Prymnesium as a threat for planktonic communities - an ecotoxicological approach for the environmental disaster in the Oder River 2022

The golden alga Prymnesium parvum plays a key role in harmful algal blooms (HABs) worldwide, including the massive fish kills that occurred in the Oder river (Poland and Germany) in 2022. However, studies addressing this ecological disaster have to date focused mainly on the physicochemical parameters of the water, and overlooked the overall impact that environmental samples could have on aquatic organisms other than fish. Therefore, the present study evaluated the toxicological effect of the bloom by subjecting samples of the river water to microbiotests comprising organisms from two trophic levels: producers (Pseudokirchneriella subcapitata), and consumers (Daphnia magna and Thamnocephalus platyurus). In addition, the study examined the relative concentrations of prymnesins and the physicochemical parameters of the water samples, and 18S rRNA gene sequencing was used to examine eukaryotic assemblages in the samples. Among the tested organisms, D. magna was found to be most sensitive to the water samples from the disaster, with a maximum mortality of 90 % after 24 h. The 18S rRNA gene analysis found a high level of P. parvum in the tested samples during the ecological disaster (up to 9.2 %) compared to one month later (0.1 %). Our data indicates a notable increase in P. parvum and prymnesin level around the time of the ecological disaster in the Oder River in 2022, and that this may have played a part in its occurrence; in addition, D. magna may be an effective bioindicator for identifying the risk of P. parvum blooms to invertebrates.