Technical note: Preventing CO.sub.2 overestimation from mercuric or copper chloride preservation of dissolved greenhouse gases in freshwater samples

The determination of dissolved gases (O.sub.2, CO.sub.2, CH.sub.4, N.sub.2 O, N.sub.2) in surface waters allows the estimation of biological processes and greenhouse gas fluxes in aquatic ecosystems. Mercuric chloride (HgCl.sub.2) 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. HgCl.sub.2 is a weak acid and interferes with dissolved organic carbon (DOC). Hence, we tested the effect of HgCl.sub.2 and two substitutes (copper(II) chloride - CuCl.sub.2 - and silver nitrate - AgNO.sub.3 ), 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 HgCl.sub.2 on CO.sub.2 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 O.sub.2 consumption. However, effective preservation of dissolved CO.sub.2, CH.sub.4 and N.sub.2 O for up to 3 months prior to dissolved gas analysis was only achieved with AgNO.sub.3 . In contrast, HgCl.sub.2 and CuCl.sub.2 caused an initial increase in CO.sub.2 and N.sub.2 O from 24 h to 3 weeks followed by a decrease from 3 weeks to 3 months. The CO.sub.2 overestimation, caused by HgCl.sub.2 acidification and a shift in the carbonate equilibrium, can be calculated from predictions of chemical speciation. Errors due to CO.sub.2 overestimation in HgCl.sub.2 -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 CO.sub.2 diffusion efflux by a factor of 20 over a month or a factor of 2 over the ice-free season. The use of HgCl.sub.2 and CuCl.sub.2 for freshwater preservation should therefore be discontinued. Further testing of AgNO.sub.3 preservation should be performed under a large range of freshwater chemical conditions.