. Summary of experimental results . The light absorbing capacity of phytoplankton, estimated from Fo , and its photosynthetic activity (estimated as Fv/Fm ) are key characteristics of the primary processes of photosynthesis. We suggested a formula for calculation of the primary production of phytoplankton from these two characteristics and underwater irradiance. The probing data were used to plot vertical profiles of phytoplankton productivity in various regions of the Baltic, Norwegian, and South China