Several aspects of a diel vertical migration (DVM) of adult Eudiaptomus gracilis in Lake Maarsseveen (The Netherlands) are described. The period of DVM lasted from the end of May until the middle of August. On May 21, 1989, the population was found divided into a deep dwelling part and a part in the upper five meter. Large shoals of juvenile perch were observed in the open water for the first time. On June 7, the whole population was down below 10 m and concentrated in a zone of high chlorphyll-a concentrations. One week later, a regular DVM was performed. The amplitude of this migration gradually decreased towards the end of the migration period. The ascent in the evening and the descent in the morning took place after sunset and before sunrise, respectively. The movements coincided with high relative changes in light intensity. Population size increased rapidly during the period of DVM but decreased again before the end of this period.

Primary productivity of four size classes of phytoplankton ( <150/~m, < 50/Jm, < 20/~m and ,~ 5/~m) was measured from March through October 1986 in Lake Maarsseveen I with an inbubator technique. The mean column production was approximately 400 mg C.m-2.day -1, with a range of values between 150 and 750 mg C.m-2.day-1. The mean contribution of the size fractions < 50 #m, < 20/~m and < 5 #m to the size fraction <. 150/~m was 80 %, 60 % and 35 %, respectively. During their appearance the grazing impact of small herbivorous zooplankton, e.g. rotifers, can give an underestimation of the size fraction <. 150/Jm. An indication of this phenomenon is given.

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