Description: Ultraviolet B radiation (UVBR, 280-315 nm) is known to be a stress factor for Antarctic benthic algae and invertebrates. However, there is almost no available information regarding these effects at the community level. A two-factorial colonization experiment (UVR, three levels and grazing, two levels) was performed at an intertidal and a subtidal site on King George Island/I. 25 de Mayo. Structural parameters of the community were followed for fifteen and ten weeks, respectively. The effects on the intertidal community are presented in detail in Zacher et al. Subtidal communities were dominated macroscopically by colonial diatoms and green algal filaments. Ultraviolet radiation (UVR, 280-400nm) did not affect diatoms but exerted a group-specific effect on the macroalgal assemblage. Overall, red algal cover was negatively impacted by UVR whereas for green algal filaments a significant interaction between grazing and UVR was detected. Grazers introduced a shift in both micro- and macroalgal species composition and reduced the community biomass, with stronger effects when UVBR was absent. When comparing intertidal and subtidal experiments, community architecture and biomass production was markedly different at both sites, with higher biomass and more complex diatom composition at the subtidal spot. However, UVR and grazing affected both sites in a similar pattern. Our findings suggest that UVR and grazing play a key role in shaping the subtidal and intertidal benthic algal communities in Antarctica. UVR impact on subtidal communities seemed to be more complex than in the intertidal, exerting both direct and indirect effects on the community structure.