Our earlier study on the phytoplankton abundance and nutrient content in the Rybinsk Reservoir showed a weak correlation between chlorophyll a and nutrients. CHL:TP and CHL:TN ratios were used to quantitatively estimate the chlorophyll dependence on nitrogen and phosphorus. CHL:TP changed from 0.01 to 0.57 mg/mg in May-October 1981, 1982 and reached 1.46 in July 1989, CHL:TN ranged from 1-2 to 47 and to 56 mg/g, respectively. Variations of both indices through the ranges of CHL, TP, TN and TN:TP are given and their increment in eutrophic waters in comparison with mesotrophic ones is discussed. The present approach seems to be useful for the prediction and control of the eutrophication processes under the similar ecological conditions.