Abstract The change in optimal quantum efficiency (Fv/Fm) of the Arctic species Laminaria saccharina and Palmaria palmatawas investigated in a long-term experiment in situ under different radiation levels during the summer of 1997 in the Kongsfjord(Ny-Ålesund, Spitsbergen, Norway, 78°55.5'N, 11°56.0'E). Whole plants were incubated in an open box system made ofUV-transparent Perspex and exposed to solar radiation (>295 nm), solar radiation excluding UVB (<lambda> > 320 nm)and solar radiation excluding UVA + UVB (> 400 nm). Increasing radiation levels were simulated by transplantation of thepre-adapted algae from their growth depth at 2 m to a water depth of 1 m. Sensitivity to artificially increased UV radiation wasdetermined by exposure of algae from the three treatments to 6 h of strong UV radiation. P. palmata was relatively insensitive toincreasing UV radiation and recovered very fast and almost completely in 2 h. Even plants pre-cultured in ambient radiation levelsexcluding UVA + UVB or UVB only showed no photoinhibition after exposure to extra UV radiation in the laboratory. L. saccharinawas, in comparison to P. palmata, more sensitive and showed photoinhibition under solar radiation and solar minus UVB radiationafter transplantation from 2 to 1 m water depth. However, after 3 weeks at 1 m depth, Fv/Fm of L. saccharina was equal in alltreatments and restored to the original values at the start of the experiment. Sensitivity to extra UV radiation in the laboratory increasedin time, although recovery was also fast and occurred within 20 h.