The impact of the interannual variability in hydrodynamic conditions on the plankton development in Lake Constance in spring and summer

Long-tenn measurements (1979-1994) of meteorological parameters and of algal and crustacean biomass were used in conjunction with a comprehensive hydrodynamic model to evaluate the impact of the weather regime on plankton dynamics in a large, deep, temperate lake (Upper Lake Constance), and to identify potential causal mechanisms. The natural variability of weather conditions, including the exceptionally mild winters during the late eighties, allowed to investigate the covariation of meteorological parameters such as irradiance, air temperature, and wind with vernal algal and crustacean population growth. Crustacean zooplankton responded strongly to differences in surface water temperature, but not to mixing depth or algal biomass. Clear relationships between changes of algal biomass and meteorological factors were only found during the rare occasions when they acted together to favour or hamper algal development. Otherwise, the impact of the meteorological on the physical conditions, which were most likely conducive to phytoplankton development, could not be followed by this simple approach. This problem was overcome with a one-dimensional hydrodynamic turbulent exchange model driven by the meteorological boundary conditions at the water surface. It was used to simulate the development of the vernal density stratification and to study the relationships between meteorological parameters and exchange rates from the euphotic to the aphotic zone. The beginning of the spring algal bloom was shown to depend on the stabilization of the upper part of the water column. As soon as mixing below 20 m was inhibited, confining the algae to the euphotic zone for prolonged periods of time, substantial increases in algal standing stock occurred consistently. In contrast, during periods when high vertical mixing rates were computed with the model, no substantial enhancement of algal biomass was found. This tight coupling between the estimates of vertical mixing intensity and observed algal development, combined with knowledge about the impact of individual meteorological factors on mixing, enabled predictions about the response of algae to different weather conditions during spring