Mussel squeeze: Dissolved oxygen and temperature can 'squeeze' zebra mussels out of invaded reservoirs

Zebra mussels (Dreissena polymorpha) are an aquatic invasive species that cause extensive economic and ecological impacts and are a management priority in areas outside of their native range. Survivorship and distribution of zebra mussels within a waterbody are thought to be influenced by temperature and dissolved oxygen conditions, but detailed information to confirm the importance of these environmental controls is necessary to inform management efforts. We measured planktonic zebra mussel veliger density and adult survivorship in San Justo Reservoir in central California to determine distribution and timing of spawning in relation to temperature and dissolved oxygen throughout winter, spring, and summer. We found seasonal patterns in adult survivorship, with high mortality late in the summer and higher than expected survivorship during the spring when dissolved oxygen concentrations were approximately 1 mg/L. Veliger abundance peaked several meters above the thermocline from June to August. Dissolved oxygen concentrations limited veliger distribution, with few to no veligers collected in the anoxic hypolimnion. Veliger settlement out of the water column appears to be possible in San Justo Reservoir at any time of year. A better understanding of how veligers, juveniles, and adult mussels respond to fluctuating dissolved oxygen and temperature conditions will further knowledge of timing and duration of water drawdowns or other control methods for managing this harmful invasive species.