Poster

IAPSO

P02 - Physics and Biogeochemestry of Semi-Enclosed, Shelf Seas and Coastal Zones

Sequential Evolution of Changjiang Diluted Water and Its Impact on Stratification and Phytoplankton Blooms in the East China Sea during Summer 2020

The Changjiang Diluted Water (CDW) plays a critical role in shaping the hydrography and ecosystem dynamics of the East China Sea (ECS), particularly during summer when intensified freshwater discharge enhances stratification and modulates biogeochemical processes. However, the sequential evolution of CDW and its associated physical and ecological impacts remain poorly understood due to limited high-resolution observations. In this study, we investigate CDW progression at the Ieodo Ocean Research Station (I-ORS) during summer 2020 using in situ high-resolution observations. CDW evolution was classified into five distinct phases, characterized by abrupt shifts in low-salinity intensity and vertical structure, reflecting intrusion and dissipation of CDW driven by monsoonal winds, tidal forcing and typhoon-induced mixing. CDW intrusion strengthened upper-ocean stratification through barrier layer formation, which suppressed vertical mixing, trapped heat within the surface layer, and increased sea surface temperatures. The nutrient influx from CDW enriched nitrate and silicate concentrations, favoring diatom proliferation over picoplankton, with the subsurface chlorophyll maximum progressively shoaling into the low-salinity surface layer as CDW influence grew. Despite the phosphate limitation inherent to CDW, internal wave-induced mixing facilitated nutrient replenishment from deeper layers, partially alleviating phosphate deficiency and enhancing phytoplankton productivity. The dissipation of CDW, triggered by Typhoon BAVI, resulted in strong vertical mixing, which eroded stratification, homogenized the water column, and restored pre-intrusion hydrographic conditions. This study provides a comprehensive assessment of the sequential evolution of CDW and its cascading effects on stratification and phytoplankton blooms in the ECS, highlighting the necessity of sustained high-resolution monitoring to improve our understanding of freshwater-driven oceanographic processes and their ecological implications.