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We investigated distributions of surface water CO₂ partial pressure (pCO₂), dissolved oxygen (DO) and associated carbonate parameters in the Pearl River estuary, a large subtropical estuary under increasingly anthropogenic pressure in China, in the summer of 2000 and late spring of 2001. pCO₂ levels, measured underway using a continuous measurement system, were high during both seasons, with levels of >4000 μatm at salinity <0.5. pCO₂ distribution overall mirrored DO across the salinity gradient. Using the linear relationship between excess CO₂ and apparent oxygen utilization (AOU) in surface water, we conclude that aerobic respiration is the most important process in maintaining such high pCO₂ measured upstream. The material being respired is likely in a close association with the organic pollutants discharged into the system. Based on the measured excess CO₂ vs. AOU plots, we estimate that the …