Distribution and control mechanism of pCO and water-air CO efflux in the Pearl River Estuary.

Estuaries are generally considered to be important sources of atmospheric CO. However, the differences between estuaries, and inadequate observations of partial pressure of CO in estuarine water (pCO) hamper global estuarine CO budgeting. In this study, the longitudinal distribution of CO in the waters of Modaomen (MSE) and Lingdingyang (LSE), two sub-estuaries of the Pearl River Estuary (PRE), and its influencing mechanism are studied. The change in the distribution of pCO along the distance from the upstream estuary to the ocean between LSE and MSE was significantly different. pCO at the LSE ranges from 238 to 7267 µatm, whereas the MSE ranges from 406 to 3078 µatm. Stronger microbial respiration and relatively long water retention times were the main influences that led to higher pCO at LSE than at MSE. Seasonally, the increase of soil CO into the water in the upstream basin caused by precipitation is the potential influencing factor that the water pCO in the flood season is higher than in the dry season. PRE was a net source of atmospheric CO with an average annual water-air flux of 41.2 ± 33.3 mmol m day. Our results suggest that the differences in longitudinal gradients of pCO between estuaries in the same region and the effects of different gas transport velocity models on CO emission estimates need to be considered in estuarine CO emission budgeting.