This publication presents the results obtained from observations on the daily cycles in the River Seine, downstream from the water purification plant at Acheres. Most of these observations concern six stations spread over 59 km. They were made in 1989 at three different times of the year (summer, autumn and winter) for periods lasting from 1 to 24 h according to the stations and the parameters studied: light, temperature, pH, dissolved oxygen, conductivity, chlorophylls, pheopigments, chlorides, bicarbonates, as well as different forms of nitrogen and phosphorus. Regarding conductivity and the concentrations of certain nutrients, an undeniable link was found between their daily rhythms and those of the flows from the Acheres sewage plant. In the absence of heavy rainfall, simple continuous measurements of conductivity along the river are sufficient to follow these flows and study their longitudinal dispersion as they moved downstream. The origin of the daily cycles of dissolved oxygen and chlorophyll are more complex. In June and October, they are mainly linked to light, the concentration of chlorophyll being moreover very closely related to temperature. In December on the other hand, the situation alters completely (minimum light, important fall in temperature, low chlorophyll concentrations). A daily cycle persists for dissolved oxygen but has nothing to do with light, being entirely subject to the discharges from the Acheres sewage plant.
