Fig. 8

Conceptual diagram showing the pattern of maximum biomass of P. rubescens in the metalimnion of Rappbode Reservoir in summer, under different combinations of withdrawal water volume and withdrawal start time. The pattern is generated from the scenario analysis, which indicates that the phytoplankton biomass would be lower with higher withdrawal volumes and earlier withdrawal (given as color scale, i.e., decreasing towards the upper left corner). The solid line is the maximum withdrawal water volume that allows adequate water storage in the reservoir. The maximum allowable water withdrawal decreases over the season, because the stored water volume declines from spring to summer. The phytoplankton biomass threshold (dashed line) is a critical upper level of P. rubescens that can prevent the subsequent development of high concentrations. It increases over the season, since earlier metalimnetic withdrawal can more effectively suppress P. rubescens growth compared to the later withdrawal. Between the two lines is the safe operating space as a tradeoff between mitigating harmful cyanobacteria blooms and maintaining water resources