The drift of zooplankton in a lake-outlet (Glatt) in a day-night-rhythm depending from the water level

To give an indication on the influence of the dynamics and behavior of lake plankton on the seston composition in the outlet, four 24-hour sampling sessions were made in the Greifensee (situated near Zürich, Switzerland) and its outlet (River Glatt). The most important crustacean species studied wereEudiaptomus gracilis, Cyclops abyssorum, Cyclops vicinus, Mesocyclops leuckarti andDaphnia spp.Leptodora kindtii was found during one sampling only. No evidence of shore avoidance was found. The vertical migration in the lake especially in the upper layer (0–6 m) is believed to have an influence on the amount of plankton which is flushed into the outlet. Although the composition and concentration of the plankton at the deepest point of the lake have been known, no accurate predictions on the quality of the plankton close to shore or in the outlet were possible.