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

Abstract

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.

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

“…Data availability. All data supporting this study are available at https://doi.org/10.4211/hs.436be40748a246269102b20211b49762(Clayer et al, 2024).https://doi.org/10.5194/bg-21-190321- -202421- Biogeosciences, 21, 190321- -192121- , 2024 …”