Auffällige biologische Folgen von Sprungschichtneigungen im Zürichsee

Summary

• (1) Continued north-easterly winds in June 1949 caused a transverse tilt of the thermocline in Lake Zürich and upwelling of deep water containing nutrients on the right shore. This resulted in mass development of littoral filamentous algae (Cladophora glomerata, Spirogyra sp.).

• (2) Over a distance of 3 km the difference in level of the 19,5° isotherm was 3.5 m, while that for the lower metalimnion was ca. 2.5 m.

• (3) TheOscillatoria rubescens layer, which at this season in Lake Zürich normally lies at a depth of 12.5 m, actively overcame the thermocline tilt and took up a horizontal position; so that the maximum numbers on the left-hand and right-hand sides of the lake lay in water of 10° and 8,7° respectively, while the latter were able, over a considerable period, to extract from the elevated hypolimnion nutrients which are normally unavailable to phytoplankton during thermal stratification.

• (4) Oscillatorio rubescens takes up nitrate and CO₂ avidly, but can be driven out of layers rich in nitrate by light conditions (right-hand side of lake in June 1949).

• (5) About 300 m from the lee shorePhacotus lenticularis occurred in swarms, which are assumed to have been produced by wind drift. It is possible that wind drift at night would also lead to swarming of zooplankton.

• (6) A severe storm from the west on 3rd August 1949 produced a considerable thermocline tilt so thatOscillatoria rubescens and cold water down to temperatures of 7° C rose to the surface on the left shore. A week later the thermocline was still practically obliterated.