Mechanism and driving factors of phytoplankton community construction in the upper and middle reaches of the Yellow River

<p indent="0mm">The phytoplankton community is an important primary producer in river ecosystems, and understanding its structural characteristics and the mechanism of community assembly is crucial for understanding the health status and evolution of aquatic ecosystems. Phytoplankton community assembly and driving mechanisms in the Yellow River, a typical heavily sediment-laden river in China, remain unclear. This study selected 21 sampling sites from typical reservoirs and river sections in the upper and middle reaches of the Yellow River in the summer of 2022 (July–August) to explore the structural characteristics of phytoplankton communities and the mechanisms and driving factors of their community construction. The results showed that 53 phytoplankton species from 7 phyla and 35 genera occurred in the upper and middle reaches of the Yellow River, with Bacillariophyta and Chlorophyta as the dominant phyla. The phytoplankton density and biomass ranged from <sc>8.64×10⁵ to 1.95×10⁷ cells L⁻¹</sc> and <sc>0.57–9.93 mg L⁻¹,</sc> respectively, and tended to increase from the tail to the head of the reservoirs. The deterministic process represented by ecological niche differentiation dominated the construction of the phytoplankton community, with environmental factors explaining most of the changes (16%). Notable environmental and geographic distance decay relationships of the phytoplankton community were observed, and the geographic distance decay relationships in the reservoir areas were more substantial than those in the free-flowing river sections. Total dissolved solids and total phosphorus were the key factors influencing the composition of the phytoplankton community in the upper and middle reaches of the Yellow River, whereas total phosphorus, total dissolved solids, and altitude significantly influenced phytoplankton diversity in free-flowing sections. This study deepens our understanding of phytoplankton community succession in the heavily sediment-laden river and demonstrates the biogeographic patterns and construction mechanism of the phytoplankton community in various water bodies of the Yellow River.