Environ. Sci. Technol. 2005, 39, 8150-8155
Enhanced Deposition and Bioaccumulation of Mercury in Antarctic Terrestrial Ecosystems Facing a Coastal Polynya ROBERTO BARGAGLI,* CHIARA AGNORELLI, FRANCESCA BORGHINI, AND FABRIZIO MONACI Dipartimento di Scienze Ambientali, Universita` di Siena, Via P.A. Mattioli, 4; 53100 Siena, Italy
Mercury emitted by anthropogenic and natural sources occurs in the atmosphere mostly in the gaseous elemental form, which has a long lifetime in tropical and temperate regions. Once deposited in terrestrial and aquatic ecosystems the metal is partly re-emitted into the air, thus assuming the characteristics of global pollutants such as persistent volatile chemicals. In polar regions, during and after the sunrise, the photochemically driven oxidation of gaseous Hg by reactive halogens may result in areas of greatly enhanced Hg deposition. Mercury concentrations in soils, lichens, and mosses collected in a stretch between 74°30′ S and 76°00′ S, in ice-free coastal areas of Victoria Land facing the Terra Nova Bay coastal polynya, were higher than typical Antarctic baselines. The finding of enhanced Hg bioaccumulation in Antarctic terrestrial ecosystems facing a coastal polynya strongly supports recent speculations on the role of ice crystals (“frost flowers”) growing in polynyas as a dominant source of sea salt aerosols and bromine compounds, which are involved in springtime mercury depletion events (MDEs). These results raise concern about the possible environmental effects of changes in regional climate and sea ice coverage, and on the possible role of Antarctica as a sink in the mercury cycle.
Introduction In the context of global metal cycles, Hg is unique in that it is emitted mainly as a vapor by natural and anthropogenic sources and it biomagnifies through food chains. The atmosphere is the main pathway for Hg transport and distribution because the metal has a low aqueous solubility and because of the relative inertness toward oxidation of the dominant atmospheric species (gaseous elemental Hg0), which in tropical and temperate regions has residence time between 6 months and one year (1, 2). The long atmospheric lifetime allows the transport of Hg0 over large distances to regions far from the main sources such as active volcanoes and facilities for waste incineration, coal combustion, or smelting (3-5). Atmospheric Hg0 can be oxidized in the gaseous or aqueous phase to form gaseous divalent compounds (usually denoted as reactive gaseous mercury, RGM) which are readily deposited on the Earth’s surface via wet or dry processes. Once deposited in terrestrial and aquatic * Corresponding author phone: +39(0577)232828; fax +39(0577)232930; e-mail:
[email protected]. 8150
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environments Hg is partly re-emitted into the air, thereby enhancing its distribution on continental and global scales like the so-called POPs (persistent organic pollutants). Part of the deposited metal can be naturally converted into more toxic methylmercury compounds; these bioaccumulate in food chains, even posing a threat to target organisms in remote areas (6, 7). In Arctic ecosystems, for instance, Hg concentrations are known to have increased with time and to be rather high (8). In the 1990s several hypotheses were put forward to explain environmental contamination in remote regions with no significant natural or anthropogenic sources of Hg. Recent discoveries suggest that in several Arctic regions, following the polar sunrise, frequent mercury depletion events (MDEs) reduce the atmospheric lifetime of Hg0 with respect to that at lower latitudes. During a MDE, Hg0 undergoes photochemically driven oxidation by reactive halogens and is converted to RGM which rapidly deposits on snow (8-10). During a three-year survey (1987-1989) on total concentrations of gaseous Hg, episodic decreases in Hg0 concentrations were also observed at the southern tip of Ross Island (77° S). However, very few atmospheric samples were collected during the Antarctic springtime, and at the time it was supposed that the marked departure of Hg0 levels from background concentrations was due to the influx of air masses containing very low concentrations of the metal (11). However, springtime MDEs have recently been reported at the German Antarctic research station Neumayer (12). These events were limited to the time after polar sunrise and they end suddenly during the Antarctic summer (13). Simultaneous measurements of both Hg0 and RGM concentrations (from the middle of November 2000 to the middle of January 2001) at a Victoria Land coastal site (near the Italian Antarctic station “M. Zucchelli”) indicated that the oxidation of Hg0 may produce RGM concentrations (up to 334.2 pg m-3; mean value 116.3 ( 77.8 pg m-3) as high as those measured in some industrial environments (2). Despite evidence of enhanced Hg deposition during and after polar sunrise in some Antarctic coastal sites, the postdeposition dynamics of the metal in snow and its ultimate fate in terrestrial ecosystems are unknown. In general, the few available data on Hg concentrations in Antarctic snow, soil, and terrestrial biota are among the lowest ever reported (e.g., 14-16). However, lichens are one of the most effective biomonitors of airborne metals and particularly Hg (17). A preliminary survey of Hg distribution in epilithic macrolichens collected in ice-free areas of coastal Victoria Land (East Antarctica) during austral summer 1989-90, revealed surprisingly enhanced bioaccumulation of the metal in some samples collected from ice-free areas in and around the Nansen Ice Sheet (18). This was an unexpected finding because lichens receive elements mainly from atmospheric deposition, and the atmospheric input of Hg throughout a remote and pristine region such as Victoria Land was expected to be fairly uniform. At the time we hypothesized that the bioaccumulation of Hg in lichens from the Nansen Ice Sheet region was probably due to local environmental features. The main factor distinguishing this area from the rest of the Victoria Land coast is the strong and persistent katabatic wind which continuously clears the sea ice, determining the recurring formation of a coastal polynya in Terra Nova Bay. We therefore supposed that the wind was responsible for the very dry microhabitats and the minimal growth rates of lichens, and for the possible advection of air masses enriched in Hg either by emission from Mt. Erebus (an active volcano 3794 m high, about 300 km to the south) and/or by enhanced 10.1021/es0507315 CCC: $30.25
2005 American Chemical Society Published on Web 09/29/2005
marine productivity in the coastal polynya. However, the discovery in the past decade of the dynamics and chemistry of MDEs in polar regions, the very recent hypothesis on the role of ice crystals (“frost flowers”) growing in polynyas as the dominant source of sea salt aerosol and bromine compounds (19) to the Antarctic environment, and the finding of high Hg concentrations in snow and frost flowers along sea ice leads in Alaska (20) suggest the involvement of other processes. Thus, in austral summer 2002 we performed a large-scale sampling of surface soils, epilithic macrolichens, and mosses throughout ice-free areas of coastal Victoria Land in order to establish spatio-temporal patterns of Hg bioaccumulation in Victoria Land terrestrial ecosystems, and verify if soils and cryptogams from ice-free areas facing the coastal polynya (where frost flowers form) are affected by enhanced Hg accumulation.
Experimental Section Field Sampling. In East Antarctica, scattered permanently or seasonally ice-free areas only occur in coastal regions, on the steep slopes of the Transantarctic Mountains and in nunataks. Most of these areas are typical cold desert environments with sparse microbial and cryptogamic communities in sheltered coastal microhabitats, where some water and nutrients are available in summer. Lichens and mosses constitute the bulk of plant biomass in these ecosystems, and their metabolism is largely dependent upon atmospheric deposition of major and trace elements from the marine environment (21). Since these organisms are perennial and have very slow growth rates, they accumulate metals, radionuclides, and other persistent atmospheric contaminants to levels well above those in snow or aerosols. Several species of Antarctic cryptogams have wide geographical ranges, thereby enabling the establishment of largescale biomonitoring networks (21). To map Hg deposition in coastal Victoria Land, samples of the epilithic macrolichen Umbilicaria decussata (Vill.) Zahlbr. and the terricolous moss Bryum pseudotriquetrum (Hedw.) Schwaegr. were collected in January and February 2002, from coastal ice-free areas between Quartermain Point (72°02′ S, 170°10′ E) and Granite Harbor (77°02′ S, 162°32′ E; Figure 1). Umbilicaria decussata is a cosmopolitan epilithic macrolichen, and one of the most widespread species in coastal habitats of continental Antarctica. In central and northern Victoria Land, thalli of reduced dimensions can be found in niches and fissures of north-facing rocks at altitudes of up 700-800 m and 40-50 km from the coastline. The moss B. pseudotriquetrum is another cosmopolitan and widespread species in ice-free areas of Victoria Land. In contrast to U. decussata, the moss grows on sand and gravel substrates subject to flooding by overflow of small melt pools and in seepage areas. Moss samples were therefore collected only in coastal habitats where in December and/or January the melting of ice fields or snow banks produces ephemeral streams, small ponds, and seeps. Samples were collected from each ice-free area, maintaining a distance of at least 500 m from the helicopter landing site and using new pairs of disposable plastic gloves and clean plastic or stainless steel tools. Specimens included composite samples (n ) 42) of U. decussata (thalli with different dimensions and from different rocks), 28 samples of B. pseudotriquetrum shoots (from different zones of the same cushion and from different cushions), and 43 soil samples (the top 3 cm of fine particles accumulating at the base of rocks bearing lichens and near moss cushions). Sample Preparation and Analysis. Immediately upon return to the laboratory in the Italian Antarctic station M. Zucchelli, all samples were air-dried, sorted to remove as much extraneous materials as possible (e.g., rock and soil particles from cryptogams), and then stored and sealed in
FIGURE 1. Terra Nova Bay coastal polynya (December 2004), sampling sites of soils and cryptogams in ice-free costal areas of Victoria Land, and a schematic representation of katabatic wind direction. acid-cleaned glass jars with Teflon lined caps, in a laminar flow chamber. Once in Italy, each sample was unsealed in a chamber under N2 flow, and 150 mg of homogenized lichen thalli, moss shoots, and the
