Carbon dioxide outgassing from Amazonian aquatic ecosystems in the Negro River basin

Outgassing of biogenic gases from river systems is observed widely, and tropical rivers and their extensive floodplains are likely to be large sources. By combining the first regional survey of CO2 outgassing from rivers and floodplains in the Amazonian Negro River basin with areal coverage of habitats, we demonstrate large CO2 fluxes from these dissolved organic carbon-rich waters. We conducted measurements of pCO(2) and calculations of CO2 outgassing from the water to the atmosphere in river channels and floodplains, including seasonally flooded forests (igap) and shrubs (campinas), of the Negro River and 21 of its tributaries, in both high (2011) and low (2012) water phases. CO2 outgassing averaged 96 +/- 48 mmol C m(-2) day(-1) during low water and 253 +/- 193 mmol C m(-2) day(-1) during high water. We observed lower rates of CO2 outgassing in vegetated habitats in comparison to the river channels, due mainly to lower wind speeds. Regional CO2 outgassing was determined based on average CO2 emissions in each habitat and the area covered by each category of aquatic habitat in the basin. Our estimate of total CO2 emission from the basin is 35.1 +/- 4.3 Tg C year(-1). These results add to the current understanding of the role of Amazonian rivers and floodplains in local, regional and global estimates CO2 emission from inland aquatic ecosystems.