Two field experiments were carried out using a custom built flow-through laboratory to test the effect of elevated pH on dreissenid mussels as a potential control method. Both experiments tested the ability of dreissenid pediveligers to settle under conditions of elevated pH and the long-term survival of adult dreissenids under the same conditions. The two experimental sites had different water quality and different species of dreissenids present. The settlement of quagga mussel pediveligers at the lower Colorado River was inhibited with increasing pH. At the maximum achieved pH of 9.1, there was approximately 90% reduction compared to the maximum settlement observed in the controls. Since the settlement was almost as low in pH 8.9 as at pH 9.1, the inhibition in settlement may have been due to the presence of a precipitate formed under high pH conditions rather than the increase in background pH. No mortality of quagga mussel adults was observed in the experimental pH levels at the lower Colorado River. At San Justo Reservoir, zebra mussel settlement decreased with increasing pH. New settlement was almost entirely absent at the highest pH tested (pH 9.6). The observed mortality of adult zebra mussels was low, but did tend to increase with increasing pH. We also tested the response of adult zebra mussels to short-term exposure to very high pH levels (i.e. pH 10, 11, and 12). Adult mussels in poor physical condition experienced 90% mortality after 12 hours at pH 12. For unstressed adult zebra mussels, 90% mortality was reached after 120 hours at pH 12. Significant mortalities were also observed both at pH 10 and pH 11. From this study, we conclude that pH elevation could be used both as a preventative treatment to eliminate settlement by dreissenid mussels and as an end of season treatment to eliminate adults. The high pH treatment would have to be tailored to the site water quality to prevent formation of precipitate during treatment and to minimize corrosive action on materials of construction.
Management of Biological Invasions (2013) 4(2): 101–111

A field experiment was conducted in 2009 using Lake Ontario water in a continuous flow through system to determine the impact of low pH on dreissenid mussel (zebra mussel, Dreissena polymorpha Pallas, 1771, and quagga mussel, Dreissena rostriformis bugensis Andrusov, 1897) settlement and survival in calcium rich waters. Raw water containing veligers was pumped to the field laboratory where the incoming water was divided into four streams. Three of the streams had the pH adjusted using phosphoric acid to pH 7.3, 7.1 and 6.9. The fourth stream was used as a control. Three replicates of each pH resulted in 9 treatment tanks and three control tanks. Three bags of caged adults were placed in each tank. Adult mortality of almost 40% was recorded at a pH of 6.9 after 10 weeks of exposure. Analysis of the weight/length relationship of adult mussels confirmed that for any given length the mussels were significantly lighter at all three pH levels when compared to the controls. Visually, erosion and perforation of the shells was noted, leading to the conclusion that the loss in weight was primarily due to loss of calcium from the shells of the adults. The visual loss of calcium was the greatest at a pH of 6.9. New settlement was essentially prevented at a pH of 7.1. Based on these results, downward adjustment of pH in calcium rich waters may be a viable treatment for prevention of dreissenid fouling in industrial cooling water systems and raw water conveyances.
Aquatic Invasions (2012) 7(1): 21–28