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

• In the summer of 2022, an ecological disaster occurred on the River Odra in Europe, caused by Golden Algae toxins, resulting in an unprecedented mass mortality of fish, bivalves, and water snails.
• The disaster led to an estimated 65 million dead bivalves, with the Unionidae mussels population declining by 86%, and the native Anodonta anatina species particularly affected with a 95% decrease.
• The invasive mussel Sinanodonta woodiana suffered the least damage, highlighting the potential for an invasive species to take over the ecological niche left by the native mussels.
• The disaster resulted in a minimum of 144 million dead water snails washed ashore (85% decline).
• An estimated 3.3 million dead fish (1,025 tons) were found in the lower section of the river, with a significant 60% reduction in fish population biomass along the entire section affected by the disaster.