Quantifying a mass mortality event in freshwater wildlife within the Lower Odra River: Insights from a large European river.

In the summer of 2022, the River Odra in Europe witnessed a significant ecological disaster, leading to an unprecedented mass mortality among fish, bivalves, and water snails. The disaster was attributed to toxins released by the haptophyte golden algae, Prymnesium parvum. This study primarily focused on the river's lower section, where the disaster's impacts were exacerbated by the downstream flow of deceased organisms. The Unionidae mussels' mortality rate in this section was estimated at 65 million individuals, marking an 88 % decline in their population. The native mussel, Anodonta anatina, saw the steepest decline at 95 %, while the invasive Sinanodonta woodiana decreased by 15 %. Additionally, a minimum of 147 million dead water snails, predominantly Viviparus viviparus, were found ashore, indicating an 85 % population decline. An estimated 3.3 million fish, predominantly ruffe (Gymnocephalus cernua), bream (Abramis brama), and perch (Perca fluviatilis), were found deceased along the lower Odra, amounting to a biomass of 1025 tons. Across the entire 560 km affected stretch of the river, the estimated fish mortality was 1650 tons, a 60 % decline from pre-disaster levels. The swift deterioration of the river's ecosystem underscores the need for further studies on its adaptive capacity and potential recovery.