CYANOPHYCEAN BLOOMS IN THE RESERVOIR OF VAL-JOLY (NORTHERN FRANCE) AND THEIR DEVELOPMENT IN DOWNSTREAM RIVERS

A study was carried out in 1992 on the Val Joly reservoir and the rive rs situated downstream: the River Helpe Majeure and the canalized Samb re into which it flows 40 km below the reservoir. Each year, blooms of blue-green algae occur in the reservoir. They are dominated by specie s such as Aphanizomenon flos-aquae (L.) Ralfs in 1991 or Pseudanabaena tenuis Koppe and Oscillatoria pseudogeminata G. Schmid in 1992. A typ ical fluvial composition of the phytoplankton was mostly restored abov e the confluence with the Sambre. Nevertheless, significant quantities of Oscillatoria and Pseudanabaena have been observed in the River Sam bre, just below its confluence with the Herpe Majeure. The environment al conditions in the River Sambre do not allow Aphanizomenon to develo p, but could favour Oscillatoria blooms.