Articles | Volume 13, issue 15
Biogeosciences, 13, 4595–4613, 2016

Special issue: Effects of rising CO2 on a Baltic Sea plankton...

Biogeosciences, 13, 4595–4613, 2016

Research article 15 Aug 2016

Research article | 15 Aug 2016

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

Alison L. Webb1,2, Emma Leedham-Elvidge1, Claire Hughes3, Frances E. Hopkins4, Gill Malin1, Lennart T. Bach5, Kai Schulz6, Kate Crawfurd7, Corina P. D. Brussaard7,8, Annegret Stuhr5, Ulf Riebesell5, and Peter S. Liss1 Alison L. Webb et al.
  • 1Centre for Ocean and Atmospheric Sciences, School of Environmental Science, University of East Anglia, Norwich, NR4 7TJ, UK
  • 2Groningen Institute for Evolutionary Life Sciences, University of Groningen, 9700 CC Groningen, the Netherlands
  • 3Environmental Department, University of York, York, YO10 5DD, UK
  • 4Plymouth Marine Laboratory, Plymouth, PL1 3DH, UK
  • 5GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24148 Kiel, Germany
  • 6Centre for Coastal Biogeochemistry, School of Environment, Science and Engineering, Southern Cross University, Lismore, NSW 2480, Australia
  • 7Department of Biological Oceanography, NIOZ – Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, the Netherlands
  • 8Aquatic Microbiology, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, P.O. Box 94248, 1090 GE Amsterdam, the Netherlands

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.

Short summary
This paper presents concentrations of several trace gases produced by the Baltic Sea phytoplankton community during a mesocosm experiment with five different CO2 levels. Average concentrations of dimethylsulphide were lower in the highest CO2 mesocosms over a 6-week period, corresponding to previous mesocosm experiment results. No dimethylsulfoniopropionate was detected due to a methodological issue. Concentrations of iodine- and bromine-containing halocarbons were unaffected by increasing CO2.
Final-revised paper