Effects of low-level chlorination on zebra mussel, Dreissena polymorpha

Mortality pattern of different size groups (5-20 mm shell lengths) of the zebra mussel Dreissena polymorpha (Pallas), was studied in the laboratory under different chlorine concentrations (0.25-3.0 mgl(-1)). Results showed that exposure time for 100% mortality of mussels significantly decreased with increasing chlorine concentration. For example, mussels in the 10mm size group exposed to 0.25mg l(-1) chlorine residual took 1080h to reach 100% mortality whereas those exposed to 3 mg l(-1) chlorine took 252 h. All size groups (between 5 and 20 mm shell length) took identical exposure time to reach 100% mortality at given chlorine concentration (between 1 and 3mg l(-1)). The effect of acclimation temperature on D. polymorpha mortality in the presence of chlorine was significant. For example, 1026 h is required to reach 95% mortality using 0.5mg l(-1) residual chlorine at 10 degrees C, compared to 456h at 0.50 mg l(-1) chlorine and 25 degrees C. Resistance of D. polymorpha to chlorine appeared to be lower than that for other mussel species in The Netherlands viz., Mytilus edulis L. and Mytilopsis leucophaeata (Conrad). The present study also suggests that 100% mortality data for European populations of D. polymorpha at different chlorine concentrations are similar to those for the North American populations.