore we determined the temperature- dependent UV- susceptibility of zoospores of Saccharina latissima and Laminaria digitata from the Arctic (Spitsbergen) as well as from temperate waters (Helgoland), of Arctic Alaria esculenta and of temperate L. hyperborea. The zoospores were exposed to three radiation conditions (photosynthetic active radiation PAR, PAR + UV A radiation or PAR + UV A + UV B radiation) and four temperatures (2, 7, 12, 18 °C). Subsequently the germination rates were assessed. Under elevated water temperatures the tolerance of zoospores of A. esculenta, S. latissima and L. hyperborea to UV B- stress were not changed. Laminaria digitata from the Arctic, however, improved its UV-tolerance with increasing temperature. Zoospores of the summer reproducing L. digitata from Helgoland reached under the whole light spectrum their sublethal limit at 18 °C, a temperature often recorded in summer at Helgoland today. So in the Arctic the upper distribution limit of L. digitata will increase with rising temperatures and in the North Sea the southern distribution boundary of this species will be shifted north. This shows the considerable effect global change has on seaweed distribution.