Seasonal speciation of dissolved germanium in Bohemian reservoirs with contrasting chemistry and trophic status

Abstract

Although germanium is increasingly important for modern technologies, its occurrence and biogeochemical behaviour in natural waters remain insufficiently characterized, particularly in freshwater environments; here we investigate its speciation and seasonal dynamics in two contrasting drinking water reservoirs. The speciation of dissolved germanium, including inorganic, methyl-, and dimethyl-germanium, was monitored monthly over the course of a year in two drinking water reservoirs. The sampling covered both the productive and winter periods. The study sites were Vrchlice, a eutrophic reservoir in Central Bohemia that becomes anoxic in summer, and Souš, an oligotrophic reservoir in the mountainous region of North Bohemia, which was historically affected by acid rain. Speciation was measured using hydride generation–cryotrapping–inductively coupled plasma–mass spectrometry (HG-CT-ICP-MS/MS). The two reservoirs showed markedly different concentrations of inorganic Ge (iGe). In winter, before snowmelt, iGe concentrations were 3.1 (± 0.1) ng L–1 in Vrchlice and 21.3 (± 0.3) ng L–1 in Souš. Throughout the year, in Vrchlice iGe developed nutrient-type vertical profiles resembling those of silicon, consistent with the presence of diatoms. In contrast, no such pattern was observed in Souš, where diatoms were absent. Both reservoirs showed evidence of Ge fluxes from sediments during summer. Methylated Ge species exhibited conservative behaviour. In Vrchlice, average concentrations were 0.34 ng L–1 (± 0.04) for methylgermanium and 0.09 ng L–1 (± 0.02) for dimethylgermanium. In Souš, concentrations, were lower, with methylgermanium near the limit of quantification at ~0.07 ng L–1, and dimethylgermanium below detection limit. These findings are consistent with earlier observations from Lake Geneva. They confirm that freshwater systems lack an apparent in situ source of methylated Ge species, and that the inorganic form dominate, in contrast to marine environments.