AbstractDuring June 1981 zooplankton composition was investigated in samples collected with Clarke-Bumpus plankton samplers from 0.5, 2, 5, 10, 15, and 20 m depths. Most taxa were limited to specific depth zones. Diel variations in vertical range were observed for the majority of species inhabiting the uppermost part of the water column, whereas diel variations in median depth could be detected for only 3 out of 11 taxa. These were adult Temora longicornis and adult Pseudocalanus elongatus, which ascended during the night, and Evadne nordmanni which tended to descend during the night. A similarity analysis divided the samples into a surface layer group (0.5, 2, and 5 m) and a deeper group (10, 15, and 20 m) separated by the thermocline. The composition and distribution of the zooplankton living deeper down changed little with time, in contrast to the surface-dwelling zooplankton which was more variable. Multivariate analysis indicated that two factors were responsible for 79 % of the total variance in the species abundance data. In a stepwise multiple regression analysis, temperature and primary production were found to account for more than 75 % of the variations in the factors.