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The influence of chronic exposure to UV‐B and UV‐A radiation on growth and photosynthesis of two polar marine diatoms (Pseudonitzschia seriata and Nitzschia sp.) was investigated in cultures exposed to moderate photon fluences for 3–7 days. Population growth rates were diminished 50% by UV‐B. Fluorescence induction kinetics of photo‐system II (PSII) revealed that UV‐B caused lower F_v/F_m ratios and half‐rise times, indicating damage to the reaction center of PSII and to related elements of the photosynthetic electron transport chain. Carbon assimilation rates per cell and per chlorophyll a were nonetheless highest for UV‐B—exposed populations, which also had the highest chlorophyll a content per cell. The UV‐B—exposed cells were, however, more vulnerable to visible light‐induced photoinhibition. Exposure to UV‐A in the absence of UV‐B had little effect on growth, fluorescence induction of PSII, or …