Ocean acidification and its regulating factors in the East China Sea off the Yangtze River estuary

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 COQ and eutrophication to acidification. Carbonate parameters data were collected during summer 2019 and combined winter 2011. During winter, acidification is primarily driven by rising atmospheric COQ, 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 COQ increases, while in bottom waters, the combined pressures of atmospheric COQ and intensified aerobic respiration leads to more severe acidification. Notably, biological processes now contribute more to acidification than increasing atmospheric COQ in the bottom waters. Our projections indicate that the summer bottom waters will experience the most pronounced acidification, with average pH levels expected to decline from 8.04 to 7.82 and aragonite saturation state (52ar) values decreasing from 2.24 to 1.38 between 2000 and 2100. Additionally, our study indicates that winter acidification trends are also concerning, with pH only slightly higher than in summer bottom waters. The buffering capacity and the DIC:TA ratio play significant roles in determining the rate of future pH and 52ar declines. The strong buffering capacity in summer surface waters mitigates the pH decline, while the low DIC:TA ratio results in a rapid drop in 52ar.