Salinity and Mixotrophy as Key Drivers of HAB Species Distributions in the Changjiang Estuary

The Changjiang Estuary is a major hotspot for harmful algal blooms (HABs) in China. Metabarcoding analysis was conducted during four seasonal cruises (March, August, October 2025, and January 2026) at 14 stations to reveal the composition, dynamics, and environmental drivers of harmful algal bloom species in this region. A total of 63 HAB species were detected, with diatoms dominating year-round but dinoflagellates increasing markedly from spring (21.6%) to summer and autumn (34.8–44.6%). Salinity emerged as the primary factor structuring spatial patterns: the euryhaline diatom Skeletonema costatum dominated low-salinity nearshore waters, whereas diverse dinoflagellates, including Karlodinium veneficum, Prorocentrum micans, Heterocapsa rotundata, and Alexandrium catenella, prevailed offshore. Seasonally, the diatom-to-dinoflagellate ratio declined from 10.6 in spring to 1.6 in autumn, coinciding with rising temperature and declining N:P ratios. Notably, most high-abundance dinoflagellates are mixotrophic, a trait that likely confers competitive advantages. Our findings demonstrated that HAB species dynamics were shaped by the interplay between salinity-driven habitat filtering and trophic functional traits, advocating for the inclusion of mixotrophy as a key criterion in future HAB monitoring and early warning frameworks. Future monitoring would benefit from integrating these molecular trends with classical quantitative methods for validation.