Since 1975, different patterns of the growth of Fragilaria and Cyanobacteria in the mid or late summer have been observed in the Saidenbach Reservoir. In most of the years, there was a mutual exclusion of mass growths of these two groups. High yields of Fragilaria caused low yields of blue-greens and vice versa. In the first years, Fragilaria was always the first to achieve a mass growth, followed by the Cyanobacteria. Then, in the last years, the blue-greens were succeeded by Fragilaria. Only in some years, there was a large and simultaneous growth of both groups. It has been shown, that the supply of silicon and phosphorus immediately before controlled the pattern of succession in the midsummer period. Mostly, the Si:P ratio was the regulating factor, but there were also years in which the absolute concentration level of these two nutrients were either non-limiting high or below the minimum resource concentration necessary to compensate for the losses. In these cases, the Si:P ratio lost its controlling function. Other influencing factors are discussed, in particular the effect of turbulent mixing. The special nutrient conditions in the midsummer were not only determined by the external load but also by the Si depletion during the spring mass development of diatoms other than Fragilaria, which mostly is physically controlled. Therefore, the succession pattern in the midsummer is sensitive to the meteorological conditions to a large extent.