This study evaluates temporal variations of physicochemical variables and densities of phytoplankton and zooplankton, under different flow conditions. Four samplings were carried out at four sites on the Lower Delta of the Paraná River (Argentina). This first approach was developed in 1995 and included a flood of high amplitude and average duration. Daily data analysis along the year qualified sampling water levels as very high, medium (two opportunities) and low level. Triplicate subsuperficial water samples were taken in the middle of the navigation channel. Dissolved oxygen, pH, conductivity and temperature were determined in situ; nutrients and concentration of major solutes in the laboratory. Quantitative samples of phytoplankton (concentrated by sedimentation) and zooplankton (80 l; 48 µm) were obtained. Temporal variation of most physical, chemical and biological variables exceeded the spatial variation. Physicochemical variables were mainly associated in time with the hydrological regime. In the main channel water exchange originated by the flood of marshy environments caused dissolved oxygen, nitrate and sulphate to decrease and phosphate, conductivity, bicarbonate, calcium and magnesium to increase. The dilution–concentration action of flow was evidenced in chloride variation (conservative ion). The plankton variables would be affected directly and indirectly by water level, and also by seasonal climatic condition. The densities of phytoplankton and zooplankton increased during low flow. For some organisms, the variation in density exceeded the simple concentration effect of flow decrease. The lowest plankton densities coincided with a temperature minimum. Copyright © 2003 John Wiley & Sons, Ltd.