The spatial and temporal dynamics of zooplankton populations were investigated in a highly engineered and regulated section of the River Great Ouse from 1989 to 1993. In addition to main river sites some secondary channels (back channels) and a marina were also included in the study. At main river sites Rotifera were strongly dominant, with peak numbers of 2000 1 71 to more than 18000 1 71 , corresponding closely with spring and early summer maximum concentrations of chlorophyll a. Copepoda (mostly nauplii) were recorded at consistently lower densities (maxima, 20ยฑ125 1 71 ) whereas Cladocera were rarely recorded and never exceed 10 1 71 . Most Cladocera belonged to species generally associated with macrophytes rather than truly planktonic species. The situation in main river and back channel situations contrasted markedly with that found in marinas, where densities of Rotifera, although extremely variable spatially, were generally higher than those in the adjacent main river. Copepoda and Cladocera were also much more abundant than at the river sites with taxa such as Bosmina and other typically planktonic taxa predominant. As rotifers are the main food for newly hatched cyprinids, spatial and temporal variation in their abundance relative to the spawning locations and hatching times of cypinid eggs may have important consequences for the growth and survival of young ยฎsh.