Greater niche overlap and species association of phytoplankton in dry season than in wet season in Wujiang River, Yungui Plateau, China

Phytoplankton plays a crucial role in the energy flow and nutrient cycling of aquatic ecosystems. To understand the spatial and temporal distribution of phytoplankton in the Wujiang River, Yungui Plateau, SW China, samples were collected in 12 locations in wet and dry seasons and analyzed. We hypothesized that phytoplankton assemblages would exhibit significant temporal variability, with niche breadths of dominant species fluctuating seasonally, leading to distinct patterns of species association and community stability. Results show differences in community structure between the two seasons, but such changes did not cause non-significant differences in α-diversity. Diatoms were dominant in the assemblages in terms of biomass, while the numerical abundance of Cyanobacteria was highest in the wet season due to their relatively small cells. Rainfall-driven changes in runoff significantly altered nutrient availability, which in turn strongly affected phytoplankton structure. The more intense water flow contributed to a greater β-diversity in the wet season, driven primarily by species replacement, with stochastic processes played a more important role during the dry season. In the dry season, dominant species exhibited a broader niche breadth and greater niche overlap, along with more positive species associations, suggesting a more stable and resilient community structure. Conversely, in the wet season, species had narrower niche breadth and less niche overlap, leading to a less stable community. Both negative and positive species associations were observed, indicating a complex balance between environmental filtering and competition within the assemblages. These findings provide important insights into how seasonal environmental changes, particularly water flow and nutrient dynamics, shape phytoplankton communities in aquatic ecosystems. Understanding the mechanisms driving changes in community and stability is critical for predicting the impacts of climate change and managing aquatic biodiversity, as fluctuations in water flow and nutrient input may alter ecosystem functioning and productivity.