The geographical distribution of marine macroalgae is substantially influenced by the sea water temperatures affecting both growth and reproduction of the algae. The sublitoral rocky coast of Helgoland, North Sea, is dominated by brown macroalgae of the genus Laminaria (L. digitata and L. hyperborea) which are key structuring elements. Laboratory experiments on the induction of fertile tissue and casual observations during the hot summers 2003 and 2006 led to the assumption that elevated water temperatures above 19 °C may have a negative effect on reproduction of Laminaria digitata. This was investigated in more detail during this study. In a laboratory experiment, six water temperatures between 10 and 22 °C were applied to sporogenous tissue of the alga. The amount and germination capacity of the spores were determined after 3 and 7 days of temperature incubation. It became evident that the release of spores was triggered by high temperatures above 17 °C within 3 days. After 7 days temperature incubation, only a low amount of spores was still released at temperatures between 19 and 22 °C. In contrast, the germination rate of spores decreased with increasing temperatures. At the threshold temperature for thesurvival of Laminaria digitata (22 °C) only a few spores have been released, and none of them germinated. In the field study, in a set of 50 marked algae the amount of released spores and their germination capacity under optimal conditions was followed over the season (May to October). While the amount of released spores did not changed significantly during the season, the germination rate revealed a significant decrease during Juli and August when sea water temperatures rose above 16 °C. Germination rates increased again in September when sea water temperatures had fallen below 16 °C. Although laboratory and field experiments show the same trend, the expected lag phase for recovery of the sporogenous tissue and the effective temperature leading to a decrease in germination rate were dissimilar suggesting additional factors acting in the field.In summary it became evident that 'normal' summer temperatures at Helgoland as had been present during 2007, already have a negative effect on reproduction of Laminaria digitata. Thus the expected increase of summer water temperatures may become a threat to the Helgolandic kelp population of Laminaria digitata.