Transgenerational effects (parent environment effects on offspring traits) have recently gained attention as a means of fast response to changing environmental conditions under climate change. In temperate and polar rocky coastal ecosystems, kelps form the base of complexly structured and highly diverse species associations. This study investigates the potential for temperature-related within-generation and transgenerational plasticity in a geographically isolated population of the brown alga Laminaria digitata from the island of Helgoland (North Sea). We obtained separate strains (genetic lines) from fertile field material and assessed sporophyte growth and gametogenesis at 5 and 15 °C. Rearing the resulting sporophytes in a full-factorial design at these two temperatures resulted in four temperature history treatments. A concluding experiment on growth, photosynthetic characteristics (Fv/Fm, rapid P-I curves, pigments) and storage compounds of five parental strains should allow for the separation of phenotypic plasticity from genetic effects in the response to 5 and 15 °C. Initial results indicate seasonality in the temperature response, with higher plasticity in material collected in spring than in summer. Gametogenesis was faster at 15 than at 5 °C, but recruitment was higher at 5 °C. A difference in morphology between sporophytes raised at 5 and 15 °C indicates that the temperature during growth had a stronger impact on morphology than the temperature during gametogenesis. Here we will show preliminary results of whether performance is related to genetic background or phenotypic plasticity among strains.