Sulfide and mercury species profiles in two Ontario boreal shield lakes

•Sensitive method for the analysis of nanomolar levels of sulfide in freshwater.•Selective purge and trap preconcentration, followed by HPLC-UV/Vis detection.•Measured sulfide profiles for two lakes in Ontario mirror dissolved oxygen.•Positive correlation between methylmercury and sulfide in lake profiles.

The cycling of sulfur in freshwater environments plays an important role in the cycling of metals. In this study, acid volatile sulfides were measured at nanomolar levels using a purge-and-trap preconcentration, followed by methylene blue derivatization with HPLC separation and UV-Vis detection. The limit of detection using the preconcentration step was 7.5 ng L−1 or 0.23 nM sulfide. Profiles of sulfide and methylmercury were generated for two Ontario lakes. Sulfide concentrations were inversely related to dissolved oxygen concentrations and significant levels of anoxia had developed in both lakes. In both Plastic Lake and Lake 658, mercury concentrations also increased below the oxycline. Lake 658 showed a strong positive correlation between sulfide and methylmercury (CMeHg = 2 × 10−6 ⋅ Csulfide + 0.198; r = 0.96, p = 1.2 × 10−5), at the time of sampling.