Responses of protist communities to spatio-temporal changes in water masses in the northeastern East China Sea in 2023

<p style="line-height: 1.6;"> This study explores the dynamics of protist communities in the northeastern East China Sea (ECS) focusing on their responses to changing water masses and oceanographic conditions during spring, summer, and autumn in 2023. Water masses were classified using K-means clustering based on physico-chemical parameters, revealing significant seasonal variation. Protist community spatial, temporal, and vertical distributions were analyzed using 18S rRNA gene amplicon sequencing and morphological identification along the 33° N latitude, a region characterized by the confluence of multiple water masses, including Kuroshio surface (KSW) and sub-surface (KSSW) waters, Yellow Sea water (YSW), Changjiang diluted water (CDW), and ECS Shelf water. Protist diversity, community structure, and cell abundance exhibited pronounced seasonal variations influenced by temperature, salinity, and nutrient availability. In spring, YSW, characterized by phosphate deficiencies and, consequently, increased N : P ratios, exhibited a prevalence of dinoflagellates, whose phagotrophic capabilities likely conferred a competitive advantage in such nutrientfluctuating environments. During summer, diatoms were prevalent in the ECS Shelf water, underscoring the role this water plays in shaping seasonal community structure. In contrast, CDW exhibited low species diversities, likely due to unstable salinity. In autumn, water column mixing increased overall diversity and cell abundance, particularly in YSW, where <i>Chaetoceros socialis</i> and other diatoms thrived. Potential phosphorus limitation, especially in the western waters, may have further influenced the spatial structuring of communities across seasons. These findings highlight complex interactions between oceanographic factors and protistan communities in the northeastern ECS, contributing to a deeper understanding of their distributions and responses to environmental changes. </p>