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: A. L. Webb, E. Leedham-Elvidge, C. Hughes, F. E. Hopkins, G. Malin, L. T. Bach, K. Schulz, K. Crawfurd, C. P. D. Brussaard, A. Stuhr, U. Riebesell, P. S. Liss
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2016
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Ocean acidification
• Online Access: https://doi.org/10.5194/bg-13-4595-2016; https://doaj.org/article/c75c214e48dd4a54bb71e45edb6831bc

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 Tvärminne Research Station, Finland, in summer 2012. During the second half of the experiment, dimethylsulfide (DMS) concentrations in the highest- f CO 2 mesocosms (1075–1333 µatm) were 34 % lower than at ambient CO 2 (350 µatm). 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 Phases I 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 µatm f CO 2 . After this ...