Controls on methylmercury concentrations in lakes and streams of peatland‐rich catchments along a 1700 km permafrost gradient

Abstract

Permafrost thaw may increase the production of neurotoxic methylmercury (MeHg) in northern peatlands, but the downstream delivery of MeHg is uncertain. We quantified total mercury (THg) and MeHg concentrations in lakes and streams along a 1700 km permafrost transect in boreal western Canada to determine the influence of regional permafrost extent compared to local lake and catchment characteristics. In lakes, we assessed sediment microbial communities and modeled potential rates of water column photodemethylation (PD). Regardless of permafrost conditions, peatlands were the primary sources of MeHg across the transect as MeHg concentrations in streams increased with aromatic dissolved organic carbon (DOC), iron, and lower pH. Higher DOC and greater catchment peatland extent were further associated with higher stream %MeHg (MeHg/THg). Peatland lakes were potential MeHg sinks, with lower MeHg concentrations than streams (mean AE 1SD: 0.19 AE 0.23 and 0.47 AE 0.77 ng MeHg L À1 , respectively), and larger stream catchments had lower %MeHg where PD may occur in abundant small lakes. Microbial communities in lake sediments showed that abundance of Hg reducing genes (merA) predominated over Hg methylating (hgcA) and MeHg demethylating (merB) genes. The effects of permafrost extent on MeHg processes in lakes were secondary to the influence of local catchment characteristics, but lakes in regions with less permafrost had higher DOC concentrations, higher %MeHg, and lower potential rates of PD. Our study highlights a need to understand the impacts of climate change on MeHg source and sink processes, particularly as mediated through changes to peatland DOC, to improve projections of future MeHg concentrations in northern catchments.

Concentrations and Yields of Mercury, Methylmercury, and Dissolved Organic Carbon From Contrasting Catchments in the Discontinuous Permafrost Region, Western Canada

“…The peatland catchment had higher filtered MeHg concentrations than the mixed catchment but similar bulk MeHg concentrations. The THg and MeHg concentrations in the peatland catchment and mixed catchment were relatively low but within previously observed ranges for boreal streams in Canada (0.46–20.46 ng THg L −1 ; 0.03–4.82 ng MeHg L −1 ; Fink‐Mercier, Lapierre, et al., 2022; Lam et al., 2022; Thompson et al., 2023).…”

Nitrous Oxide Fluxes in Permafrost Peatlands Remain Negligible After Wildfire and Thermokarst Disturbance

“…We also studied five peatland ponds (Figure S2 in Supporting Information ), three at Lutose (L1, L2, and L4) and two at Smith Creek (W1 and W2), all of which have been part of previous studies (Kuhn et al., 2021; Thompson et al., 2023). All pond depths ranged from 0.5 to 2.5 m and were between 0.5 and 5 ha in area.…”