The Jiulong River is a major river in Fujian Province, closely linked to the local residents' water supply, but suffered from frequent blooms of Unruhdinium between 2009 and 2011. To investigate the factors influencing the occurrence of Unruhdinium blooms and their associated community structure, this study employed a DNA metabarcoding approach, targeting the large subunit ribosomal (LSU D1–D2) to analyze the diversity, community structure, and environmental drivers of Unruhdiniums in Xipi Reservoir and the North Creek of the Jiulong River in 2020 and 2021. The results showed that the alpha diversity of dinoflagellates in Xipi Reservoir was higher in autumn compared to other seasons. Similarly, in the North Creek, upstream areas exhibited higher diversity than midstream and downstream areas. In terms of community composition, a total of 36 dinoflagellate genera (made up of 6 Unruhdinium species) were identified in Xipi Reservoir, while 44 dinoflagellate genera were identified in North Creek, including six Unruhdinium species. Redundancy analysis (RDA) indicated that temperature, dissolved inorganic nitrogen (DIN), and dissolved reactive phosphorus (DRP) were the primary environmental factors affecting the abundance of Unruhdinium. In particular, DRP levels showed significant differences between bloom and non-bloom periods, suggesting that DRP could be a key driver of Unruhdinium blooms. This study not only identifies the key environmental factors driving Unruhdinium blooms but also highlights the temporal and spatial variations in dinoflagellate community structure, providing valuable insights into the causes and ecological impacts of harmful algal blooms in the Jiulong River.
 

dinoflagellate blooms, Unruhdinium, high-throughput sequencing, environmental factors, Jiulong River