Carbon biogeochemistry and CO2 emissions in a human impacted and mangrove dominated tropical estuary (Can Gio, Vietnam)

The quantitative contribution of tropical estuaries to the atmospheric CO2 budget has large uncertainties, both spatially and seasonally. We investigated the seasonal and spatial variations of carbon biogeochemistry downstream of Ho Chi Minh City (Southern Vietnam). We sampled four sites distributed from downstream of a highly urbanised watershed through mangroves to the South China Sea coast during the dry and wet seasons. Measured partial pressure of CO2 (pCO(2)) ranged from 660 to 3000 mu atm during the dry season, and from 740 to 5000 mu atm during the wet season. High organic load, dissolved oxygen saturation down to 17%, and pCO(2) up to 5000 mu atm at the freshwater endmember of the estuary reflected the intense human pressure on this ecosystem. We show that releases from mangrove soils affect the water column pCO(2) in this large tropical estuary (similar to 600 m wide and 10-20 m deep). This study is among the few to report direct measurements of both water pCO(2) and CO2 emissions in a Southeast Asian tropical estuary located in a highly urbanised watershed. It shows that the contribution of such estuaries may have been previously underestimated, with CO2 emissions ranging from 74 to 876 mmol m(-2) day(-1) at low current velocity (< 0.2 m s(-1)). Corresponding gas transfer velocities k(600), ranging from 1.7 to 11.0 m day(-1), were about 2 to 4 times of k(600) estimated using published literature equations.