AbstractPhytoplankton primary production was measured using the14C method once per month from 1973 through 1976 as part of an intensive ecosystem study of a small eutrophic soft‐water lake, under restoration since 1970. Relationships among phytoplankton production, species composition, chlorophyll a content, bacteria, zooplankton and a variety of abiotic environmental factors have been studied. Productivity normally showed one peak in spring and another in summer, whereas a minimum was reached under the ice cover in February or March. Maximum production rates in the depth profile ranged from 3 to 144 mg C · m−3· d−1, integral production from 6 to 510 mg C · m̄−2· d−1. Species of Cyanophyceae, Dinophyceae, Chlorophyceae and Chrysophyceae dominated alternately and showed significant differences in the level and variation of photosynthetic activity. Maximum activity was observed in summer. A high biomass increase during late winter and spring despite low primary productivity resulted from the immigration of the dominant blue‐green alga,Oscillatoria limosa, from the sediment. Energy efficiency increased not only with depth in the light‐limited parts of the euphotic zone but at all depths during bad weather conditions and during the decrease of ir