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 the estimation of 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 prohibition 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 water from a typical boreal lake. 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 3 months prior to dissolved gas analysis was only achieved with AgNO3. In contrast, HgCl2 and CuCl2 caused an initial increase in CO2 and N2O from 24 h to 3 weeks followed by a decrease from 3 weeks to 3 months. The CO2 overestimation, caused by HgCl2 acidification and a 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, which is common in the Northern Hemisphere, 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. The use of HgCl2 and CuCl2 for freshwater preservation should therefore be discontinued. Further testing of AgNO3 preservation should be performed under a large range of freshwater chemical conditions.