Carbon Dioxide Fluxes to the Atmosphere From Waters Within Flooded Forests in the Amazon Basin

Inundated tropical forests are underrepresented in analyses of the global carbon cycle and constitute 80% of the surface area of aquatic environments in the lowland Amazon basin. Diel variations in CO2 concentrations and exchanges with the atmosphere were investigated from August 2014 to September 2016 in two flooded forests sites with different wind exposure within the central Amazon floodplain (3°23′S, 60°18′W). CO2 profiles and estimates of air–water gas exchange were combined with ancillary environmental measurements. Surface CO2 concentrations ranged from 19 to 329 μM, CO2 fluxes ranged from −0.8 to 55 mmol m−2 hr−1 and gas transfer velocities ranged from 0.2 to 17 cm hr−1. CO2 concentrations and fluxes were highest during the high water period. CO2 fluxes were three times higher at a site with more wind exposure (WE) compared to one with less exposure (WP). Emissions were higher at the WP site during the day, whereas they were higher at night at the WE site due to vertical mixing. CO2 concentrations and fluxes were lower at the W P site following an extended period of exceptionally low water. The CO2 flux from the water in the flooded forest was about half of the net primary production of the forest estimated from the literature. Mean daily fluxes measured in our study (182 ± 247 mmol m−2d−1) are higher than or similar to the few other measurements in waters within tropical and subtropical flooded forests and highlight the importance of flooded forests in carbon budgets. Plain Language Summary Aquatic habitats in the lowland Amazon emit large quantities of carbon dioxide (CO2). However, information on CO2 fluxes from seasonally flooded forests that constitute 80% of the surface area of aquatic environments in the lowland Amazon basin is sparse. We provide the first multi‐year measurements of CO2 exchanges within flooded forests of the central Amazon basin. Our approach combines measurement of dissolved CO2 concentrations and fluxes between the water and atmosphere and ecological data. Although the rates of CO2 emission by flooded forests are lower than other aquatic habitats, such as open waters in rivers and lakes, the combination of high CO2 concentrations and a large area results in an appreciable regional out gassing of carbon dioxide from flooded forests. These fluxes can represent about half of the net primary production of flooded forests in the central Amazon basin. Key Points CO2 concentrations in near‐surface waters within flooded Amazon fo