Effect of solar ultraviolet radiation on growth in the marine macroalga Dictyota dichotoma (Phaeophyceae) at Helgoland and its ecological consequences

At Helgoland, in the North Sea, growth of the high sublittoral brown macroa lga Dictyota dichotoma (Hudson) Lamoroux was examined in October (the time of tetraspore release) in an outdoor tank by exposing 2-day-old germlings t o four solar radiation treatments achieved with different filter materials or an additional artificial light source: photosynthetically active radiati on (PAR; 395-700 nm), PAR plus ultraviolet (UV)-A (320-700 nm), full solar spectrum, or solar spectrum plus artificial UV radiation (UVR). Based on le ngth measurements over a period of 3 weeks, the growth rate in germlings st rongly decreased in conditions with UVR compared to PAR: by 14% under PAR+U V-A, by 31% under the full solar spectrum and by 65% with additional UVR. A lthough growth rates of germlings under UVR were reduced mainly in the firs t week, the plants did not regain the size of the untreated plants even aft er 9 weeks. Regardless of the exposure, no defects in morphology or anatomy including the exposed apical meristem were detected, except for a reductio n in cell division rates perhaps due to additional cost for photoprotective or repair mechanisms. Depending on the actual position of D. dichotoma pla nts in the natural habitat, individuals in high positions receive substanti al amounts of the more harmful UV-B while those lower down might only recei ve UV-A during part of the day, thus the effect of UV-B on the growth of D. dichotoma will depend on its position in the field. The effects of tidal v ariation of the light climate and the implications of our results for the z onation of D. dichotoma are discussed.