Résumé
A natural planktonic assemblage of organisms (<240 μm) was studied in mesocosm experiments for 7 d under varying conditions of ultraviolet-B radiation (UVB: 280 to 320 nm): UVB excluded, natural radiation and UVB enhanced at 2 different levels. Specifically, the changes in Prymnesiophyceae abundance and light scatter properties attributed to cell size (CS) were investigated by flow cytometry during the first 72 h with 4 h temporal resolution and were thereafter examined during the 4 following days with twice daily sampling. In addition, the specific rate of photosynthesis (Pcell) of phytoplankton <5 μm, ciliate abundance (predator 15 to 35 μm) and dynamics of nutrients were monitored. Prymnesiophyceae ranged in size between 2.7 and 4 μm and dominated the phytoplanktonic community <5 μm (more than 94%). Prymnesiophyceae exhibited marked diel variability with synchronized cell division: CS increased during the day and diminished at nighttime, indicating cell division. Short-term UVB exposures during the first 3 d of the experiment did not affect CS, probably due to vertical water mixing in the mesocosms moderating harmful UVB exposure. In contrast, long-term UVB treatments (3 to 7 d) induced progressive CS increases in Prymnesiophyceae as a function of increasing UVB doses. The successive inhibition of Pcell of phytoplankton <5 μm was also observed as a function of increasing UVB doses. The results suggest that enhanced UVB provokes the retardation of cell division and inhibition of Pcell which causes Prymnesiophyceae CS enlargement. CS enlargement and probably a change in the food quality of Prymnesiophyceae could result in food limitation for the ciliate population, although ciliates seem to be directly affected by UVB enhancement. This study highlights the ecological implications of CS changes and photosynthetic capacity of phytoplankton, with respect to predator-prey interactions in response to UVB enhancement.