In polar areas, benthic diatoms are regarded to play a major role in supplying energy to the benthic fauna, particularly prior to the release of microalgae from sea ice and the phytoplankton bloom. As phototrophs, benthic polar diatoms have to contend not only with dark polar nights but also with darkness due to sea-ice and snow cover that can prevail in the littoral zone for additional months. Upon sea ice break-tip the autotrophs are suddenly exposed to high light intensities including ultraviolet radiation. The aim Of Our study was to mimic a sudden spring-time sea ice break-up, focusing Oil the ultraviolet part of the solar spectrum. We therefore exposed a semi-natural Community of benthic diatoms to light burst after a period of total darkness. We studied the effects of different spectral qualities: photosynthetically active radiation (PAR, 400-700 nm: P treatment), PAR+ UV-A (UV-A 320-400 nm; PA treatment), and PAR+UV-A+UV-B (UV-B 280-320 nm; PAB treatment) on cell number (growth), species composition and optimum quantum yield (F-v/F-m) in 2 separate experiments where diatoms were kept in darkness for 15 and 64 days, respectively. In both experiments, the most frequently (>50%) observed species were Gyrosigma fasciola and G. obscurum. No growth was observed and no resting spores were found. In both experiments, the initial optimum quantum yield of the PSII prior to dark treatment was comparable (F-n/F-m = 0.70). The F-v/F-m was not affected after 15 days dark incubation but a significant decrease in photosynthetic efficiency was observed after 64 days in the dark (F-v/F-m = 0.39). Exposure to different light treatments (P, PA, PAB) immediately after different dark incubation periods showed higher reduction in F-v/F-m (PAB > PA > P) after the longer dark period. Estimated P-E Curve parameters showed an efficient light harvesting and photosynthetic conversion capacity (alpha= 0.20- rER(max)=14) that was significantly reduced after 64 days ill the dark (alpha= 0.06; rETR(max)=8). The reduction in these photo-physiological indices (alpha and rETR(max)) after dark incubation was compensated with higher saturating irradiance (E-k), which we suspect to be a mechanism to optimize photochemical processes. But the PSII antenna was relatively light-sensitive because photosynthesis was already photoinhibited at half the photon flux density (>= 585 mu mol photons m(-2) s(-1)) relative to light-adapted (>= 972 mu mol photons m(-2) s(-1)) diatoms. We conclude that the benthic diatoms ill our study were able to resume photosynthetic activity after 64 days ill darkness and they were able to cope with relatively high intensities of UV radiation compared with their natural habitat.