Upper pH tolerance limit of the zebra mussel (Dreissena polymorpha)

The upper pH tolerance limit of the zebra mussel (Dreissena polymorpha) has not been established experimentally. This study was designed to test the e ffect of elevated pH on the health and survivorship of zebra mussels. Dishe s that contained 250 mL of pond water were assigned to four replicates of c ontrol, zebra mussel control, low-NaOH, medium-NaOH, and high-NaOH treatmen ts. Except for the control replicates, two zebra mussels were added to each dish. Every other day, volumes of 1 M NaOH were added to low- (50 mu L), m edium- (100 mu L), and high-NaOH (200 mu L) treatments. All zebra mussels b ut one in the high-NaOH treatment died between days 17 and 31 at pH 9.3-9.5 . On the last day of the study, day 31, three zebra mussels in the medium-N aOH treatment died at pH 9.5-9.6. The zebra mussels in the high-NaOH treatm ent moved and formed byssus attachments less often and had lower mean dry b ody mass than zebra mussels in other treatments. Dishes that contained zebr a mussels had more algal genera than control dishes. In this experiment, th e upper pH tolerance limit of zebra mussels was between 9.3 and 9.6 and may have been dependent on the rate of pH change.