At Helgoland, North Sea, growth of the high sublittoral brown macroalga Dictyota dichotoma (Hudson) Lamoroux was examined in October (time of tetraspore release) in an outdoor tank by exposing 2-day old germlings to 4 solar radiation treatments achieved with different filter materials or an additional artificial light source: PAR (395-700 nm), PAR + UV-A (320-700 nm), full solar spectrum, solar spectrum + artificial ultraviolet radiation (UVR). Based on length measurements over a period of 3 weeks, growth rate in germlings strongly decreased in conditions with UVR compared to PAR: by 14% under PAR + UV-A, by 31% under the full solar spectrum and by 65% with additional UVR. Although growth rates of germlings under UVR were reduced mainly in the first week, the plants did not regain the size of the untreated plants even after 9 weeks. Regardless of the exposure, no defects in morphology or anatomy including the exposed apical meristem were detected, except for a reduction in cell division rates perhaps due to additional cost for photoprotective or repair mechanisms. Depending on the actual position of D. dichotoma plants in the natural habitat, individuals in the high positions receive substantial amounts of the more harmful UV-B while those further down might only receive UV-A during part of the day, thus the effect of UV-B on growth of D. dichotoma will depend on its position in the field. The effects of tidal variation of the light climate and the implications of our results for the zonation of D. dichotoma are discussed.