Mercury dynamics of an arctic tundra ecosystem in northern Alaska: a mass balance

Abstract:

To constrain the mercury (Hg) mass balance of a tundra ecosystem, we measured atmospheric mercury (Hg) concentrations and surface-atmosphere exchange at Toolik Field Station (68° 38' N) beginning September 2014. We also conducted automated measurements of gaseous Hg in soil pores and snow interstitial air to quantify gas exchange between soils, snow, and the atmosphere; and characterized wet and dry deposition and plant-derived Hg inputs. Results show that atmospheric Hg concentrations peak in winter, decrease in spring, and show summertime minima. Oxidized atmospheric Hg was below detection limits (0.05 ng m^-3) indicating no significant dry deposition. Summertime minima of atmospheric Hg concentrations were associated with depositional fluxes of gaseous Hg (up to 2.8 ng m^-2 hr^-1; measured by a gradient method) that emerged after complete snowmelt. In contrast, gaseous Hg fluxes were below detection limits when snowpack was present; this was supported by in situ snowpack measurements and in contrast to commonly observed gaseous emissions from temperate snowpacks. The cumulative annual gaseous deposition flux of mercury was 12 µg m^-2, in similar range as plant-derived inputs (17 µg m^-2 yr^-1) which we consider the major reason for observed gaseous Hg sink. Wet deposition was extremely low (<1 µg m^-2 yr⁻¹) compared to other sites.

Hg concentrations in plants and soils are similar to levels found at temperate sites, but terrestrial pool sizes are large in comparison ranging around 400 g ha^-1. The results suggest that: atmospheric Hg exposure is low at this site; that deposition is dominated by plant-derived deposition; and that significant Hg pools accumulate in tundra soils, likely driven by strong retention and low re-emissions after deposition. The high Hg soil pool sizes and key role of plant-productivity for Hg deposition may indicate a high sensitivity to climate change, in particular to permafrost soil thawing and increased growing season length.