Abstract Chondrus crispus and Mastocarpus stellatus both inhabit the intertidal and upper sublittoral zone of Helgoland, but with C.crispus generally taking a lower position. Measurements of chlorophyll fluorescence, activity of ribulose-1,5-bisphosphatecarboxylase/oxygenase (RubisCO), and content and composition of UV absorbing mycosporine-like amino acids (MAAs) were conducted inthe laboratory, to test whether susceptibility to UV radiation may play a role in the vertical distribution of these two species. Effective andmaximal quantum yield of photochemistry as well as maximal electron transport rate (ETRmax) in C. crispus were more strongly affected byUV-B radiation than in M. stellatus. In both species, no negative effects of the respective radiation conditions were found on total activity ofRubisCO. Total MAA content in M. stellatus was up to 6-fold higher than in C. crispus and the composition of MAAs in the two specieswas different. The results indicate that, among others, UV-B sensitivity may be a factor restricting C. crispus to the lower intertidal andupper sublittoral zone, whereas M. stellatus is better adapted to UV radiation and is therefore more competitive in the upper intertidal zone.Key words Chondrus crispus · Chlorophyll fluorescence · Mastocarpus stellatus · Mycosporine-like amino acids · UV radiation