Phytoplankton Under Pressure - The Role of Environmental Drivers in Parasitism

Fungal parasitism is attracting growing attention in phytoplankton ecology because of its outstanding importance for aquatic food webs and energy cycling. However, relatively few studies have addressed baseline data on occurrence and environmental factors associated with chytrid parasite infections in natural ecosystems. This work provides insights into occurrence, prevalence, and dynamics of parasitic infections by studying three shallow, freshwater bodies during the growing season over a period of six years. Data were collected each year from April to October, monthly or fortnightly from a central point of each waterbody from 2019 to 2024. Chytrids were detected in each of the studied waterbodies, infecting species of green algae, diatoms, and cyanobacteria. General linear model (GLM) indicated that major factors driving the occurrence of chytrid infections were water temperature, nitrates, phosphates and pH. However, recurring and prevalent infections were observed in only one waterbody, which is classified as a natural, undisturbed aquatic ecosystem. The recorded infection prevalence (IPC) ranged between 0% and 20%, while the mean infection severity remained low throughout the study. Infections were highest in summer (June-August) and were most prominent during cyanobacterial blooms, although the most infected group of phytoplankton was green algae ( Desmodesmus spp.). GLM revealed a significantly positive correlation between IPC and water temperature, precipitation and cyanobacterial bloom. Overall, our results demonstrate that a combination of abiotic and biotic parameters drives the occurrence of parasitic infection more than just indicated by the magnitude of the prevalence alone.