Fig. 9. O₂ content (O₂-data from North Rhine-Westphalia State Environment Agency) and valve closure behaviour of C. fluminea (27 specimens in parallel) inside the Rhine at Bad Honnef during July 23-31 1998, recorded by the Dreissena-Monitor®. Illustrated are the proportions of opened clams inside the two parallel flumes of the monitor. Dotted lines indicate the change of date, 0:00 h, water temperature varied between 22°C and 24°C.