Massive die offs of fish and invertebrates were observed in the lowland river Oder (Poland and Germany) in August 2022. The causes remained unclear for weeks, while speculations in the media about possible reasons flourished. Finally, a massive bloom of the haptophyte Prymnesium parvum was made responsible for the environmental disaster. Its growth was supported by high concentrations of NaCl probably originating from mining water effluents during summer low flow. We demonstrated the existence of a harmful bloom of P. parvum in several consecutive steps. A phytoplankton bloom was clearly indicated by an abrupt rise of chlorophyll a content and strong diurnal variations of oxygen concentration and pH. Prymnesium-like cells peaked at 140 × 10⁶ cells/L at river-km 661. We cultivated Prymnesium in enrichment and unialgal cultures. The cultures were taxonomically identified as P. parvum (18S rRNA and microscopy) and classified as producers of B-type prymnesins by molecular phylogenetics (ITS). Our cultures are deposited at the “Central Collection of Algal Cultures” (refs. CCAC 9414-9417). We retrospectively determined the start of the Prymnesium bloom at river-km 661 by a droplet digital PCR assay and by DNA metabarcoding indicating the presence of P. parvum in low abundances before the bloom. The presentation will not only delineate the course of the bloom, but also provide hypotheses about its origin and cover its echo in media and politics.