Mercury inputs and outputs at a small lake in northern Minnesota

Abstract

Storages and cycling of total mercury (Hg T ), methylmercury (MeHg), and Hg 0 are described for Spring Lake, a small bog lake in the Marcell Experimental Forest in north-central Minnesota. We quantified photoredox transformations, MeHg photolysis, burial to the sediments, and internal and external loadings of Hg T and MeHg. Atmospheric deposition was the main input of Hg T ; MeHg was supplied by a combination of atmospheric, near-shore wetland, and biotic (methylation) sources. Hg T outputs were dominated by burial (67%), and Hg 0 evasion accounted for 26% of Hg T outputs. The watershed of Spring Lake is small (3.7· lake surface area), and accordingly, bog and upland runoff were minor contributors to both Hg T and MeHg inputs. Wet deposition was *9% of total MeHg input, and other external inputs (runoff, sediment porewater) provided only an additional 7%, indicating that internal production of MeHg was occurring in the lake. Photolysis of MeHg, measured in the field and laboratory, removed *3· the lake mass of MeHg (20 mg) annually, and was the dominant sink for MeHg. Residence times of MeHg and Hg T in the lake were 48 and 61 days, respectively, during the open-water season, compared with only 8 days for the residence time of MeHg on settling particles (seston). Photoreduction of Hg 2+ to Hg 0 was greater than the reverse reaction (Hg 0 photooxidation), and the residence time of Hg 0 in the photic zone was short (hours). Data from this study show active cycling of all the measured species of mercury (Hg T , MeHg, and Hg 0 ) and the importance of MeHg photolysis and photo-redox processes.

Factors controlling the abiotic photo-degradation of monomethylmercury in surface waters

“…The lack of net MMHg production in any of our treatments agrees with the net photo-degradation of MMHg by sunlight reported for the majority of studies on the effect of sunlight on MMHg in natural waters (Sellers et al, 1996(Sellers et al, , 2001Krabbenhoft et al, 2002;Fitzgerald, 2006, 2010;Bergquist and Blum, 2007;Hines and Brezonik, 2007;Whalin et al, 2007;Lehnherr and St. Louis, 2009). However, these results are in contrast to a study by Siciliano et al (2005) that reported net production of MMHg by sunlight in natural waters.…”

“…Direct comparison of our results to other studies in the literature is hampered by the relative dearth of the necessary irradiance measurements in the same studies. Exceptions to this include the work of , Hines and Brezonik (2007), and Lehnherr and St. Louis (2009).…”

“…The photo-decomposition of MMHg (Inoko, 1981;Sellers et al, 1996) is the most important known abiotic process capable of demethylating MMHg, and may well be the dominant mechanism of MMHg degradation or loss in many aquatic environments (Sellers et al, 1996(Sellers et al, , 2001Krabbenhoft et al, 2002;Bergquist and Blum, 2007;Hines and Brezonik, 2007). Previous studies have reported evidence of both abiotic photochemical MMHg production (Akagi et al, 1974(Akagi et al, , 1977Siciliano et al, 2005) and decomposition, and have suggested that the abiotic demethylation of MMHg may occur via either direct or indirect photochemical processes (Inoko, 1981;Zepp et al, 1987;Suda et al, 1993;Sellers et al, 1996Sellers et al, , 2001Gardfeldt et al, 2001;Chen et al, 2003;Fitzgerald, 2006, 2010;Lehnherr and St. Louis, 2009;Zhang and Hsu-Kim, 2010).…”

Tidal exchange of total mercury and methylmercury between a salt marsh and a Chesapeake Bay sub-estuary

“…MeHg released from marshes is predominantly in the filterable phase, which may increase surface water residence time (Hines and Brezonik 2007) and consequently uptake by biota. Conversely, this may also increase the potential for photo-and biological demethylation (Hintelmann et al 2000;Li et al 2010).…”

Diffusive gradients in thin films for predicting methylmercury bioavailability in freshwaters after photodegradation

“…It has been shown that MeHg can be demethylated in the water column of lakes by the action of microorganisms (Matilainen and Verta, 1995;Schaefer et al, 2004), but recently, abiotic processes driven by sunlight have received increasing attention (Sellers et al, 1996;Lehnherr and Louis, 2009;Hammerschmidt and Fitzgerald, 2010;Black et al, 2012). The photodegradation of MeHg is the most important known abiotic process capable of demethylating MeHg and has been suggested to be the principal sink for MeHg in lakes (Sellers et al, 2001;Lehnherr and Louis, 2009) consuming up to 80% of MeHg total input Hines and Brezonik, 2007). Visible and, more efficiently, ultraviolet (UV) radiation (Lehnherr and Louis, 2009;Black et al, 2012) are able to decompose MeHg, mainly by indirect photolysis, involving reactive oxygen species (ROS, e.g., Å OH, Å O 2 H, and 1 O 2 ) (Suda et al, 1993;Gardfeldt et al, 2001;Chen et al, 2003;Zhang and Hsu-Kim, 2010a) formed by sunlight irradiation of water and its dissolved organic matter (DOM; i.e.…”