Abstract: Aims Planktonic diatoms constitute the dominant algal group in alpine river ecosystems, playing a crucial role in maintaining ecosystem stability. This study investigates the community assembly mechanisms and key drivers of planktonic diatoms in the Yarlung Tsangpo River. Methods Between 2019 and 2023, a total of 84 representative sampling sites were established throughout the Yarlung Tsangpo River basin.Identification of planktonic diatoms in summer and autumn using a combined ap-proach of live observation and fixed staining.Analyses were conducted on their community structure characteris-tics, response mechanisms to environmental changes, and the driving factors influencing community assembly. Important findings A total of 543 diatom species were identified, belonging to 2 classes, 6 orders, 10 families, and 55 genera, NMDS analysis revealed significant inter-seasonal and inter-basin differences in diatom communi-ties. The α diversity index was significantly higher in the middle reaches than in the upper and lower reaches, ex-hibiting a unimodal spatial distribution pattern. β-diversity decomposition revealed that species turnover primar-ily drove inter-community differences, though nested processes dominated in the middle reaches during autumn. Community assembly was influenced by both stochastic and deterministic processes, with deterministic processes predominating, particularly during summer. Geographic distance and altitude significantly affected diatom com-munity structure, with geographic distance exhibiting stronger attenuation effects than altitude, and spatial con-straints becoming more pronounced in autumn communities. Altitude, dissolved oxygen, and total dissolved salts are key factors driving changes in diatom community structure. The revealed community assembly mechanisms and key drivers provide scientific basis for biodiversity conservation and ecological management in alpine rivers.

Key words: planktonic diatom, Yarlung Tsangpo River, biodiversity, deterministic process, geographic pattern