Methylmercury Production and Degradation under Light and Dark Conditions in the Water Column of the Hells Canyon Reservoirs, USA

Abstract

Methylmercury (MeHg) is a highly toxic form of mercury that can bioaccumulate in fish tissue. Methylmercury is produced by anaerobic bacteria, many of which are also capable of MeHg degradation. In addition, demethylation in surface waters can occur via abiotic sunlight-mediated processes. The goal of the present study was to understand the relative importance of microbial Hg methylation/demethylation and abiotic photodemethylation that govern the mass of MeHg within an aquatic system. The study location was the Hells Canyon complex of 3 reservoirs on the Idaho-Oregon border, USA, that has fish consumption advisories as a result of elevated MeHg concentrations. Our study utilized stable isotope addition experiments to trace MeHg formation and degradation within the water column of the reservoirs to understand the relative importance of these processes on the mass of MeHg using the Water Quality Analysis Simulation Program. The results showed that rates of MeHg production and degradation within the water column were relatively low (<0.07 d -1 ) but sufficient to account for most of the MeHg observed with the system. Most MeHg production within the water column appeared to occur in the spring when much of the water column was in the processes of becoming anoxic. In the surface waters, rates of photodemethylation were relatively large (up to -0.25 d -1 ) but quickly decreased at depths >0.5 m below the surface. These results can be used to identify the relative importance of MeHg processes that can help guide reservoir management decisions.

Demethylation─The Other Side of the Mercury Methylation Coin: A Critical Review

“…MeHg can be degraded either directly or indirectly by light through photolysis or photochemical reactions, a mechanism so-called photodemethylation (Figure ). Photodemethylation is obviously only significant in surface water due to rapid attenuation of light in the water column, especially for ultraviolet (UV) radiation. ,, Additionally, the rate and efficiency of photodemethylation strongly depend on the radiation intensity and wavelength; shorter waveband UV-B light (280–320 nm) is far more effective in degrading MeHg than longer waveband UV-A light (320–400 nm) and the visible or photosynthetic active radiation (PAR) (400–700 nm). ,,,− For example, in a study of photodemethylation along a lake-wetland gradient in a boreal coniferous forest, the relative ratio of demethylation rate constants with UV-B, UV-A, and PAR was found to be 3100:43:1 in surface waters . Similarly, Black et al reported that photodemethylation rate constants were at least 400-fold greater for UV-B and 37-fold greater for UV-A than PAR in water.…”

“…Photodemethylation is obviously only significant in surface water due to rapid attenuation of light in the water column, especially for ultraviolet (UV) radiation. 159 , 160 , 165 Additionally, the rate and efficiency of photodemethylation strongly depend on the radiation intensity and wavelength; shorter waveband UV-B light (280–320 nm) is far more effective in degrading MeHg than longer waveband UV-A light (320–400 nm) and the visible or photosynthetic active radiation (PAR) (400–700 nm). 33 , 160 , 161 , 166 − 170 For example, in a study of photodemethylation along a lake-wetland gradient in a boreal coniferous forest, the relative ratio of demethylation rate constants with UV-B, UV-A, and PAR was found to be 3100:43:1 in surface waters.…”

Biogeochemical and hydrologic synergy control mercury fate in an arid land river-reservoir system

“…MeHg accumulated in these strata, in both filter-passing and particulate fractions, and increased longitudinally both up and downgradient in Brownlee Reservoir with the spread of anoxic conditions. 29 MeHg accumulation in the water column is tentatively attributed primarily to the methylation of water column f.Hg( ii ) by methylating organisms on sinking particles as they decompose, 11,62 a phenomenon enhanced by the DOM of aromatic quality 38 that is more prevalent in Brownlee Reservoir during high flow years. The processes described above occur between February through early July in this river-reservoir system, when warm inflowing water enters the epilimnion of the reservoir.…”

“…UV-vis absorption spectra were measured from 190–800 nm and decadic absorbance values were converted to absorption coefficients aswhere α λ is the decadic absorption coefficient (cm −1 ), A λ is the absorbance, and l is the path length (cm). 47 The DOM specific ultraviolet absorbance at 254 nm (SUVA 254 ), a proxy for DOM aromaticity 47 that indicates DOM source and reactivity to Hg( ii ), 11,15,39 was calculated by dividing the α 254 (m −1 ) by DOC concentration (mg L −1 ). Spectral slopes were determined at wavelengths 275 to 295 nm ( S 275–295 ) and 350 to 400 nm ( S 350–400 ).…”

“…2 Flooding associated with new reservoirs has been well-documented to produce MeHg and increased sh mercury concentrations which can last up to a few decades, 8,9 though the timing and magnitude of the response can vary between systems. 10 Older reservoirs can also exhibit enhanced MeHg formation in anoxic waters 11 and sediments and downstream MeHg export, [12][13][14][15] exacerbating MeHg uptake in proximal aquatic food webs. 12,16,17 Provided the regional and national importance of reservoirs in arid landscapes and projected increases in water stress of North American rivers due to climate drivers, 18 integrated science to support reservoir management to mitigate contaminant fate may be effective to sustain the health of freshwater ecosystems.…”