Effect of ocean acidification and elevated f CO 2 on trace gas production by a Baltic Sea summer phytoplankton community

• Published in: Biogeosciences
• Main Authors: Webb, AL, Leedham-Elvidge, E, Hughes, C, Hopkins, FE, Malin, G, Bach, LT, Schulz, K, Crawfurd, K, Brussaard, CPD, Stuhr, A, Riebesell, U, Liss, PS
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus GmbH 2016
• Subjects: Earth Sciences; Oceanography; Biological Oceanography; Ocean acidification
• Online Access: https://doi.org/10.5194/bg-13-4595-2016; http://ecite.utas.edu.au/133569

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 CO 2 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 Tvrminne Research Station, Finland, in summer 2012. During the second half of the experiment, dimethylsulfide (DMS) concentrations in the highest- f CO 2 mesocosms (10751333 atm) were 34 % lower than at ambient CO 2 (350 atm). However, the net production (as measured by concentration change) of seven halocarbons analysed was not significantly affected by even the highest CO 2 levels after 5 weeks' exposure. Methyl iodide (CH 3 I) and diiodomethane (CH 2 I 2 ) 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 CO 2 and corresponded to 30 % lower Chl a concentrations compared to Phase I. No other iodocarbons increased or showed a peak, with mean chloroiodomethane (CH 2 ClI) concentrations measured at 5.3 (0.9) pmol L −1 and iodoethane (C 2 H 5 I) at 0.5 (0.1) pmol L −1 . Of the concentrations of bromoform (CHBr 3 mean 88.1 13.2 pmol L −1 ), dibromomethane (CH 2 Br 2 mean 5.3 0.8 pmol L −1 ), and dibromochloromethane (CHBr 2 Cl, mean 3.0 0.5 pmol L −1 ), only CH 2 Br 2 showed a decrease of 17 % between PhasesI and II, with CHBr 3 and CHBr 2 Cl showing similar mean concentrations in both phases. Outside the mesocosms, an upwelling event was responsible for bringing colder, high-CO 2 , low-pH water to the surface starting on day t 16 of the experiment; this variable CO 2 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 atm f CO 2 . 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.