CO2 flux and seasonal variability in a large subtropical estuarine system, the Pearl River Estuary, China

Abstract

This paper presents the spatial distribution and seasonal changes of the carbonate system and CO2 fluxes in a complex river‐estuary system located in a subtropical region, the Pearl River Estuary, based on five surveys covering primarily a wet and dry seasonal cycle on two major subestuaries of the Pearl River, namely Lingdingyang and Huangmaohai. Significant spatial and seasonal variations of surface water partial pressure of CO2 (pCO2) were observable in these two subestuaries. While both Lingdingyang and Huangmaohai had higher pCO2 in their upper estuaries, which quickly decreased downstream as seen in many estuarine settings elsewhere, significant differences occurred between the two subestuaries in terms of pCO2 level, with much higher pCO2 in the upper Lingdingyang than the upper Huangmaohai. In terms of seasonality, substantially higher pCO2 was observed in warm and wet seasons in both upper estuaries (2100–8350 μatm in the Lingdingyang and 1040–3590 μatm in the Huangmaohai) than in cold and dry seasons (1100–7460 μatm and 560–970 μatm in the Lingdingyang and the Huangmaohai, respectively). As a consequence, CO2 emission from the Pearl River Estuary system in summer was ∼6 times of that in winter. At the same time, we observed a clear drawdown of pCO2 in the lower estuary in both summer and winter, reaching a level of water pCO2 which was below the atmospheric level. This seasonal and spatial contrast can also be seen in the distribution of dissolved inorganic carbon (DIC) and total alkalinity. On the basis of a seasonal and zonal distribution of pCO2, the annual CO2 emission from the Pearl River Estuary was estimated to be ∼3 × 1010 mol C, which is equivalent to ∼6% of the total DIC export flux to the South China Sea from the Pearl River system.