SUMMARY.1. The population densities, dynamics and production ofDaphnia hyalinaandBosmina longirostriswere studied over a 2‐year period in a shallow eutrophic reservoir in eastern England. The diet of the two species was assessed and their ecology was compared in relation to environmental factors.2.Daphnia hyalinawas characterized by small overall body size, probably due to heavy size‐selective predation by fish. The first generation produced from the overwintering population appeared in April and Depopulation peaked in late spring and early summer. Adult survival was poor. Peak egg production varied between years, while mean brood size fell in summer probably due to food limitation during the cyanobacterial bloom. At this time selection for individual algal species was high and many guts were empty. Reproductive ratios were inversely related to population density.Bosmina longirostrisexhibited a similar annual cycle.3. Patterns of production were very different in the 2 years inD. hyalina. In 1981 production was high from August to early October, but in 1982 the peak was in spring. Annual production was 32.4 g C m−2in 1981 and 13.3 g C m−2in 1982. In 1981B. longirostrisproduction peaked in spring and late summer; in 1982 there was a high production in summer only. Annual production was 3.2 g C m−2in 1981 and 2.2 g Cm−2in 1982.4. The number of adultD. hyalinawas highly correlated with water temperature and total phytoplankton biomass, but these were negatively correlated with the number of eggs, mean brood size and instantaneous birth rate. In contrast, clutch size ofB. longirostriswas positively correlated with water temperature while birth rate was positively correlated with both temperature and phytoplankton biomass. The biomass of both species was positively related to total phytoplankton biomass, but neither relationship explained much of the variation in zooplankton biomass.5. The significance of the observations on plankton in terms of managing the water quality in eutrophic reservoirs i
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