Technical Note: Preventing CO₂ overestimation from mercuric or copper (II) chloride preservation of dissolved greenhouse gases in freshwater samples

Abstract. The determination of dissolved gases (O₂, CO₂, CH₄, N₂O, N₂) in surface waters allows to estimate biological processes and greenhouse gas fluxes in aquatic ecosystems. Mercuric chloride (HgCl₂) has been widely used to preserve water samples prior to gas analysis. However, alternates are needed because of the environmental impacts and regulation of mercury. HgCl₂ is a weak acid and interferes with dissolved organic carbon (DOC). Hence, we tested the effect of HgCl₂ and two substitutes (copper (II) chloride – CuCl₂ and silver nitrate – AgNO₃), as well as storage time (24 h to 3 months) on the determination of dissolved gases in low ionic strength and high DOC lake water. Furthermore, we investigated and predicted the effect of HgCl₂ on CO₂ concentrations in periodic samples from another lake experiencing pH variations (5.4–7.3) related to in situ photosynthesis. Samples fixed with inhibitors generally showed negligible O₂ consumption. However, effective preservation of dissolved CO₂, CH₄ and N₂O for up to three months prior to dissolved gas analysis, was only achieved with AgNO₃. In contrast, HgCl₂ and CuCl₂ caused an initial increase in CO₂ and N₂O followed by a decrease. The CO₂ overestimation, caused by HgCl₂-acidification and shift in the carbonate equilibrium, can be calculated from predictions of chemical speciation. Errors due to CO₂ overestimation in HgCl₂-preserved water, sampled from low ionic strength and high DOC freshwater, could lead to an overestimation of the CO₂ diffusion efflux by a factor of >20 over a month, or a factor of 2 over the ice-free season.