Responses to high irradiance contribute to the decline of the spring diatom maximum

The effect of high irradiance was studied with cultures and samples from natural populations of the colonial diatom Asterionella formosa Hass. The fluorescence ratio Fv : Fm, where Fv is the difference between DCMU‐enhanced fluorescence (Fm) and normal, dark‐adapted fluorescence (F0), was used as a relative measure of photosynthetic performance. A. formosa grown in batch culture displayed a 70–80% decrease in Fv : Fm during 1 h of exposure to 1,600 µmol quanta m−2 s−1. In addition, sinking rate increased from a mean of 0.23 m d−1 in controls to 0.43 after high irradiance. A. formosa populations were sampled in May, the later stage of the spring abundance maximum in the north basin of Windermere (English Lake District). Diatoms in the upper 1–3 m exhibited low Fv : Fm (0.1–0.2) during near‐surface stratification (four of five dates) but no depression of Fv : Fm on the one occasion of sunny weather and strong surface winds. Near‐surface cell abundances were also significantly lower during high‐irradiance, stratified conditions. The results suggest that high irradiance lowers production rates and increases sedimentation of diatom populations during the later stages of the spring maximum.