Ocean acidification is a critical environmental issue affecting marine ecosystems globally. Estuaries are particularly vulnerable due to the combined pressures of increasing atmospheric CO₂，local eutrophication and river freshwater input. This study examines the seasonal variations in carbonate system parameters in the East China Sea (ECS) off the Yangtze River Estuary (YRE) and analyzes the contributions of anthropogenic CO₂ and eutrophication to acidification. Data were collected during summer 2019 and winter 2011 to investigate the seasonal differences in carbonate dynamics. A conservative river-ocean mixing model and the ΔDIC method were employed to estimate the influence of biological processes and anthropogenic CO₂ invasion since the pre-industrial period. During winter, acidification is primarily driven by rising atmospheric CO₂, with minimal impact from biological processes. In contrast, summer presents a different pattern: enhanced photosynthesis due to eutrophication in surface waters helps mitigate the acidification effects of atmospheric CO₂ increases, while in bottom waters, the combined pressures of atmospheric CO₂ and intensified aerobic respiration lead to more severe acidification. Notably, biological processes now contribute more to acidification than increasing atmospheric CO₂ in the bottom waters. Our projections show that the ECS will generally remain supersaturated with respect to aragonite until 2100. The summer bottom waters are expected to experience the most severe acidification, with the average pH projected to drop from 8.05 to 7.84 and Ω_ar decreasing from 2.45 to 1.58. Additionally, our study indicates that winter acidification trends are also concerning, with pH and Ω_ar only slightly higher than in summer bottom waters. The buffering capacity plays a significant role in future pH decline, with the strongest buffering capacity in summer surface waters, leading to the slowest pH decrease. Similarly, the DIC:TA ratio has a notable impact on the future drop in Ω_ar, with the lowest DIC:TA ratio in summer surface waters causing the fastest decline in Ω_ar.

coastal, acidification, eutrophication, Yangtze River estuary