The analysis of diel variations in fluorescence yield of chlorophyll-(alpha) (Chl-(alpha) ) in vivo is presented in the paper. We rely on recent field observations and laboratory studies, as well as on the biophysical description of the relationship between photosynthesis and Chl- (alpha) fluorescence. The variations in fluorescence yield due to `instant' changes and diurnal variations in ambient irradiance can be explained as the manifestations of `photochemical' and `non-photochemical' quenching of Chl-(alpha) fluorescence, respectively. Field studies of the diel variability in Chl-(alpha) fluorescence were carried out with a shipborne lidar system in the Baltic, Mediterranean, and Black Seas, and in the Atlantic Ocean. As predicted by the model, we observed the maximal values of Chl-(alpha) fluorescence yield in the near-surface layer at night and its decrease in the noon. The max/min ratio varied from 3 to 1, depending on environmental conditions and, thus, on the functional state of photosynthetic apparatus. Non-photochemical mechanisms of `photoinhibitory' and energy-dependent quenching were found to be the major mechanisms of regulation of Chl-(alpha) fluorescence yield at a diurnal scale.