A basin-scale comparative analysis of phytoplankton community responses to contrasting hydrological extremes in the Yellow River Basin

Phytoplankton are key indicators of aquatic ecosystem health, and their community dynamics are highly sensitive to hydrological fluctuations. However, understanding of how contrasting hydrological extremes (floods and droughts) shape phytoplankton communities in large sediment-laden rivers remains limited. This study examined the impacts of extreme hydrological events on phytoplankton communities in the Yellow River Basin by comparing an extreme wet year (2021) and a relative dry year (2022) across 81 sampling sites. Results showed that in the dry year, water temperature, total nitrogen and turbidity were all higher than that in the wet year. Consequently, phytoplankton density and biomass were substantially lower in the dry year compared to the wet year. A clear spatial succession was observed along the river continuum: diatom-dominated assemblages in headwaters ( Nitzschia , Cymbella , Gomphonema , Navicula ) shifted to turbidity-tolerant taxa ( Gomphonema , Coelastrum ) in the middle reaches, and finally to cyanobacteria-dominated assemblages ( Planktothrix , Chroococcus , Microcystis ) in downstream and lakes. The middle reaches were critical for sediment and nitrogen pollution, with notably higher turbidity and total nitrogen in the dry year; downstream sections were affected by organic pollution and lakes showed nutrient enrichment with potential for internal release. These findings reveal the cascading effects of hydrological extremes on phytoplankton communities through alterations in turbidity, temperature, and nutrient dynamics, providing a spatial framework for understanding large river responses to contrasting hydrological regimes.