Interannual and seasonal variability of the air-sea CO₂ exchange at Utö in the coastal region of the Baltic Sea

Abstract. Oceans alleviate the accumulation of atmospheric CO₂ by absorbing approximately a quarter of all anthropogenic emissions. In the deep oceans, carbon uptake is dominated by aquatic phase chemistry, whereas in biologically active coastal seas the marine ecosystem and biogeochemistry play an important role in the carbon uptake. Coastal seas are hotspots of organic and inorganic matter transport between the land and the oceans, and thus important for the marine carbon cycling. In this study, we investigate the net air-sea CO₂ exchange at the Utö Atmospheric and Marine Research Station, located at the southern edge of the Archipelago Sea within the Baltic Sea, using the data collected during 2017–2021. The air-sea fluxes of CO₂ were measured using the eddy covariance technique, supported by the flux parametrization based on the pCO₂ and wind speed measurements. During the spring-summer months (April–August), the sea was gaining carbon dioxide from the atmosphere, with the highest monthly sink fluxes typically occurring in May, being -0.26 μmol m^-2 s^-1 on average. The sea was releasing the CO₂ to the atmosphere in September–March, and the highest source fluxes were typically observed in September, being 0.42 μmol m^-2 s^-1 on average. On the annual basis, the study region was found to be a net source of atmospheric CO₂, and on average, the annual net exchange was 27.1 gC m^-2 y^-1, which is comparable to the exchange observed in the Gulf of Bothnia, the Baltic Sea. The annual net air-sea CO₂ exchanges varied between 18.2 gC m^-2 y^-1 (2018) and 39.1 gC m^-2 y^-1 (2017). During the coldest year, 2017, the spring-summer sink fluxes remained low compared to the other years, as a result of relatively high seawater pCO₂ in summer, which never fell below 220 μatm during that year. The spring-summer phytoplankton blooms of 2017 were weak, possibly due to the cloudy summer and deeply mixed surface layer, which restrained the photosynthetic fixation of dissolved inorganic carbon in the surface waters. The algal blooms in spring-summer 2018 and the consequent pCO₂ drawdown were strong, fueled by high pre-spring nutrient concentrations. The systematic positive annual CO₂ balances suggest that our coastal study site is affected by carbon flows originating from elsewhere, possibly as organic carbon which is remineralized and released to the atmosphere as CO₂. This coastal source of CO₂ fueled by the organic matter originating probably from land ecosystems stresses the importance of understanding the carbon cycling in the land-sea continuum.