Evolution du phytoplancton dans des bassins de culture

The concentration of chlorophyll a in large out-door continuous cul­ture was analysed in spring 1977 and 1978, and autumn - winter 1977-1978. The growth of marine phytoplankton in springsl977 is analysed with more details and shows three phases : - first, the diatoms (especially Skeletonema costatum) grow rapidely (27/4 to 3/5). The growth rate estimated by the chlorophyll measurements is 1.36 day“1. - then (3/5 to.13/5) the concentration of chlorophyll remains more or less constant (23.611.2 mg/m3) as well as the concentration of particulate organic matter. - at the end of the experiment (13/5 to 20/5) the concentration of chlo­rophyll falls as soon as Skeletonema costatum decrease in number.The declin of the phytoplankton biomass is followed by an increase of the bacteria which reach a maximum on 15/5. We assume that the rapid decrease of chlorophyll a on the 13/5 is related to the effect of the dissolved silicate deficiency on skeletonema costatum and to the grazing by rotifers and copepods.