High partial pressure of CO2 and its maintaining mechanism in a subtropical estuary: the Pearl River estuary, China

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 upper limit of pCO2 should be ∼7000 μatm in the Pearl River estuary assuming that CO2 was produced solely by aerobic respiration.