Benchmarking toxic effects on Daphnia magna of the Prymnesium parvum strain causing the 2022 Oder River fish kill

Abstract Harmful algal blooms (HABs) caused by Prymnesium parvum pose significant threats to aquatic ecosystems, as exemplified by the massive fish kill in the Oder River in 2022. In addition to fish kills, P. parvum can affect zooplankton populations that play a crucial role in top-down control of algae. We measured immobilization of Daphnia magna to compare the toxicity of the P. parvum strain that caused the 2022 Oder fish kill (ODER1, known to produce B-type prymnesins) with two well-studied P. parvum strains, UTEX2797 (known to produce A-type prymnesins), and RCC3427 (known to produce C-type prymnesins). D. magna neonates were exposed to a range of cell concentrations of the three P. parvum strains, and immobility was recorded after 24, 48, and 72 h. Overall, immobilization increased with increasing P. parvum cell concentrations and exposure times for all three strains. The ODER1 strain caused 100% immobilization of D. magna at a cell concentration of 200,000 cells/mL after 72 h. The two other P. parvum strains caused significantly higher immobilization at lower cell concentrations, with 100% immobilization at 25,000 cells/mL after 48 h for UTEX2797 and 10,000 cells/mL after 24 h for RCC3427. Our findings highlight strain-specific toxicity of P. parvum on zooplankton. Identifying the dominant prymnesin type of local P. parvum populations may be crucial to predict toxic bloom effects on zooplankton communities.