How are benthic Antarctic communities affected by changing environmental conditions such as UV radiation (UVR)? UVR is known to affect many biological processes but almost no information exists whether these effects, visible on the molecular and cellular level, impair polar ecosystem structure. In order to obtain more information we studied the colonization of benthic primary producers in the Antarctic rocky intertidal and subtidal over a period of 106 and 70 days, respectively. 32 experimental units were installed at each site in a two-factorial design (UVR, three levels and grazers, two levels). Intertidal communities were dominated by single cell diatoms and green algal propagules. Limpet grazing reduced the algal biomass and macroalgal recruit density and further introduced a shift in both macro- and microalgal species composition. There were no negative UVR effects on the diatom assemblage. In contrast, significant UVR effects on the macroalgal assemblage were detected. UVAR (315-400nm) negatively impacted density and richness of recruits, whereas additional UVBR (280-315nm) caused a shift in species composition and led to a lower diversity of the macroalgal community by the end of the intertidal experiment. Effects were species-specific and showed that particularly young propagules were sensitive to UVR. No interactive effects of UVR and grazing were found. It is postulated that UVR has the power to change ecosystem structure in intertidal Antarctic macroalgal communities, which might have consequences for higher trophic levels. The effects of UVR and grazers on the subtidal community and a comparison of both field experiments are presented by Campana et al.