Bottom-Up Control and Phytoplankton Succession in a Subtropical Upwelling System

Diagnosing whether upwelling systems are governed by bottom-up or top-down control is critical for understanding marine ecosystem productivity and trophic interactions. While temperate upwelling systems are widely studied and generally characterized by bottom-up control, the dominant regulatory mechanism in subtropical upwelling systems remains uncertain due to their unique physical and biogeochemical dynamics. This study investigated the Qiongdong upwelling system in the northwest South China Sea, aiming to elucidate the drivers of phytoplankton biomass, community composition, and trophic interactions across different upwelling phases. Three consecutive surveys conducted during the summer of 2023 revealed significant spatial and temporal variations in hydrological parameters, nutrient availability, and biological communities. Nutrient availability was closely linked to upwelling intensity, consistently supporting elevated phytoplankton and mesozooplankton biomass, indicating that bottom-up control plays a dominant role in this system. Phytoplankton community composition exhibited a distinct successional pattern: <i>Prochlorococcus </i>dominated during early upwelling stages, driven by vertical mixing that transported it from nutrient-poor subsurface waters to the surface, where its ability to utilize reduced nitrogen provided a competitive advantage. As upwelling intensified, diatoms prevailed, supported by increased nitrate availability and turbulent mixing. This succession highlights how physical and chemical processes interact to shape phytoplankton dynamics. Additionally, minor taxa such as cryptophytes and prasinophytes enhanced community diversity and contributed to ecosystem stability under fluctuating environmental conditions. These findings highlight the Qiongdong upwelling system as a highly productive subtropical ecosystem governed by nutrient-driven processes, offering critical insights into the mechanisms regulating such systems and their potential responses to environmental variability.