The light dependence of photosynthesis intensity was used to determine the two photosynthetic parameters of phytoplankton: the maximum intensity (P super(B) sub(max), mg C/mg chl a h) and the efficiency of photosynthesis ( alpha super(B), mg C/mg chl a h W/m super(2)). The results of the measurements of the vertical profiles of P super(B) sub(max) and alpha super(B) clearly showed that they change in opposite modes. With the depth growth, the values of P super(B) sub(max) decrease, while those of alpha super(B) increase. During the period of stable water stratification, the variations of the photosynthetic parameters of phytoplankton with depth are more pronounced than those in the absence of the stratification mentioned. Weak or temporary stability of the upper layer promotes the appearance of physiological heterogeneity of the phytoplankton. During the cold season of year (December-April), the vertical homogeneity of the values of the maximum quantum yield and light absorption of phytoplankton was found in the layer of photosynthesis. In summer, the maximum intensity value regularly increases from the surface to the bottom of the euphotic zone, while the efficiency of photosynthesis decreases. The role of temperature, nitrate concentration, and light in the variability of P super(B) sub(max) is quite important, but the temperature and optical depth provide the greatest effects. The efficiency of phytoplankton photosynthesis at different depths is mainly controlled by nitrates. The results of the measurements and the quantitative regularities obtained on their basis rather correctly represent the vertical distribution of the phytoplankton photosynthetic and biooptical characteristics in the Black Sea, determining the potentiality of their practical applicability to modeling the primary production from satellite observations.