Seasonal dynamics and regional distribution patterns of CO₂ and CH₄ in the north-eastern Baltic Sea

Abstract. Significant research has been carried out in the last decade to describe the CO₂ system dynamics in the Baltic Sea. However, there is a lack of knowledge in this field in the NE Baltic Sea, which is the main focus of the present study. We analysed the physical forcing and hydrographic background in the study year (2018) and tried to elucidate the observed patterns of surface water CO₂ partial pressure (pCO₂) and methane concentrations (cCH₄). Surface water pCO₂ and cCH₄ were calculated from continuous measurements during six monitoring cruises onboard R/V Salme, covering the Northern Baltic Proper (NBP), the Gulf of Finland (GoF) and the Gulf of Riga (GoR) and all seasons in 2018. The general seasonal pCO₂ pattern showed oversaturation in autumn-winter and undersaturation in spring-summer in all three areas, but it locally reached the saturation level during the cruises in April, May and August in the GoR and in August in the GoF. cCH₄ was oversaturated during the entire study period, and the seasonal course was not well exposed on the background of high variability. Surface water pCO₂ and cCH₄ distributions showed larger spatial variability in the GoR and GoF than in the NBP for all six cruises. We linked the observed local maxima to river bulges, coastal upwelling events, fronts, and occasions when vertical mixing reached the seabed in shallow areas. Seasonal averaging over the CO₂ flux based on our data suggest a weak sink for atmospheric CO₂ for all basins, but high variability and the long periods between cruises (temporal gaps in observation) preclude a clear statement.