The seasonal variation in primary production, individual numbers, and biomass of phyto-and zooplankton was studied in the River Danube in 1981 . The secondary production of two dominant zooplankton species (Bosmina longirostris and Acanthocyclops robustus) was also estimated . In the growing season (April-Sept .) individual numbers dry weights and chlorophyll a contents of phytoplankton ranged between 30-90 x 10 6 individuals, l -', 3-12 mg 1 -', and 50-170µg 1 -1 , respectively . Species of Thalassiosiraceae (Bacillariophyta) dominated in the phytoplankton with a subdominance of Chlorococcales in summer . Individual numbers and dry weights of crustacean zooplankton ranged between 1400-6500 individuals M-3 , and 1.2-12 mg m-3, respectively . The daily mean gross primary production was 970 mg C m-3 d -1 , and the net production was 660 mg C m-3 d -' . Acanthocyclops robustus populations produced 0 .2 mg C m-3 d -1 as an average, and Bosmina longirostris populations 0 .07 mg C m-3 d -'. The `ecological efficiency' between phytoplankton and crustacean zooplankton was 0 .03 % . M J J J A J A S S ----Bosmina longirostris

Analysis of the population dynamics of Bosmina longirostris (0. F. Mtiller) was carried out in the River Danube in the years 1981 and 1982. An estimation method of secondary production elaborated for standing waters was adapted to river conditions. In the River Danube, compared to lakes, standing crop and production values were smaller, while fecundity, P/B values were larger and turnover time was shorter. Possible reasons for this are discussed.

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