Growth of Blue‐Green Algae in the Saidenbach Reservoir and its Relationship to the Silicon Budget

Abstract

AbstractThe phytoplankton succession during the summer in the mesotrophic reservoir Saidenbach since 1975 may well be explained by the resource ratio hypothesis. Until 1980, only phosphorus controlled the phytoplankton growth, and diatoms prevailed, because an excesses of silicon existed. From 1981 to 1986, the ratio Si:P often was smaller than 90, a value, critical for the development of the diatom Fragilaria crotonensis. Its reduced growth caused an increased occurrence of blue‐greens (mostly Aphanothece clathrata) immediately after the diatom mass development. During these years at first silicon limited phytoplankton growth in summer, later on the growth again was limited by phosphorus.Because of increased Si and P load since 1987 a simultaneous limitation of both nutrients occurs. This leads now to parallel mass developments of diatoms and blue‐greens.In order to maintain the positive effect of diatoms (phosphorus transport into the sediment), it is to guarantee a sufficiently high Si:P ratio. If a reduction of P load isn't possible, Si remobilization from the sediment could be increased by artificial changes of the water level.