Ice cover drives seasonal restructuring of larger-bodied heterotrophic protist communities in a hypereutrophic lake

ABSTRACT Comprehensive annual studies on the seasonal dynamics of heterotrophic protists in ice-covered hypereutrophic systems are scarce, despite their vital roles in aquatic microbial food webs. We examined the community composition and abundance of larger-bodied (>15 μm) heterotrophic protists over 13 consecutive months in a northern temperate hypereutrophic lake that experiences four months of ice cover. The abundance of larger-bodied heterotrophic protists exhibited considerable seasonal variation, with an 852-fold annual fluctuation (from 100 to 85 200 individuals per liter), significantly surpassing the typical 10–50-fold variations observed in less extreme systems. Community functional composition underwent complete restructuring between the ice-covered and open-water periods: sessile Vorticella dominated under ice (17.7 times more abundant), while planktonic Strobilidium was 171 times more abundant during the open-water season. Testate amoebae (Difflugia) were absent under ice. Correlation analyses identified 17 significant associations between taxa and environmental parameters, with the strongest being Tintinnopsis sp.1 and redox potential (ρ = 0.898). These findings highlight that ice cover prompts a fundamental restructuring of larger-bodied protist communities, with different functional groups prevailing under contrasting seasons. Climate-driven reductions in ice cover duration may alter the structure of protist communities in northern temperate lakes, potentially triggering cascading effects on microbial food webs.