Harmful algal blooms (HABs) are increasingly prevalent in the Baltic Sea, with potentially serious impacts on ecosystem services, marine biodiversity and human health. An extreme example is the mass mortality of fish in the Oder River in August 2022, caused by the HAB species Prymnesium parvum, which could occur more frequently along the Baltic Sea coast in the future. In addition, a dinoflagellate (Alexandrium pseudogonyaulax) is currently spreading, which produces toxins and could also cause harm in the Baltic Sea region. It is unclear which factors favor the spread of P. parvum and A. pseudogonyaulax, what effects the toxins produced have on human health and at what concentrations harmful effects on other organisms are to be expected. HABBAL Phase II brings together a consortium of biologists, chemists, social scientists and local stakeholders to investigate the risks of harmful algal blooms for marine biodiversity and human health. The aim is to better understand the distribution of A. pseudogonyaulax and P. parvum and to characterize their toxic effects on other plankton species, fish and humans. This includes the determination of toxin properties and ecotoxicological thresholds. The combination of this research with social science and the involvement of stakeholders provides the knowledge base for the development of risk management strategies.