Temporal dynamics and environmental controls of carbon dioxide and methane fluxes measured by the eddy covariance method over a boreal river

Abstract. Boreal rivers and streams are significant sources of carbon dioxide (CO2) and methane (CH4) for the atmosphere. Yet the controls and the magnitude of these emissions remain highly uncertain, as current estimates are mostly based on indirect and discrete flux measurements. In this study, we present and analyse the longest CO2 and the first ever CH4 flux dataset measured by the eddy covariance (EC) technique over a river. The field campaign (Kitinen Experiment, KITEX) was carried out during June–October 2018 over the river Kitinen, a large regulated river with a mean annual discharge of 103 m3 s−1 located in northern Finland. The EC system was installed on a floating platform, where the river was 180 m wide and with a maximum depth of 7 m. The river was on average a source of CO2 and CH4 for the atmosphere. The mean CO2 flux was 0.36 ± 0.31 µmol m−2 s−1, and the highest monthly flux occurred in July. The mean CH4 flux was 3.8 ± 4.1 nmol m−2 s−1, and it was also highest in July. During midday hours in June, the river acted occasionally as a net CO2 sink. In June–August, the nocturnal CO2 flux was higher than the daytime flux. The CH4 flux did not show any statistically significant diurnal variation. Results from a multiple regression analysis show that the patterns of daily and weekly mean fluxes of CO2 are largely explained by partial pressure of CO2 in water (pCO2w), photosynthetically active radiation (PAR), water flow velocity and wind speed. Water surface temperature and wind speed were found to be the main drivers of CH4 fluxes.