AbstractThe examinations showed that the phosphate supply for the planktonic primary production comes almost exclusively from the influxes. The nutrient pool in the hypolimnion and in the sediment does not play a leading part. Evidently, that is often so in reservoirs.Both the phytoplankton (the most frequent species areAsterionella formosaandFragilaria crotonensis) and the concentration of phosphate fractions are subject to considerable seasonal variations. TheAsterionellaspring maximum, typical of this reservoir, is released directly or indirectly by meteorological factors. An adequate orthophosphate concentration is a basic condition which is accomplished, however, nearly every year. The autumnalFragilariamaximum begins when a definite mixing depth hae been achieved. The breakdown of a diatom maximum is caused almost at all times by exhaustion of the phosphate reserves.