Biological and photochemical production of dissolved gaseous mercury in a boreal lake

We used in situ experiments and measured depth profiles of dissolved gaseous mercury (DGM) to investigate the relative contribution of photochemical versus biological processes on the production of DGM in an oligomesotrophic lake of the Canadian Shield. At the surface, DGM production was mainly photomediated, with reduction rates being twice as high in the wetland than in the lake. In the water column, the distribution of DGM concentrations was not strictly related to light but followed a multimodal distribution, with peaks encountered below the epilimnion at depths receiving <5% of the incident light. Those peaks were recorded in the middle and at the bottom of the metalimnion during the ice‐free season, as well as under ice cover and at the bottom of an anoxic hypolimnion. Rather than being a consequence of the bacterial mercuric reductase activity, metalimnetic DGM peaks were associated with the intensity and duration of phytoplankton blooms. In situ incubation experiments also showed that DGM production ceased when samples were kept in the dark, filtered, or when an inhibitor of photosynthesis was added. Our results illustrate the important role of phytoplankton on Hg redox dynamics in the water column of lakes.