RESUMEN
Vegetation uptake of atmospheric mercury (Hg) is an important mechanism enhancing atmospheric Hg deposition via litterfall and senescence. We here report Hg concentrations and pool sizes of different plant functional groups and plant species across nine tundra sites in northern Alaska. Significant spatial differences were observed in bulk vegetation Hg concentrations at Toolik Field station (52⯱â¯9⯵gâ¯kg-1), Eight Mile Lake Observatory (40⯱â¯0.2⯵gâ¯kg-1), and seven sites along a transect from Toolik Field station to the Arctic coast (36⯱â¯9⯵gâ¯kg-1). Hg concentrations in non-vascular vegetation including feather and peat moss (58⯱â¯6⯵gâ¯kg-1 and 34⯱â¯2⯵gâ¯kg-1, respectively) and brown and white lichen (41⯱â¯2⯵gâ¯kg-1 and 34⯱â¯2⯵gâ¯kg-1, respectively), were three to six times those of vascular plant tissues (8⯱â¯1⯵gâ¯kg-1 in dwarf birch leaves and 9⯱â¯1⯵gâ¯kg-1 in tussock grass). A high representation of nonvascular vegetation in aboveground biomass resulted in substantial Hg mass contained in tundra aboveground vegetation (29⯵gâ¯m-2), which fell within the range of foliar Hg mass estimated for forests in the United States (15 to 45⯵gâ¯m-2) in spite of much shorter growing seasons. Hg stable isotope signatures of different plant species showed that atmospheric Hg(0) was the dominant source of Hg to tundra vegetation. Mass-dependent isotope signatures (δ202Hg) in vegetation relative to atmospheric Hg(0) showed pronounced shifts towards lower values, consistent with previously reported isotopic fractionation during foliar uptake of Hg(0). Mass-independent isotope signatures (Δ199Hg) of lichen were more positive relative to atmospheric Hg(0), indicating either photochemical reduction of Hg(II) or contributions of inorganic Hg(II) from atmospheric deposition and/or dust. Δ199Hg and Δ200Hg values in vascular plant species were similar to atmospheric Hg(0) suggesting that overall photochemical reduction and subsequent re-emission was relatively insignificant in these tundra ecosystems, in agreement with previous Hg(0) ecosystem flux measurements.
Asunto(s)
Mercurio/análisis , Contaminantes del Suelo/análisis , Alaska , Cambio Climático , Líquenes/química , Mercurio/química , Isótopos de Mercurio/análisis , Isótopos de Mercurio/química , TundraRESUMEN
Anthropogenic activities have led to large-scale mercury (Hg) pollution in the Arctic. It has been suggested that sea-salt-induced chemical cycling of Hg (through 'atmospheric mercury depletion events', or AMDEs) and wet deposition via precipitation are sources of Hg to the Arctic in its oxidized form (Hg(ii)). However, there is little evidence for the occurrence of AMDEs outside of coastal regions, and their importance to net Hg deposition has been questioned. Furthermore, wet-deposition measurements in the Arctic showed some of the lowest levels of Hg deposition via precipitation worldwide, raising questions as to the sources of high Arctic Hg loading. Here we present a comprehensive Hg-deposition mass-balance study, and show that most of the Hg (about 70%) in the interior Arctic tundra is derived from gaseous elemental Hg (Hg(0)) deposition, with only minor contributions from the deposition of Hg(ii) via precipitation or AMDEs. We find that deposition of Hg(0)-the form ubiquitously present in the global atmosphere-occurs throughout the year, and that it is enhanced in summer through the uptake of Hg(0) by vegetation. Tundra uptake of gaseous Hg(0) leads to high soil Hg concentrations, with Hg masses greatly exceeding the levels found in temperate soils. Our concurrent Hg stable isotope measurements in the atmosphere, snowpack, vegetation and soils support our finding that Hg(0) dominates as a source to the tundra. Hg concentration and stable isotope data from an inland-to-coastal transect show high soil Hg concentrations consistently derived from Hg(0), suggesting that the Arctic tundra might be a globally important Hg sink. We suggest that the high tundra soil Hg concentrations might also explain why Arctic rivers annually transport large amounts of Hg to the Arctic Ocean.