Subsurface seawater methylmercury maximum explains biotic mercury concentrations in the Canadian Arctic

Abstract

Mercury (Hg) is a contaminant of major concern in Arctic marine ecosystems. Decades of Hg observations in marine biota from across the Canadian Arctic show generally higher concentrations in the west than in the east. Various hypotheses have attributed this longitudinal biotic Hg gradient to regional differences in atmospheric or terrestrial inputs of inorganic Hg, but it is methylmercury (MeHg) that accumulates and biomagnifies in marine biota. Here, we present high-resolution vertical profiles of total Hg and MeHg in seawater along a transect from the Canada Basin, across the Canadian Arctic Archipelago (CAA) and Baffin Bay, and into the Labrador Sea. Total Hg concentrations are lower in the western Arctic, opposing the biotic Hg distributions. In contrast, MeHg exhibits a distinctive subsurface maximum at shallow depths of 100–300 m, with its peak concentration decreasing eastwards. As this subsurface MeHg maximum lies within the habitat of zooplankton and other lower trophic-level biota, biological uptake of subsurface MeHg and subsequent biomagnification readily explains the biotic Hg concentration gradient. Understanding the risk of MeHg to the Arctic marine ecosystem and Indigenous Peoples will thus require an elucidation of the processes that generate and maintain this subsurface MeHg maximum.

Mercury in Ringed Seals (Pusa hispida) from the Canadian Arctic in Relation to Time and Climate Parameters

“…Many factors such as geological influences, river and atmospheric input, and long‐range transport of Hg emissions from Asian sources may be influencing the region‐specific accumulation of Hg in seals (Kirk et al 2012; Cole et al 2014; Brown et al 2018). Wang et al (2018) have also suggested that differences in MeHg enrichment in subsurface waters may explain the higher biotic Hg concentrations in western regions where the MeHg peaks have been found to occur at shallower depth. Moreover, climate can be a major influence on Hg bioavailability and accumulation in biota (Wang et al 2019).…”

Seasonal variation of mercury contamination in Arctic seabirds: A pan-Arctic assessment

“…Hg levels in Pacific seabirds are closely associated with the base of the food web (Elliott and Elliott 2016), and variation in food webs among ocean basins may be one cause of lower Hg levels in the Pacific relative to the Atlantic. For example, a recent survey comparing Pacific to Atlantic waters in the Canadian Arctic (where Hg levels are higher in predators in the Pacific) found that variation in Hg in top predators was associated with higher levels of methylation in the Pacific despite lower levels of total Hg (Wang et al 2018). Clearly, food web processes that affect methylation of Hg are important for explaining region differences, and that study, which linked disparate predator information to a single ship-based transect, might be strengthened by similar pan-Arctic predator information such as that present in our study.…”

“…We based these predictions on known foraging ecologies (Gaston and Jones, 1998) and previously documented patterns of Hg exposure in the Pacific, and West and East Atlantic. The northern Pacific has higher Hg deposition rates due high emissions from Asia (Selin et al, 2007), but levels in biota are more closely linked to methylation rates by bacteria (Elliott and Elliott, 2016;Wang et al, 2018), which led us to expect high levels in the western Atlantic based on previous work (Albert et al, 2019;AMAP, 2018).…”

Mercury Export Flux in the Arctic Ocean Estimated from 234Th/238U Disequilibria

“…Elevated Hg levels in arctic biota (Dietz et al, 2013, Wang et al, 2018, Dudarev et al, 2019 have been explained by enhanced inorganic Hg inputs to the Arctic Ocean (Outridge et al, 2008, Fisher et al, 2012. Several recent studies aimed at refining Hg inputs from the atmosphere (Schroeder et al, 1998, Soerensen et al, 2016, rivers (Sonke et al, 2018), coastal erosion (Leitch et al, 2007, Schuster et al, 2019, Lim et al, 2020 and other oceans (Cossa et al, 2018, Petrova et al, 2020.…”