A High-Density Bloom of Prymnesium parvum with No Apparent Toxicity in the Oder After the Ecological Disaster of 2022

In the summer of 2022, a massive toxic algal bloom of the haptophyte Prymnesium parvum devastated the Oder River in Poland and Germany. Between the end of July and the beginning of September, an estimated hundred thousand fish, mussels, and aquatic snails died. Since then, Prymnesium parvum has been established in the river network, as confirmed by monthly sampling, and there is the potential that such a catastrophe could repeat itself. Here, we report the occurrence of an apparently non-toxic bloom of the ‘golden algae’ Prymnesium parvum in May and June 2024 at comparable abundances to the ones observed during 2022, with a maximum abundance of 120,000 cell equivalents (CE)/mL in the main stem river without any associated fish deaths. Interestingly, in July 2024, local blooms in the West Oder even exceeded these abundances (160,000 CE/mL). The main-stem abundances decreased to 850–4250 CE/mL in July and 5–500 CE/mL in August. During the bloom, no mass mortalities of aquatic organisms were reported despite high abundances. Nutrients (total nitrogen TN, total phosphorus TP, N:P ratio) were inconspicuous and comparable to the disaster in 2022. To our knowledge, this is the first record of a massive Prymnesium bloom that did not result in mass fish deaths.