Algal scavenging of mercury in preindustrial Arctic lakes

Abstract

The geochemical speciation of total mercury (THg) was examined in pre-1800 Arctic lake sediments to improve understanding of the factors controlling "natural baseline" THg. Solid-phase binding forms of THg were determined by sequential extraction of dated cores from three lakes in different ecozones (barren tundra, grassy tundra, and boreal forest). Sediment organic matter (OM) was mostly of algal origin. Mercury was highly concentrated in the sediment OM fraction (OM-Hg), comprising 60-87% of THg, while OM (as total organic carbon) constituted only 0.6-13% of sediment dry weight (DW). OM-Hg concentrations were equivalent to 159 AE 13 to 776 AE 215 ng Hg g −1 DW in algal OM and were enriched 2-39 times compared to sediment THg, indicating that even small changes in algal OM inputs could significantly alter THg. OM-Hg explained 76-96% of the variation in THg concentrations over many centuries. Concentrations of S2 carbon (an algal productivity proxy) and OM-Hg were significantly correlated in two lakes but not in the boreal forest lake possibly because of OM remineralization in its deep water column. Fluxes of S2 carbon, OM-Hg, and THg were highly correlated in the barren tundra lake but could not be calculated for the other lakes. The results overall indicate that high algal Hg concentrations due to scavenging of available Hg controlled OM-Hg flux to sediments, thus driving changes in THg concentrations and fluxes. These findings improve our understanding of the long-term stability of baseline THg in northern lakes under a changing climate, including in the modern era. †

Why productive lakes are larger mercury sedimentary sinks than oligotrophic brown water lakes

“…Our results showed that differences in atmospheric Hg deposition fluxes, catchment area, or erosional fluxes cannot explain the high Hg accumulation rates in the HZ lakes. Similar to what has been observed in other lakes (Biester et al 2018; Outridge et al 2019) or the ocean (Zaferani et al 2018), we suggest that water column Hg scavenging by sinking algal OM strongly increases Hg export from the water phase to the sediment in the HZ lakes. The chemical composition of the trap material of the HZ lakes (low C/N ratios, polypeptides, amides, and proteins indicated by the FTIR spectra) as well as in the water phase (high pH, low nitrate) gives a clear indication that OM in the HZ lake sediments is dominated by algal‐derived OM in contrast to the BF lakes, where most OM is derived from catchment soils (Fig.…”

“…Our results indicate that algae OM appear to dilute Hg concentrations in trap materials but also greatly enhance Hg accumulation rates. High Hg accumulation in sediments as a result of Hg scavenging by algal OM in the water column has been described in previous studies (Outridge et al 2007(Outridge et al , 2019Zhang et al 2014;Biester et al 2018;Zaferani et al 2018), although the source and mass balance of Hg in these studies was mostly unknown.…”

“…Outridge et al (2019) argued in a recent study that inconsistent findings in sediment Hg concentrations vs. the extent of primary productivity relationship in some northern lakes and shallow ponds might be due to differences in lake and catchment properties which limit the amount of Hg that is available for scavenging in the water column by algae. Accordingly, at similar atmospheric Hg deposition rates and comparable soil Hg content, a larger catchment area would result in higher Hg accumulation rates in the lake sediments.…”

“…Furthermore, higher OM fluxes from the surrounding catchment resulted in higher Hg to C ratios (Supporting Information Table S1) in the BF sediment traps because terrigenous OM is generally older and more degraded than autochthonous OM (Ouellet et al 2009;Jiskra et al 2015). Outridge et al (2019) argued in a recent study that inconsistent findings in sediment Hg concentrations vs. the extent of primary productivity relationship in some northern lakes and shallow ponds might be due to differences in lake and catchment properties which limit the amount of Hg that is available for scavenging in the water column by algae. Accordingly, at similar atmospheric Hg deposition rates and comparable soil Hg content, a larger catchment area would result in higher Hg accumulation rates in the lake sediments.…”

Paleolimnological characterization of metal(loid) sources and transport processes in Canadian subarctic lakes

“…In aquatic environments, warmer water temperature can increase primary production (Häder et al, 2014;Rantala et al, 2016), with potential indirect consequences on underwater solar radiations, dissolved oxygen and redox gradients (Paerl and Huisman, 2009;Sahoo and Schladow, 2008). These impacts can affect metal biogeochemistry by increasing the rate of biological sequestration of dissolved and particulate metals in lakes (Outridge et al, 2007;Outridge et al, 2019;Stern et al, 2009) and favor the mobility of metals in lake sediment (Boyle, 2001). The capacity of sediments to acquire more atmospheric Hg0 from the uptake by algae during periods of increased primary production is termed "algal scavenging" (Outridge et al, 2007).…”