Environmental drivers of phytoplankton community structure and diatom size restructuring in the Yellow Sea.

The Yellow Sea (YS) is a productive marginal sea undergoing substantial environmental change associated with warming and hydrographic variability. We analyzed total and size-fractionated phytoplankton assemblages using high-performance liquid chromatography pigment analysis and microscopy across 12 seasonal surveys in 2020, 2022, and 2023. Diatoms dominated throughout the study period (56.7 ± 15.0%), with cryptophytes subdominant in winter, spring, and autumn and cyanobacteria and prymnesiophytes more prominent under warmer, nutrient-imbalanced summer conditions. Realized niche analysis separated these groups along temperature and nutrient gradients, with cryptophytes occupying a niche adjacent to but distinguishable from that of diatoms. Microscopy indicated that the diatom assemblage was mainly composed of chain-forming taxa. Despite persistent diatom dominance at the group level, size-fractionated analysis revealed marked restructuring within the diatom assemblage. Small-sized diatoms dominated throughout the study, accounting for 68.1 ± 11.1% of total diatoms on average, and seasonally adjusted analyses suggested a progressive shift from the large-to the small-sized fraction over the three-year period. Despite belonging to the same taxonomic group, the two size fractions occupied different thermal niches. Large-sized diatoms occupied a cooler optimum (μ = 13.2 °C) and declined monotonically with temperature, whereas small-sized diatoms occupied a warmer optimum (μ = 16.0 °C) and persisted across a broader thermal range. These findings suggest that, within this three-year dataset, phytoplankton variability in the YS was expressed not only among major taxonomic groups but also through niche differentiation between size fractions within the persistently dominant diatom assemblage, a pattern that group-level composition alone cannot resolve.