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

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Title:	Sequential Evolution of Changjiang Diluted Water and Its Impact on Stratification and Phytoplankton Blooms in the East China Sea During Summer 2020.
Authors:	Lee, Seung‐Woo¹ (AUTHOR), Lee, Dabin¹ (AUTHOR) ldb1370@kiost.ac.kr, Noh, Suyun² (AUTHOR), Kim, Go‐Un¹ (AUTHOR), Park, Sung‐Hwan¹ (AUTHOR), Jeong, Jin‐Yong³ (AUTHOR), Lee, Howon⁴ (AUTHOR), Noh, Jae Hoon⁴ (AUTHOR), Jeong, Jongmin¹ (AUTHOR), Lee, Jaeik¹ (AUTHOR), Min, Yongchim¹ (AUTHOR), Lee, Su‐Chan¹ (AUTHOR), Min, In‐Ki¹ (AUTHOR), Choi, Jin‐Yong¹ (AUTHOR)
Source:	Journal of Geophysical Research. Oceans. Aug2025, Vol. 130 Issue 8, p1-20. 20p.
Subject Terms:	Algal blooms, Nutrient cycles, Ocean, Oceanography, Typhoons, Vertical mixing (Earth sciences)
Geographic Terms:	East China Sea, Yangtze River (China)
Abstract:	The Changjiang Diluted Water (CDW) plays a crucial role in shaping the hydrography and ecosystem dynamics of the East China Sea (ECS), particularly during summer when freshwater discharge enhances stratification and modulates biogeochemical processes. Despite its importance, the detailed progression of CDW and its short‐term impacts remain poorly understood due to the limited availability of high‐resolution observations. Using high‐resolution in situ observations from the Ieodo Ocean Research Station in summer 2020, we examined the sequential evolution of CDW and its effects on the upper‐ocean structure and phytoplankton blooms. CDW evolution was categorized into six distinct phases, characterized by abrupt shifts in salinity and stratification, driven by monsoonal winds, tides, and typhoon‐induced mixing. CDW intrusion formed a persistent barrier layer that suppressed vertical mixing and trapped surface heat, leading to increased sea surface temperatures. Nutrient influx from CDW, enriched in nitrate and phosphate, promoted the proliferation of diatoms relative to picoplankton and dinoflagellates, although all groups increased during the CDW‐influenced period. Internal wave‐induced mixing facilitated nutrient replenishment from deeper layers, partially alleviating phosphate deficiency inherent in CDW and enhancing phytoplankton productivity. Typhoon Bavi disrupted stratification through strong vertical mixing, resulting in a high‐salinity state that differed from preintrusion hydrography. This study provides a time‐resolved view of CDW‐driven variability and demonstrates how episodic freshwater inputs, coupled with physical forcing, regulate stratification, nutrient supply, and ecosystem response in the ECS. Plain Language Summary: The Changjiang (Yangtze) River delivers a large volume of freshwater into the East China Sea, influencing ocean conditions and marine ecosystems. In summer 2020, record‐breaking rainfall led to an unusually strong discharge, forming an extensive low‐salinity plume known as the Changjiang Diluted Water (CDW), which caused significant changes in salinity, temperature, and biological productivity in the ECS. Using high‐resolution data from the Ieodo Ocean Research Station, this study tracked the evolution of CDW and its influence on ocean stratification, temperature, and biological productivity. CDW progressed through six distinct phases, shaped by winds, tides, and typhoons. As CDW expanded, it formed a barrier layer that suppressed vertical mixing and trapped heat in the surface ocean, increasing sea surface temperatures. It also carried nutrients, such as nitrate and phosphate, stimulating the growth of diatoms, although other phytoplankton groups, including dinoflagellates, also increased. Initially, low phosphate levels limited phytoplankton blooms, but later, internal wave‐driven mixing transported phosphate from deeper layers, supporting further growth. Eventually, Typhoon Bavi disrupted the stratification, mixing the water column, and bring in high‐salinity offshore water, shifting the system to a new state. This study highlights the value of sustained, high‐frequency observations for understanding changing marine environments under climate variability. Key Points: High‐resolution time series revealed six‐phase evolution of Changjiang Diluted Water (CDW) during an extreme freshwater discharge in 2020CDW‐driven barrier layer intensified stratification and nutrient retention, initiating phytoplankton blooms under phosphate limitationInternal wave‐induced vertical mixing alleviated phosphate limitation by transporting subsurface nutrients, sustaining bloom development [ABSTRACT FROM AUTHOR]
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Database:	GreenFILE

Description
Abstract:	The Changjiang Diluted Water (CDW) plays a crucial role in shaping the hydrography and ecosystem dynamics of the East China Sea (ECS), particularly during summer when freshwater discharge enhances stratification and modulates biogeochemical processes. Despite its importance, the detailed progression of CDW and its short‐term impacts remain poorly understood due to the limited availability of high‐resolution observations. Using high‐resolution in situ observations from the Ieodo Ocean Research Station in summer 2020, we examined the sequential evolution of CDW and its effects on the upper‐ocean structure and phytoplankton blooms. CDW evolution was categorized into six distinct phases, characterized by abrupt shifts in salinity and stratification, driven by monsoonal winds, tides, and typhoon‐induced mixing. CDW intrusion formed a persistent barrier layer that suppressed vertical mixing and trapped surface heat, leading to increased sea surface temperatures. Nutrient influx from CDW, enriched in nitrate and phosphate, promoted the proliferation of diatoms relative to picoplankton and dinoflagellates, although all groups increased during the CDW‐influenced period. Internal wave‐induced mixing facilitated nutrient replenishment from deeper layers, partially alleviating phosphate deficiency inherent in CDW and enhancing phytoplankton productivity. Typhoon Bavi disrupted stratification through strong vertical mixing, resulting in a high‐salinity state that differed from preintrusion hydrography. This study provides a time‐resolved view of CDW‐driven variability and demonstrates how episodic freshwater inputs, coupled with physical forcing, regulate stratification, nutrient supply, and ecosystem response in the ECS. Plain Language Summary: The Changjiang (Yangtze) River delivers a large volume of freshwater into the East China Sea, influencing ocean conditions and marine ecosystems. In summer 2020, record‐breaking rainfall led to an unusually strong discharge, forming an extensive low‐salinity plume known as the Changjiang Diluted Water (CDW), which caused significant changes in salinity, temperature, and biological productivity in the ECS. Using high‐resolution data from the Ieodo Ocean Research Station, this study tracked the evolution of CDW and its influence on ocean stratification, temperature, and biological productivity. CDW progressed through six distinct phases, shaped by winds, tides, and typhoons. As CDW expanded, it formed a barrier layer that suppressed vertical mixing and trapped heat in the surface ocean, increasing sea surface temperatures. It also carried nutrients, such as nitrate and phosphate, stimulating the growth of diatoms, although other phytoplankton groups, including dinoflagellates, also increased. Initially, low phosphate levels limited phytoplankton blooms, but later, internal wave‐driven mixing transported phosphate from deeper layers, supporting further growth. Eventually, Typhoon Bavi disrupted the stratification, mixing the water column, and bring in high‐salinity offshore water, shifting the system to a new state. This study highlights the value of sustained, high‐frequency observations for understanding changing marine environments under climate variability. Key Points: High‐resolution time series revealed six‐phase evolution of Changjiang Diluted Water (CDW) during an extreme freshwater discharge in 2020CDW‐driven barrier layer intensified stratification and nutrient retention, initiating phytoplankton blooms under phosphate limitationInternal wave‐induced vertical mixing alleviated phosphate limitation by transporting subsurface nutrients, sustaining bloom development [ABSTRACT FROM AUTHOR]
ISSN:	21699275
DOI:	10.1029/2025JC022655