The highest concentrations of N, P and Si appear in the surface water of Lake
Zürich every spring from about the end of January (1983, 1986, 1987: NO3-N:
850 μg l–1, PO4-P: 50 μg l–1, SiO2: 2.4 mg l–1) until April. In the course of the phytoplankton
development in April the concentrations of N and Si sink to 450 μg N l–1
and 1.0 mg SiO2 l–1 respectively until about the beginning of May – at peak spring
bloom. At the same time phosphorus is detected in the uppermost water layer
(0–7.5 m) at a minimum concentration of 4 μg PO4-P l–1. Thereafter the values sink
below 2 μg PO4-P l–1 for a few days (detection limit).