Ecological Impacts of Calcium Enrichment and Sediment Sterilization on Algal Bloom Dynamics and Microbial Communities in the Three Gorges Reservoir

Sediment-targeted interventions may affect algal blooms by altering nutrient availability associated with microbial communities. We tested this hypothesis using an in situ mesocosm experiment in the Three Gorges Reservoir, China, with an experiment composed of unaltered control, sterilized sediment, and calcium (Ca2+) enrichment at 100 mg L−1 and 300 mg L−1, respectively. Phytoplankton were examined microscopically, and bacterial and microeukaryotic communities were characterized by 16S and 18S rRNA gene amplicon sequencing. Phytoplankton composition shifted from early Cryptophyta dominance toward persistent dominance by the dinoflagellate Ceratium hirundinella. Ca2+ enrichment increased TN:TP ratios, consistent with reduced phosphorus availability and increased bacterial richness, while favoring decomposer-associated taxa. Sediment sterilization reduced microbial diversity and promoted opportunistic bacterial and fungal taxa. These results indicate that sediment modification can redirect, rather than suppress, bloom development through concurrent changes in nutrient availability and algal–microbial community structure. Bloom management strategies should therefore account for indirect microbial feedbacks and the potential replacement of one dominant community by another.