Response of halocarbons to ocean acidification in the Arctic

Abstract. The potential effect of ocean acidification (OA) on seawater halocarbons in the Arctic was investigated during a mesocosm experiment in Spitsbergen in June–July 2010. Over a period of 5 weeks, natural phytoplankton communities in nine ~ 50 m³ mesocosms were studied under a range of pCO₂ treatments from ~ 185 μatm to ~ 1420 μatm. In general, the response of halocarbons to pCO₂ was subtle, or undetectable. A large number of significant correlations with a range of biological parameters (chlorophyll a, microbial plankton community, phytoplankton pigments) were identified, indicating a biological control on the concentrations of halocarbons within the mesocosms. The temporal dynamics of iodomethane (CH₃I) alluded to active turnover of this halocarbon in the mesocosms and strong significant correlations with biological parameters suggested a biological source. However, despite a pCO₂ effect on various components of the plankton community, and a strong association between CH₃I and biological parameters, no effect of pCO₂ was seen in CH₃I. Diiodomethane (CH₂I₂) displayed a number of strong relationships with biological parameters. Furthermore, the concentrations, the rate of net production and the sea-to-air flux of CH₂I₂ showed a significant positive response to pCO₂. There was no clear effect of pCO₂ on bromocarbon concentrations or dynamics. However, periods of significant net loss of bromoform (CHBr₃) were found to be concentration-dependent, and closely correlated with total bacteria, suggesting a degree of biological consumption of this halocarbon in Arctic waters. Although the effects of OA on halocarbon concentrations were marginal, this study provides invaluable information on the production and cycling of halocarbons in a region of the world's oceans likely to experience rapid environmental change in the coming decades.