Regulation of mercury and methylmercury migration and transformation at the sediment-water interface of a seasonally ice-covered lake by temperature and illumination: A mesocosm experiment.

Under snow-covered conditions during the ice-covered period, lakes remain in a low-temperature environment with weak light or even complete darkness for an extended period. These conditions markedly influence the regulatory effects of temperature and illumination on the migration and transformation of mercury and methylmercury. Taking Wuliangsuhai Lake, the largest natural wetland at 40°N latitude, as the study object, a mesocosm experiment was conducted to determine the concentrations of mercury and methylmercury in overlying water and pore water under different light and temperature conditions. Their migration fluxes were then calculated using Fick's first law. The results showed that mercury concentrations were most sensitive to temperature variation, whereas methylmercury concentrations and methylmercury production efficiency exhibited relatively smaller changes. This indicates that temperature is a key environmental factor controlling the migration and transformation of mercury at the sediment-water interface. Dark conditions reduced the growth rates of both mercury and methylmercury, but promoted their migration and transformation processes, suggesting that illumination is a critical environmental factor influencing mercury demethylation. Further analysis based on migration fluxes revealed that low-temperature conditions weakened the sink effect of sediments for mercury while enhancing the source effect for methylmercury. Illumination promoted the source-sink effects of both mercury and methylmercury. In addition, during the lake-freezing process, the sink effect for mercury was also weakened, whereas the source effect for methylmercury was enhanced.