Divergent Planktonic Microbial Assembly Mediates the Natural Resolution of Freshwater Spring Blooms

Global warming is intensifying spring algal blooms in freshwater ecosystems, posing significant threats to water security and biodiversity. Planktonic microbial communities are pivotal to the stability of aquatic ecosystems, yet how community assembly mediates the natural resolution of spring algal blooms remains poorly understood. Here, the dynamics of planktonic microbial communities were tracked across the initiation, peak, and decline stages of a spring algal bloom in a freshwater reservoir. The results showed that a critical shift in planktonic communities, marked by reduced diversity, narrowed niche breadth, and intensified species competition, triggered a community reassembly process that ultimately facilitated bloom resolution. Community turnover was strongly governed by environmental gradients, particularly by water temperature and chlorophyll-a concentrations. Phylogenetic bin-based null model analysis revealed divergent assembly processes across planktonic microbial groups: bacterial assembly was dominated by homogeneous selection (43.5–53.6%), while microeukaryotic communities were constrained by dispersal limitation (32.8–52.3%). This divergence fostered a bloom-resolving consortium comprising algivorous metazoa (e.g., Cyclopoida ), organic matter-degrading bacteria (e.g., Bacillaceae ), and saprotrophic fungi (e.g., Mucoromycota ), whose synergistic interactions collectively accelerated bloom resolution. Collectively, these findings indicate that taxon-specific, divergent assembly processes underpin ecosystem resilience and highlight the need for management strategies tailored to distinct microbial functional groups and seasonal bloom dynamics.