Spatial Characteristics of Net Methylmercury Production Hot Spots in Peatlands

Abstract

Many wetlands are sources of methylmercury (MeHg) to surface waters, yet little information exists about the distribution of MeHg within wetlands. Total mercury (THg) and MeHg in peat pore waters were studied in four peatlands in spring, summer, and fall 2005. Marked spatial variability in the distribution of MeHg, and %MeHg as a proxy for net MeHg production, was observed, with highest values occurring in discrete zones. We denote these zones "MeHg hot spots", defined as an area where the pore water %MeHg exceeded the 90th percentile of the data set (n=463) or >22% of THg as MeHg. MeHg hot spots occurred near the interface between peatland and the upland watershed with few exceptions. The %MeHg in pore water was significantly less in peatland interiors compared to upland-peatland interface zones, with the significance of these differences related to the delineation of the boundary between the two areas. Although further research is necessary, our data suggest that the occurrence of MeHg hot spots is related to the transport of solutes in upland runoff to the peatland perimeter and not to the accumulation of MeHg in this zone as a result of transport from either the peatland interior or the surrounding upland watershed. These findings augment the understanding of peatland MeHg production in upland-peatland watersheds, provide guidance for more accurate quantification of MeHg pool sizes in the landscape, and a spatial framework forthe further study of mercury methylation processes in peatlands.

A Synthesis of Three Decades of Eco-Hydrological Research at Scotty Creek, NWT, Canada

“…Similar results are reported for 15 wetlands in non-permafrost regions (e.g. Branfireun and Roulet, 2002;Mitchell et al, 2008), suggesting that minerotrophic wetlands may be 'hot spots' for MeHg production, a major toxin that has become more prevalent in the food chain in recent years, and is of major concern to northern communities (Laird et al, 2018;Reyes et al, 2017). Understanding the trajectory of permafrost thaw-induced land cover change, including the development of such hot spots, therefore has important implications for water quality.…”

Detailed Assessment of the Kinetics of Hg-Cell Association, Hg Methylation, and Methylmercury Degradation in Several Desulfovibrio Species

“…Methylmercury production is an anaerobic process that occurs in saturated soils and wetlands (26,44,45,53), decaying periphyton mats (1, 14, 31), aquatic bottom sediments (16,27,33,36), and anaerobic bottom waters (56). Early investigations, prior to the advent of modern methylmercury (MeHg) analyses, reported a wide variety of aerobic and anaerobic Gram-positive and Gram-negative bacteria (30,49,55,58) and fungi (55) to be capable of MeHg production.…”

“…(21,37), all belonging to the Deltaproteobacteria. Many field studies, using selective microbial stimulants (1,10,26,44,57), inhibitors (1,16,24,26,59), and biogeochemical correlates (6,39,40,45), have buttressed the paradigm of SRB and FeRB as the dominant Hg methylators in natural aquatic systems (16,24,59), though recent studies have hypothesized that methanogens may be significant in some systems (31).…”

The Essential Role of the Lagg in Raised Bog Function and Restoration: A Review

“…Bubier (1991) similarly found that specific conductance increased significantly in the transition from open bog to rand forest and lagg. Mitchell et al (2008) found "hotspots" of Methylmercury (MeHg) in the upland/peatland interface of bogs in Ontario and Minnesota, particularly within 5 m of the upland interface. MeHg was higher in the lagg than in either the upland or peatland, suggesting that these hotspots are a result of net MeHg production within the lagg itself, rather than transport into the lagg from either the upland or the bog, although they do allow for the possibility of accumulation of MeHg-rich runoff in the lagg zone in addition to in situ production.…”