Effects of photodemethylation on the methylmercury budget of boreal Norwegian lakes

Abstract

Methylmercury (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 in the MeHg budget of 4 Norwegian lakes. Photodemethylation rates were determined using 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, ultraviolet-A radiation, and ultraviolet-B radiation) at 1 clear-water (Sognsvann) and 1 humic (Langtjern) site. No significant differences in photodemethylation rates between the sites were found, and the authors' observed rates agreed with available literature for lake and wetland waters. The authors paired experimentally derived photodemethylation rates with lake-specific incident irradiation, light attenuation, and MeHg concentrations to estimate MeHg loss through photodemethylation for the study sites. For Langtjern, losses through photodemethylation equalled 27% of total annual inputs, highlighting the importance of photodemethylation in the MeHg budget. Furthermore, the authors assessed how changes in terrestrial dissolved organic carbon (DOC) exported to freshwaters and climate-driven reductions in ice cover duration may affect MeHg losses through photodemethylation. 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 significant decreases in the loss of MeHg through photodemethylation due to increased light attenuation.

Factors controlling the photochemical degradation of methylmercury in coastal and oceanic waters

“…Despite the greater importance of UVB and UVA radiation at the surface these short wavelengths are attenuated the fastest, especially in coastal waters with higher levels of CDOM, chlorophyll, and suspended particulate matter; therefore, visible light often plays a larger role than its corresponding degradation rate constant would suggest. These results are consistent with previous studies that have suggested that most of the CH 3 Hg degradation is due to PAR, primarily because these studies were completed in turbid shallow waters, such as the salt marsh studies of Black et al (2012) and the lake and wetland studies of others (Fernández-Gómez et al, 2013; Poste et al, 2015). Regardless, the model results indicate that the previous conclusions about the dominant role of PAR in CH 3 Hg degradation do not extend to the open ocean.…”

“…Light attenuation, controlled by absorption and scattering within the water column, varies with wavelength so wavelength specific rate constants must be used in such an analysis. Several studies have determined the relative importance of UVB, UVA, and PAR wavelengths in the photodegradation of CH 3 Hg (Black et al, 2012; Lehnherr and St Louis, 2009; Li et al, 2010; Poste et al, 2015). The percent contribution of these wavelength regions ranged from 7 to 36% for UVB, 40–85% for UVA, and 0–42% for PAR (Table 7).…”

The Global Marine Selenium Cycle: Insights From Measurements and Modeling

“…The importance of PAR decreases from 42% to 9% over the same transition. Results are consistent with other studies of photochemical degradation in natural systems showing greater importance of PAR in turbid shallow waters than its corresponding degradation rate constant would suggest due to the rapid attenuation of UV radiation (Black et al, 2012;Fernández-Gómez et al, 2013;Poste et al, 2015).…”

Quantifying the effects of photoreactive dissolved organic matter on methylmercury photodemethylation rates in freshwaters

“…Similarly, there were significantly greater photodemethylation efficiencies in samples with less photoreactive DOM (treatment 2 > treatments 1 and 3; Figure B). These data further support the idea that freshwaters with lower DOM concentration and photobleached DOM will have greater photodemethylation potential than waters with more photoreactive and overall DOM concentration . These data also support studies that have found lower rates of photodemethylation with depth in water columns, as less energy will be available for photoreactions that involve MeHg .…”

“…Rates of photodemethylation are consistently dependent on availability of incoming solar radiation and absorbance in water columns [5,[8][9][10][11], and therefore, environmental variables that affect solar radiation availability can also influence the mechanisms underlying this photoreaction and still require further research. In particular, the role of dissolved organic matter (DOM), a major factor regulating water column radiation availability [12,13], in MeHg photodemethylation reactions is important to consider [11,[14][15][16][17].…”