Warming increases richness and shapes assemblages of eukaryotic parasitic plankton

Anthropogenic activities have led to a global rise in water temperatures, leading to an increased interest in how warming affects infectious disease ecology. While most studies have focused on individual host-parasite systems, there is a gap in understanding the impact of warming on multi-host, multi-parasite assemblages in natural ecosystems. To address this gap, we examined freshwater eukaryotic parasite communities in ten natural lakes near Konin, Poland: five artificially heated and five nearby non-heated “control” lakes. Since 1958, the heated lakes have experienced a mean annual temperature increase of 2°C due to hot water discharge from two adjacent power plants. We collected seasonal environmental DNA (eDNA) samples from surface waters over a two-year period and applied targeted metabarcoding to compare the richness and distribution of eukaryotic parasites across lake types, with a focus on protists and fungi. Using literature searches and sequence metadata from GenBank, we identified putative parasites which included Alveolates, Stramenopiles, basal Fungi and Ichthyosporeans, as well as their associated hosts. Heated lakes exhibited distinct parasite assemblages, with higher richness of parasitic chytrids and aphelids particularly during summer, suggesting thermal preferences among some freshwater parasite species. A general linear model suggests that increased chytrid richness in heated lakes may be partly due to higher richness of their green algal, diatom- and dinoflagellate hosts. These findings highlight how warming can drive shifts in parasite richness and assemblages, potentially altering host-parasite dynamics in freshwater ecosystems.