Factors affecting mercury emission from aquatic systems

The reduction of aqueous mercury (Hg2+) to sparingly soluble Hg0 (DGM) results in partitioning of mercury from the water column to the atmosphere. Mercury emission is ecologically significant because it represents one pathway for decreasing mercury loads in aquatic systems, where it might otherwise be available for methylation and bioaccumulation. Photochemical reactions with solar radiation play an important role in mercury speciation, through direct interactions with dissolved mercury as well as indirect interactions with other photochemically active substances such as dissolved organic carbon (DOC) and iron (Fe). This study addresses several factors that can affect the formation of DGM and emission in aquatic systems.In the first experiment, the effect of water column shading by floating vegetation (duckweed, Lemna minor) is assessed using flux chambers with varying percent cover of duckweed. Fluxes under duckweed were 17 to 67% lower than in controls, with lower fluxes occurring at higher percent cover. The decrease in mercury emission suggests that duckweed limits emission via the formation of a physical barrier to diffusion. Increased dissolved mercury concentrations as a result of hindered emission may increase bioaccumulation potential in lakes with duckweed cover.The second experiment measures changes in emission of Hg0 from a temperate lake during autumn turnover, with respect to mixing-related changes in water chemistry and declining solar radiation. Findings indicate that mercury emission was controlled both by solar radiation and surface water mercury concentration (r2 = 0.96, p < 0.01). Partial mixing events delivered mercury from the hypolimnion to the epilimnion, resulting in increases in the amount of mercury emitted per joule of solar radiation.The final series of experiments assesses the role of photochemical reactions of suspended particulate matter (SPM) through a series of laboratory experiments comparing DGM formation in whole water samples and particle-free waters in both solar and UV-only exposures. DGM formation was significantly lower in the whole water treatments than the particle-free treatments (p < 0.05) in both the UV-only and solar exposures. Three processes may have contributed to the lower DGM production in observed WW treatments. These include (1) limitation of Hg photoreduction in WW treatments due to particle-induced optical effects, (2) enhanced Hg photoreduction due to higher dissolved Fe concentrations in PF treatments, and (3) sequestration of Hg on particle surfaces in WW treatments.