Strong sources of CO2 in upper estuaries become sinks of CO2 in large river plumes

An extensive search of the literature and data banks identified studies of water-to-air CO2 exchange in 106 estuaries. Generally, pCO(2) in upper estuaries is highly supersaturated with respect to the atmospheric CO2, and so a large amount of CO2 is released to the atmosphere per unit surface area. Wider mid and lower estuaries are associated with slower river flow and lower turbidity, and therefore greater biological productivity. Further, mixing with low-pCO(2) seawater reduces pCO(2) and, thereby, the water-to-air CO2 flux on the ocean side. All of the globe's estuaries release 0.26 Pg C/y to the atmosphere. However, nutrients that are provided by large rivers, such as the Amazon and Changjiang (Yangtze), and those entrained by the river plumes promote photosynthesis to such an extent that the water becomes undersaturated. Accordingly, the large river plumes become a CO2 sink even many hundred kilometers beyond the river mouth.