In November 2011, the partial pressures of carbon dioxide (pCO2) in water and air in a floodplain lake of the Amazon River in Brazil were 800 ± 75 and 387 ± 8 ppmv, respectively. Turbulent CO2 fluxes from the lake measured with eddy covariance ranged from 0.05 to 2.2 µmol m−2 s−1. The corresponding gas transfer velocities k600 ranged from 1.3 to 31.6 cm h−1, averaging 12.2 ± 6.7 cm h−1. At moderate to high wind speed, k600 increased with wind speed, with values above parameterizations for other lake ecosystems. During the prevailing tropical low wind speed (below 2.7 m s−1) and high insolation conditions, unexpected high k600 values (up to 20 cm h−1) were obtained and correlated with latent heat and sensible heat fluxes. In Amazonian open lakes, owing to long quiescent periods of low wind speed but extremely high daytime insolation and heat fluxes, thermal enhancement makes time-integrated gas transfer velocities four to five times higher than those computed from classic wind parameterization.