Photosynthesis of natural phytoplankton under high frequency light fluctuations simulating those induced by sea surface waves1

The photosynthetic 14C assimilation of phytoplankton from the St. Lawrence estuary was measured under various artificial light regimes, combining different intensities, colors, and high frequency fluctuations. The light fluctuations consisted of rapid alternation of high and low light intensities of equal duration. In the first set of experiments, the photosynthetic response was measured under red, green, blue, and white light, steady or fluctuating (2.5 Hz). Under fluctuating light, the color: white 14C assimilation ratios were significantly higher than those under steady light. In the second set of experiments, light saturation curves were made under fluctuating (10 Hz) and steady lights. No differences were recorded for PBmax but the photosynthetic efficiencies α were 30% higher under fluctuating light. Vertical distribution of rapid light fluctuations, together with light intensities and color, is therefore of ecological significance. These experiments also raise the problem of deck incubations, where not only the color is modified but also the frequency of light fluctuations, leading to a median error as high as 30%.