Spatial and temporal variability of <i>p</i>CO₂ and CO₂ emissions from the Dongjiang River in South China

CO2 efflux at the water–air interface is an essential component of the riverine carbon cycle. However, the lack of spatially resolved CO2 emission measurement still hinges the accuracy of estimates on global riverine CO2 emissions. By deploying floating chambers, seasonal changes in river water CO2 partial pressure (pCO2) and CO2 evasion from the Dongjiang River in South China were investigated. Lateral soil CO2 input and dilution effect caused by precipitation played critical roles in controlling riverine pCO2 in small rivers, while the decomposition of allochthonous organic carbon is responsible for pCO2 variability in large rivers. Temperature-normalized gas transfer velocity (k600) in small rivers were 8.29 ± 11.29 m d−1 and 4.90 ± 3.82 m d−1 for the wet season and dry season, respectively, which were nearly 70 % higher than that of large rivers (3.90 ± 5.55 m d−1 during the wet season and 2.25 ± 1.61 m d−1 during the dry season). A significant correlation was observed between k600 and flow velocity but not wind speed regardless of river size. Majority of the surveyed rivers were net CO2 source, exhibiting substantial seasonal variations. The mean CO2 flux was 300.1 and 264.2 mmol m−2 d−1 during wet season for large and small rivers, respectively, 2-fold larger than that during dry season. The absence of commonly observed higher CO2 fluxes in small rivers could be associated with the depletion effect caused by abundant and consistent precipitation in this subtropical monsoon catchment.