Driving mechanisms of distinct seasonal phytoplankton dynamics in a low-latitude basin revealed by multi-platform...

Phytoplankton forms the foundation of marine food webs, and their seasonal dynamics shape ocean ecosystem functioning and carbon cycling. In low-latitude basins such as the South China Sea (SCS), these dynamics have traditionally been regarded as stable and primarily controlled by warm-cold seasonal oscillations, often evaluated using single metrics such as chlorophyll-a concentration. However, this paradigm fails to capture the significant regional heterogeneity within a single basin and the complex, sometimes contradictory, responses of different phytoplankton community parameters. To address this, we conducted a comprehensive analysis of a 20-year multi-platform dataset (remote sensing, ship-based, and Biogeochemical-Argo) to reveal how regional physical processes drive the seasonal dynamics of phytoplankton chlorophyll-a, community composition, and primary production in the basin of SCS. In the northern basin, winter monsoon coupling with strong Kuroshio intrusion elevated surface chlorophyll-a by 200 % compared to summer, yet contributed only 31.2 % to the annual primary production, revealing a significant decoupling driven by light limitation and low-temperature suppression of photosynthesis. In the southwestern basin, summer upwelling stimulated diatom blooms and sustained primary production comparable to its winter levels. Niche models confirmed that regional physical processes (Kuroshio intrusion vs. upwelling) select for distinct phytoplankton assemblages within the basin. We conclude that regional physical forcing, rather than basin-wide monsoon seasonality alone, is the primary driver of phytoplankton dynamics and carbon cycling in this low-latitude basin, supplementing the traditional seasonal oscillation paradigm for such systems.