Effect of ocean acidification and elevated fCO₂ on trace gas production by a Baltic Sea summer phytoplankton community

Abstract. The Baltic Sea is a unique environment as the largest body of brackish water in the world. Acidification of the surface oceans due to absorption of anthropogenic CO₂ emissions is an additional stressor facing the pelagic community of the already challenging Baltic Sea. To investigate its impact on trace gas biogeochemistry, a large-scale mesocosm experiment was performed off Tvärminne Research Station, Finland, in summer 2012. During the second half of the experiment, dimethylsulfide (DMS) concentrations in the highest-fCO₂ mesocosms (1075–1333 µatm) were 34 % lower than at ambient CO₂ (350 µatm). However, the net production (as measured by concentration change) of seven halocarbons analysed was not significantly affected by even the highest CO₂ levels after 5 weeks' exposure. Methyl iodide (CH₃I) and diiodomethane (CH₂I₂) showed 15 and 57 % increases in mean mesocosm concentration (3.8 ± 0.6 increasing to 4.3 ± 0.4 pmol L⁻¹ and 87.4 ± 14.9 increasing to 134.4 ± 24.1 pmol L⁻¹ respectively) during Phase II of the experiment, which were unrelated to CO₂ and corresponded to 30 % lower Chl a concentrations compared to Phase I. No other iodocarbons increased or showed a peak, with mean chloroiodomethane (CH₂ClI) concentrations measured at 5.3 (±0.9) pmol L⁻¹ and iodoethane (C₂H₅I) at 0.5 (±0.1) pmol L⁻¹. Of the concentrations of bromoform (CHBr₃; mean 88.1 ± 13.2 pmol L⁻¹), dibromomethane (CH₂Br₂; mean 5.3 ± 0.8 pmol L⁻¹), and dibromochloromethane (CHBr₂Cl, mean 3.0 ± 0.5 pmol L⁻¹), only CH₂Br₂ showed a decrease of 17 % between Phases I and II, with CHBr₃ and CHBr₂Cl showing similar mean concentrations in both phases. Outside the mesocosms, an upwelling event was responsible for bringing colder, high-CO₂, low-pH water to the surface starting on day t16 of the experiment; this variable CO₂ system with frequent upwelling events implies that the community of the Baltic Sea is acclimated to regular significant declines in pH caused by up to 800 µatm fCO₂. After this upwelling, DMS concentrations declined, but halocarbon concentrations remained similar or increased compared to measurements prior to the change in conditions. Based on our findings, with future acidification of Baltic Sea waters, biogenic halocarbon emissions are likely to remain at similar values to today; however, emissions of biogenic sulfur could significantly decrease in this region.