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A continuous culture experiment was conducted with the marine diatom Ditylum brightwellii to assess the photo-oxidative effects of transition from dim light to high light at three successive levels. The focus was on temporary stress by ambient UVA at the water surface, via a simulation of the periodic ascent of diatoms caused by vertical mixing. Within a 14 h dim-light period, cells were exposed daily to 4 h of: (A) moderate PAR (100 μmol photon m-2 s-1), (B) high PAR (400 μmol photon m-2 s-1) and (C) high PAR (400–700 nm) plus 6·7 μmol photon m-2 s-1 (2·2 W m-2) UVA. Cell shape, life cycle and sinking rates were not affected by high PAR per se (mode B). The daily 4 h high-irradiance mode C, with additional UVA (UVA: PAR = 0·016), caused an increase in cell diameter, a decrease in length axes, and a production of vegetative resting stages. Superoxide dismutase (SOD) activities, oxidized glutathione (GSSG) pools and malondialdehyde (MDA) contents increased as well, which indicates that this biologically effective UVA dose (0·78 W m-2) promoted active oxygen production, oxidative stress and lipid peroxidation. Sinking rates in UVA-exposed D. brightwellii were higher than in those grown without UVA. A close correlation between sinking rates and MDA contents suggests that UVA-induced lipid peroxidation has altered the membrane functions which regulate buoyancy in D. brightwellii. Although UVA-induced formation of resting cells will contribute to accelerated sinking, it is not evident that this can be considered as a stress avoidance mechanism under adverse light (UVA) conditions. D. brightwellii proved to be sensitive to transitions from shade to natural surface irradiance conditions. In coastal waters, such large diatom species may experience oxidative stress after a few hours' residence near the water surface at ambient UVA levels.