The relative importance of climate and nutrients in controlling phytoplankton growth in Saidenbach Reservoir

After the dramatic reduction of the P import into the Saidenbach drinking water reservoir since 1990, an improvement of the trophic state from eutrophic to mesotrophic or even oligotrophic regarding P-concentrations was observed, and corresponding decreases of algal standing crops could be expected. However, the annual mean phytoplankton biomass nearly doubled (mainly caused by higher spring mass developments) and the summer blooms of cyanobacteria still prevailed. The 27-year data set reveals the decisive importance of the spring overturn duration for the spring phytoplankton yield. The spring full circulations were markedly longer in the 1990s than in the years before and enabled the almost complete conversion of P-stores into algal biomass. In the 1970s and 1980s, however, the spring blooms mostly started late and ended before all nutrients could be utilized. Thus, the duration of spring overturn, depending on the weather conditions, controls the time remaining for the diatoms to utilize the nutrients incorporated. Until 1990, summer blooms of Fragilaria and/or cyanobacteria were mainly controlled by the Si supply, which strongly depends on the Si import, i.e., as well on the meteorological conditions. However, since 1991 extremely low SRP concentrations prevented the usual Fragilaria growth in summer, in spite of sufficient Si supply and convenient turbulence conditions. The new cyanobacteria species dominating now are obviously better competitors for P. The results suggest that it is difficult to reliably forecast or control the plankton dynamics because the impact of hydrophysical factors is decisive even in strongly nutrient limited water bodies. Further reduction of P import is necessary to successfully counteract the unpredictable meteorological stimulation of diatom growth in spring and to avoid summer blooms of cyanobacteria.