What Is the Potential of Daphnia (Water Flea) Predation as a Means of Biological Suppression of Prymnesium parvum (Golden Algae) Blooms in Ecologically Relevant Conditions?

This study explores the interaction between <i>Prymnesium parvum</i> and <i>Daphnia magna</i> under low-salinity conditions. <i>P. parvum</i> showed reduced growth below 0.4 PSU and peaked at 1.0 PSU within the tested 0.2-1.0 PSU range. <i>D. magna</i>, exposed to <i>P. parvum</i> across 0.0-6.0 PSU, experienced increased mortality at 4.0 and 6.0 PSU, but tolerated 0.0-1.0 PSU well and grazed actively on <i>P. parvum</i> without significant vitality loss. This range reflects conditions observed in the Oder River during the 2022 fish die-off. The count of <i>P. parvum</i> cells did not vary significantly across the 0.2 to 1.0 PSU range of salinities in <i>D. magna</i> presence, except at 0.6 PSU. All daphnids survived even at <i>P. parvum</i> densities of 1 × 10⁵ cells/mL, though increasing algal concentrations reduced juvenile growth rates. Direct observation under a microscope confirmed algal ingestion. Toxin accumulation in cells and medium likely reduced grazing efficiency via allelopathic effects. The study assessed whether <i>D. magna</i> can tolerate prymnesins while maintaining feeding under varying salinities. Results suggest that <i>Daphnia magna</i> could act as a biological suppressor of golden algae under certain environmental conditions, though further work is needed to quantify grazing efficiency and prymnesins concentrations.