John Wiley & Sons, Ltd.

Effects of photodemethylation on the methylmercury budget of boreal Norwegian lakes

MeHg concentrations in freshwater fish from southeastern Norway continue to increase, highlighting the need for a comprehensive understanding of MeHg sources, cycling, and degradation in the aquatic environment. The authors assessed the importance of photodemethylation (PD) in the MeHg budget of four Norwegian lakes. PD rates (kpd) were determined through incubation experiments with MeHg‐spiked natural lake water. The authors determined full spectrum exposure rates at all study sites and waveband specific rates (photosynthetically active radiation (PAR), ultraviolet‐A (UV‐A) radiation, and ultraviolet‐B UV‐B radiation) at one clear‐water (Sognsvann) and one humic (Langtjern) site. No significant differences in PD rates between sites were found, and the authors' observed rates were in agreement with available literature for lake and wetland waters. The authors paired experimentally derived PD rates with lake‐specific incident irradiation, light attenuation, and MeHg concentrations to estimate MeHg loss through PD for the study sites. For Langtjern, losses through PD were equal to 27 % of total annual inputs, highlighting the importance of PD in the MeHg budget. Furthermore, the authors assessed how changes in terrestrial dissolved organic carbon (DOC) export to freshwaters and climate‐driven reductions in ice cover duration may affect MeHg losses through PD. Results suggest that future increases in DOC may lead to higher aqueous MeHg concentrations in boreal lakes due to increased DOC‐associated MeHg inputs paired with strong decreases in losses of MeHg through PD due to increased light attenuation. This article is protected by copyright. All rights reserved

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