AbstractThe irradiance reachingNitellaat 11.5 m depths was almost the same during three years: 1000 cal cm−2year−1PAR, or 2.2% of the surface irradiance PAR. Green light (500–600 nm) constituted most (62–88%) of the irradiance onNitella.Net photosynthesis byNitellawas linearly correlated to irradiance.Nitellahad a positive oxygen balance from mid‐May to late September and a minor negative one at other times. The annual turnover of biomass was potentially high, and the dark‐respiration was very important in the metabolic balance. The contribution ofNitellato total autotrophic production and to oxygen conditions in the hypolimnion i