Algal blooms in ice-covered marine ecosystems usually start at the bottom and inside the sea ice. Pelagic bloomsoccur after ice break up in a strongly stratified layer close to the sea surface. During this period, algae mightbe exposed to fast changing and very high irradiances. Previous studies have shown that this can be ratherdetrimental for their growth and food quality (measured in terms of fatty acid composition). To investigatethese effects under controlled conditions, we performed in situ experiments with four different Arctic diatomspecies (3 pelagic, 1 ice algae) exposed at 0.5 and 8 m depth for 40 hours, respectively, in a high Arctic fjord(79°N) during spring. Experiments were performed with and without previous acclimation to ambient light. Thespecific impact of UV radiation was also addressed. Three out of four species showed a significant reduction incell numbers at 0.5 m compared to 8 m after 40 hours. In two of them, we also found a reduction of growth dueto UV radiation at 0.5 m. A moderate reduction of the relative amount of polyunsaturated fatty acids was foundin three species, but it was less pronounced than anticipated. It therefore seems that diatoms can adapt ratherwell to high irradiances, although the amount and quality of biomass produced might be impaired slightly.