The zebra mussel (Dreissena polymorpha) has all traits required to effectively colonize the aquatic environment and consequently reduce the diversity of native bivalves We hypothesized that the zebra mussel chooses lake outlets characterized by medium current velocity and good food conditions. Here, we analyzed differences between bivalve abundances in lake outlets with varying environmental conditions such as the Carlson Index (trophy status), depth, width, current velocity, bed vegetation coverage, and type of bottom substrate. The results showed that the zebra mussel inhabits outlets that provide food (high trophy outlets) and have a mineral bed and a medium current velocity (ca. 0.2–0.3 m s-1). The following main factors seem to be favorable for colonizing such outlets: (1) easy access to high amounts of food due to the increased density of the suspension drifting from the lake and (2) easy transport of the zebra mussel larvae from the lake to the downstream. The zebra mussel larvae drifting with the current may colonize the downstream. An increase in lake trophy may indirectly cause an increase in biological invasions in rivers.

The study was performed in the catchment area of the Drawa River (GPS: 53°20ʹ25ʺ N; 15°46ʹ30ʺ E – the middle Drawa), which is a 190-km-long quaternary tributary of the Odra River. Samples of bivalves were collected from 55 lake outlets in July 2016. An area of approximately 50 m² was selected for the study, starting from the shoreline of the lake downstream. Samples of macrozoobenthos were collected at the point where the water current changed from the cumulative to the rectilinear. The samples were collected using a Surber sampler (sampling surface 0.0625m²) by disturbing the bottom at 4 places: 2 places were located at the middle of the outlet and one at each shore. All samples were joined together as one sample. The abundance of the organisms was counted per square meter. We decided to use a Surber sampler because of the small size of Sphaeriidae clams. To avoid “double zeros,” an entire 50-m section of the outlet was checked for the absence of dreissenids.