Spatiotemporal variation in marine plankton communities driven by environmental changes in the East China sea

Unicellular plankton are the foundation of marine food webs and biogeochemical cycles. This study analyzed long-term observations (2009-2021) of the taxonomic composition and size structure of nano- to microplankton in the East China Sea to assess the impact of environmental changes on these communities. Over the past decade, aloricate ciliate and chain-forming diatom abundances have declined, likely due to rising sea surface temperatures and decreasing concentrations of phosphate, nitrate, and silicate. Sea surface concentrations of these nutrients and dinoflagellate biomass were positively correlated with the Pacific Decadal Oscillation (PDO) index, while the diatom-to-dinoflagellate ratio decreased as the PDO index increased. These findings reveal the largescale climatic drivers of nutrient dynamics and plankton composition. The normalized biomass size spectrum (NBSS) slope was shallower in areas with higher salinities, diatom and ciliate biomasses, and diatom-todinoflagellate ratios, and it was steeper in areas with higher nutrient concentrations. The NBSS slopes were steeper (i.e. more smaller organisms) during winters with high nutrient concentrations, likely due to reduced light availability from high turbidity and increased grazing pressure. The 2017 summer cruise identified three spatial clusters of plankton communities that were influenced by different water masses. Cluster 1, influenced by the China Coastal Current, was dominated by athecate dinoflagellates. Clusters 2 and 3, influenced by the Taiwan Warm Current and Kuroshio Current, respectively, were dominated by diatoms. This study highlights the importance of long-term monitoring and comprehensive analyses to better understand how plankton communities will respond to climate change and anthropogenic activity.