Hydrogeochemistry and greenhouse gases of the Pearl River, its estuary and beyond

The Pearl River (Zhujiang) ranks as the 13th or the 14th largest river in the world and is the 2nd largest in China in terms of annual water discharge. It is also the second largest river entering into the South China Sea. The three major tributaries, namely, the Xijiang (West River), the Beijiang (North River) and the Dongjiang (East River) are highly eutrophicated in the region of the Pearl River Delta because of the high nutrient and sediment loading. At places, the dissolved oxygen saturation level becomes as low as 2%. Naturally, CH4, N2O, and CO2 are highly supersaturated.Once the river water discharges into the main estuary, the Lingdingyang, it tends to flow out on the western side while open ocean waters with lower nutrient contents and pCO2 but higher salinity and pH values enter on the eastern side. Largely as a result of mixing with the open ocean water but partly because of biological consumption, pCO2 quickly reduces to near saturation only tens of kilometers off the mouth of the estuary. The % saturation of CH4 and N2O is also greatly reduced but the continental shelf remains as a source of these greenhouse gases to the atmosphere.