IsPseudanodonta complanatathe most vulnerable of widespread European species of unionids? An intense stress test leading to a massive die‐off

This article reports a large‐scale die‐off of approximately one‐third of the unionid mussels inhabiting a shallow, naturally eutrophic, floodplain lake (Zalew Pinczowski), which took place within the space of just a few days. Mortality was the highest in Pseudanodonta complanata (81%), followed by Unio pictorum (58%), Anodonta cygnea (26%), Unio tumidus (15%), and Anodonta anatina (5%). It was significantly related to mussel size (age) only in A. cygnea, with the mortality rate higher in smaller (young) mussels. Pseudanodonta complanata was already the rarest species in the lake, and the number of survivors decreased to such an extent as to trigger a possible Allee effect. The most likely cause of the die‐off was a motor‐boating incident that had directly preceded the die‐off. Analysis of the water samples collected just after the die‐off revealed an extremely high concentration of phosphate (up to 0.5 mg dm⁻³) and nitrite (up to 0.06 mg dm⁻³). We suggest that the boat movements must have disturbed the lake sediments, thereby disrupting the mussels' physiology and causing mass mortality. The high sensitivity of P. complanata to environmental stress suggests that it may be an effective umbrella species for the protection of co‐occurring mussel species in eutrophic lakes.