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Harmful Algal Blooms (HABs) caused by the haptophyte Prymnesium parvum represent an ecological and socio-economic threat in brackish waters worldwide. In summer 2022, a catastrophic bloom in the Oder River (Germany–Poland) caused mass fish kills (~360 t). The Oder River discharges into the Oder (Szczecin) Lagoon, a region with fisheries tradition and growing importance for tourism and recreation. Understanding how the bloom affected the lagoon is important for future risk assessment.We combined long-term (1972-2024) phytoplankton monitoring data from Polish and German environmental authorities, high-resolution (200m horizontal grid from MOM – Modular Ocean Model) hydrodynamic modeling, and Lagrangian particle tracking (Parcels framework) to (1) assess historical occurrence of Prymnesiophyceae in the lagoon, (2) simulate decay and transport of the 2022 bloom from the river into the lagoon, (3) evaluate connectivity between different regions in the lagoon and the Baltic Sea, and (4) generate ecological and socio-economic risk maps.Phytoplankton time series show that Prymnesiophyceae have been present in the lagoon since 2007, with the higher abundance (~ 100 million cells L-1) recorded in July 2022, in the German side of the lagoon. Regarding the 2022 bloom, we released virtual water parcels with a P. parvum initial abundance of 150 million cells L-1 from the river mouth. We started the simulation on July 15 2022, applying different decay scenarios (no decay, 5-day and 10-day half-life). Particles were tracked for 30 days to identify hotspots and connectivity.Even under slow decay, all water parcels remained in the Polish sector (Wielki Zalew), affecting beaches like Plaża w Czarnocinie about 6km from the river mounth. Connectivity matrix based on releasing water parcels from German and Polish sides supported the low connectivity between lagoon portions and the Baltic in a one-month time frame. This suggests that P. parvum observed on the German side in 2022 likely originated from local or previously established populations rather than direct influence by the bloom event.We integrated modeled bloom dispersion with ecological subjects (key fish species and habitats) and socio-economic features (fisheries harbors, bathing beaches) to produce risk maps. Polish side areas were more affected from the bloom regardless the decay rate and presented higher risk.However, in future scenarios, increasing drought frequency may support long-term risk of toxic algae blooms in the Oder River. Monitoring identifying Prymnesiophyceae and our risk maps could serve as important management information. Also, our particle tracking applied to different hydrodynamic conditions could help to improve the understanding of risk areas.