MEASUREMENT OF PHYTOPLANKTON PHOTOSYNTHESIS RATE USING A PUMP-AND-PROBE FLUOROMETER . ... A biophysical model was used to describe the relationship between photosynthesis, underwater radiation, and the intensity of phytoplankton fluorescence excited by an artificial light source. ... Parameters of the model that could not be measured experimentally were determined by calibrating fluorescence and radiation data against the primary production measured in the Baltic Sea by radioactive carbon method. ...