Seasonal Shifts from Water Depth to Nitrate Reorganize Protistan Communities Following Lake Freeze–Thaw Events

The seasonal freeze-thaw cycle induces a fundamental regime shift in lake ecosystems, primarily through the restructuring of microbial communities. This study investigated the dynamics and mechanisms of species diversity maintenance in protistan communities in Gonghai Lake, a shallow subalpine lake in China, across both ice-covered and ice-free periods. During ice cover, the protistan community exhibited a vertically stratified structure dominated by cryotolerant diatoms such as <i>Stephanodiscus</i>. Following thaw, the community transitioned to a more homogeneous, resource-driven assembly. Concurrently, the key environmental controls shifted from physical stratification (proxied by depth) to resource availability (notably NO₃^- and TOC), a change reflected in the taxonomic succession from Ochrophyta to Chlorophyta. Nevertheless, depth retained ecological relevance mediated by benthic-pelagic coupling, which supported a distinct near-sediment community. Our findings demonstrate that freeze-thaw-mediated terrestrial nutrient inputs directly modified protistan diversity and community structure. These alterations have fundamental implications for ecosystem functions in subalpine lakes, including nutrient cycling rates and energy flow through the microbial loop.