Spatial and temporal pattern of phytoplankton community assembly processes in the largest tributary of the Yangtze River, China

Background Elucidating phytoplankton community assembly processes is crucial for understanding the mechanisms of river bloom formation. The blooms in the Hanjiang River, the largest tributary of the Yangtze River, have garnered significant attention. However, research on the phytoplankton community assembly processes and their driving mechanisms in this river remains limited. Results Based on a comprehensive survey of phytoplankton communities in the Hanjiang River during both dry and wet seasons in 2023, this study employed statistical methodologies, including similarity and correlation analyses, to investigate these assembly processes. Furthermore, it quantified the relative contributions of stochastic and deterministic processes. The results indicated significant heterogeneity in the distribution and composition of phytoplankton communities. Phytoplankton community dissimilarity increased significantly with geographical distance. VPA results showed that spatial variables had a stronger effect on phytoplankton communities than environmental factors. Fitting results from the neutral community and null models revealed that stochastic processes played a more significant role in phytoplankton community construction in the Hanjiang River than deterministic processes, with undominated stochastic processes being predominant during both the dry (82%) and wet (64%) seasons. The roles of diatoms and cyanobacteria in community construction were systematically analyzed based on the dominant species in the Hanjiang River blooms. Compared to other phytoplankton species (such as diatoms), cyanobacteria were more influenced by deterministic factors, suggesting a risk of cyanobacteria blooms in the middle and lower reaches of the Hanjiang River during the wet season. Conclusions This study elucidated the phytoplankton community assembly processes and their influencing factors in the Hanjiang River, playing an important role in revealing bloom outbreak mechanisms and providing insights for future studies on the assembly processes of phytoplankton and other aquatic organisms.