Temporal variability of air–sea carbon dioxide exchange in the coastal water of the Baltic Sea

Abstract. Coasts are globally identified as major sinks of carbon. Still, the carbon balance in the coastal Baltic Sea remains unsolved. We analysed almost three years of continuous eddy covariance measurements of carbon dioxide fluxes in the coastal zone of the northern Baltic Sea in 2022–2024. Our results show that this coastal site acted as a small source of carbon dioxide into the atmosphere. The gap-filled carbon balances for 2023 and 2024 were 24.3 ± 0.3 g C m⁻² and 19.2 ± 0.3 g C m⁻², respectively. These results are comparable with earlier results by eddy covariance in the Baltic Sea. The measurements also reveal a consistent seasonal pattern of fluxes among the years, where the sea acts as a sink of carbon dioxide in April–May, as a variable source and sink in June–September, and a source in October–March. Maximum rates of monthly uptake were measured in April and the maximum emission values in November. Data gaps were filled using a random forest model which identified wind speed and sea salinity as the most important environmental co-variates of the carbon dioxide flux. Additionally, it was shown that the non-gapfilled monthly and yearly sums are offset due to lack of night-time data and thus uneven diurnal data coverage and that gap-filling is essential in calculating the yearly balances of fluxes.