Mass mortality of invasive zebra and quagga mussels by desiccation during severe winter conditions

Within impounded sections of the rivers Rhine and Meuse, epibenthic macroinvertebrate communities are impoverished and dominated by non-native invasive species such as the zebra mussel (Dreissena polymorpha) and quagga mussel (Dreissena rostriformis bugensis). In the winter of 2012 management of the water-level resulted in a low-water event in the River Nederrijn, but not in the River Meuse. Low-water levels persisted for five days with average daily air temperatures ranging from -3.6 to -7.2 ̊C. We assessed the effects of this low-water event on settled dreissenid mussel populations using a before-after-control-impact (BACI) design. Desiccation had a negative effect on the overall density of dreissenids. Six months after the water level recovered, mussel density had increased slightly. After 18 months, mussel density had recovered to pre-event level. Mussels collected after the event were smaller than specimens collected before the event, indicating recolonization originating from upstream river sections. At the control site, the dreissenid populations showed no significant change in density or shell size distribution. In total, 99.4% of empty shells washed up and sampled directly after the low-water event consisted of invasive bivalves, including zebra mussel, quagga mussel and Asian clams (Corbicula fluminea and Corbicula fluminalis). The high number of empty dreissenid shells stranded on the river banks directly after the event coincided with a mass mortality of mussels in the littoral zone. Imposing a sudden water-level drawdown during severe winter conditions could be a tool for the temporary reduction of invasive mollusc density in impounded river sections. Full recovery of the population structure likely may require a period of two to three years. Therefore, we recommend that the long-term effects of recurrent water-level drawdowns on the diversity of invasive and native species be assessed.