Importance of vegetation type for mercury sequestration in the northern Swedish mire, Rödmossamyran

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Geochimica et Cosmochimica Acta 74 (2010) 7116–7126 www.elsevier.com/locate/gca

Importance of vegetation type for mercury sequestration in the northern Swedish mire, Ro¨dmossamyran Johan Rydberg a,⇑, Jon Karlsson a, Roger Nyman a, Ida Wanhatalo a, Kerstin Na¨the b, Richard Bindler a b

a Department of Ecology and Environmental Science, Umea˚ University, SE-901 87 Umea˚, Sweden Institute of Environmental Geology (IUG), Technical University of Braunschweig, Post Box 3329, D-38023 Braunschweig, Germany

Received 13 April 2010; accepted in revised form 17 September 2010; available online 1 October 2010

Abstract Even if mires have proven to be relatively reliable archives over the temporal trends in atmospheric mercury deposition, there are large discrepancies between sites regarding the magnitude of the anthropogenic contribution to the global mercury cycle. A number of studies have also revealed significant differences in mercury accumulation within the same mire area. This raises the question of which factors, other than mercury deposition, affect the sequestration of this element in peat. One such factor could be vegetation type, which has the potential to affect both interception and retention of mercury. In order to assess how small-scale differences in vegetation type can affect mercury sequestration we sampled peat and living plants along three transects on a northern Swedish mire. The mire has two distinctly different vegetation types, the central part consists of an open area dominated by Sphagnum whereas the surrounding fen, in addition to Sphagnum mosses, has an understory of ericaceous shrubs and a sparse pine cover. A few main patterns can be observed in our data; (1) Both peat and Sphagnum-mosses have higher mercury content (both concentration and inventory) in the pine-covered fen compared to the open Sphagnum area (100% and 71% higher for peat and plants, respectively). These differences clearly exceed the 33% difference observed for lead210, which is considered as a good analogue for atmospheric mercury deposition. (2) The differences in mercury concentration between peat profiles within a single vegetation type can largely be attributed to differences in peat decomposition. (3) When growing side by side in the open Sphagnum area, the moss species Sphagnum subsecundum has significantly higher mercury concentrations compared to S. centrale (24 ± 3 and 18 ± 2 ng Hg g1, respectively). Based on these observations we suggest that species composition, vegetation type and decomposition can affect the mercury sequestration in a peat record, and that any changes in these properties over time, or space, have the potential to modify the mercury deposition signal recorded in the peat. Ó 2010 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Since (Pheiffer-Madsen, 1981) first investigated the use of peat as an archive of mercury (Hg) deposition a generation ago, a large number of studies using peat to decipher the deposition history of mercury have been conducted (Jensen and Jensen, 1991; Benoit et al., 1994; Norton et al., 1997; Benoit et al., 1998; Martinez-Cortizas et al.,

⇑ Corresponding author. Tel.: +46 90 786 7101.

E-mail address: [email protected] (J. Rydberg). 0016-7037/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2010.09.026

1999; Biester et al., 2002; Roos-Barraclough et al., 2002b; Bindler, 2003; Givelet et al., 2003; Shotyk et al., 2005; Roos-Barraclough et al., 2006; Ettler et al., 2008; Farmer et al., 2009). The general temporal trend displayed in most peat mercury records is relatively consistent and largely in step with the pace of industrialization and the burning of fossil fuels (i.e., low mercury concentration and accumulation in peat pre-dating 1850, followed by increasing trends during the industrial period and a decrease since about 1980). However, the magnitude of the anthropogenic maxima varies greatly between sites, from 10 to 400 times

Importance of vegetation type for mercury sequestration

background concentration (Bindler, 2006; Biester et al., 2007), and recent studies have shown that quantitative differences in mercury concentrations and fluxes occur within individual peatlands (Bindler et al., 2004; Coggins et al., 2006; Ettler et al., 2008). For example, Bindler et al. (2004) found that lead and mercury concentrations and their inventories varied by two- to four-fold in replicate cores from one hummock string in a southern Swedish bog, while Coggins et al. (2006) noted similar within-bog variations in triplicate cores from two locations in Ireland. Such within-bog variations have also been shown for atmospherically supplied lead-210 (210Pb; e.g., Malmer and Holm, 1984; Urban et al., 1990; Oldfield et al., 1995; Olid et al., 2008), which is considered as a good deposition analogue for mercury (Lamborg et al., 2000). Such large differences in mercury accumulation between and within sites implies that in order to be able to fully interpret peat mercury records we need a better understanding of factors influencing how mercury is incorporated in peat. Indeed, there are many factors that have the potential to affect mercury concentrations and accumulation. For example, the concentration of an element bound to peat, such as halogens and mercury, is known to be affected by peat decomposition (Biester et al., 2003, 2004; Franzen et al., 2004; Keppler et al., 2004), and the decomposition in itself can be affected by an array of factors (i.e., wetness, temperature, litter quality, microbial activity). However, also other factors have the potential to significantly affect mercury accumulation; one of those is differences in mercury sequestration between plant species and vegetation types (Roos-Barraclough et al., 2002a). Differences between and within species can occur due to a wide variety of reasons, including both physical and physiological factors (e.g., canopy height, speed of growth, wetness, temperature), which have the potential to influence the interception, incorporation or retention of mercury on and within the plant (Lindberg et al., 1998; Gustin et al., 2002; Gbor et al., 2006; Zhang et al., 2009). That variation in mercury sequestration between different plant species does occur has been recognized both when plants are used for mineral ore prospecting (Kovalevskii, 1986), and in biomonitoring programs using plants and mosses to study mercury deposition (Steinnes, 1980; Aboal et al., 2001; Harmens et al., 2004). However, these studies were done on relatively large spatial scales and did not address local differences both between and within species. The few studies looking at local differences have been conducted only on a single species (Rasmussen, 1995) or in a non-systematic way (Roos-Barraclough et al., 2002a). In this study we have collected vegetation samples systematically in order to assess how mercury concentrations differ, over short distances, due to variations in plant species and vegetation type. We have also collected a number of peat cores to study whether any differences in vegetation mercury concentrations are manifested as spatial differences in mercury accumulation in the peat. 2. SITE DESCRIPTION AND SAMPLE COLLECTION Ro¨dmossamyran is a small (7 ha), nutrient-poor (oligotrophic, pH in pore water is 3.5–4) fen located near the

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present-day coastline of the Gulf of Bothnia in northern Sweden (63° 470 N, 20° 200 E; 40 m a.s.l.; Fig. 1). The boreal forest surrounding the mire is underlain by thin, poorly-tomoderately sorted till, re-worked by water during the isostatic rebound of the region. Based on the basal ages of ˚ myran two other mires in the region – 2400 BP for Stor A (63° 440 N, 20° 060 E; 35 m a.s.l.) and 3800 BP for Sjulsmyran (64° 020 N, 20° 400 E; 60 m a.s.l.) (Oldfield et al., 1997; Nilsson et al., 2001), which were formed through the same process of isostatic rebound and emergence from the sea – we estimate that Ro¨dmossamyran transitioned to a mire about 2500–2800 years ago. The mire area consists of two distinctly different vegetation types. In the southern end of the mire there is an open area dominated by different Sphagnum species (mainly Sphagnum centrale and S. subsecundum), whereas the majority of the mire consists of a mixture of Sphagnum and different ericaceous shrubs and has a sparse pine-cover. The border between the open Sphagnum area and the pinecovered fen is quite sharp (Fig. 2). Annual mean temperature for the Umea˚ region is 2–3 °C, and total annual precipitation is 650 mm, of which about one third falls as snow. Winds are predominantly blowing in a north-south direction, with westerly to south westerly winds more frequent during autumn and winter (SNA, 1995). 2.1. Peat In autumn 2005 we collected peat profiles from five sites along a southwest transect (transect A) from the open Sphagnum area in the deepest section of the mire to the much thinner peat at the mire edge (Fig. 1). We collected a complete profile (i.e., both vegetation and peat) in the open Sphagnum area (labeled S1) and four complete profiles at four sites on the adjacent nutrient-poor, pine-covered fen area (labeled F1–F4). S1 and F1 were collected only 2 m apart on each side of the boundary between the pine-covered fen and the open Sphagnum area (Fig. 2). For S1 and F1–F3 we used a combination of a Wardenaar-corer (Wardenaar, 1987), for the uppermost 0.75 m, and an overlapping series of Russian peat cores (either a 0.75-mlong, 5-cm-diameter corer or a 1-m-long, 7.5-cm-diameter corer) for deeper peat sections. The cores overlapped by 20 cm. For the fifth site (F4) at the edge of the mire we removed a 30-cm deep monolith with a spade. The thickness of the peat along transect A was 270, 260, 245, 150 and 30 cm for S1, F1, F2, F3 and F4, respectively. The Russian cores were cut in the field into 10-cm sections and placed in polyethylene bags. In the laboratory all field-sectioned samples were weighed and then stored frozen at 18 °C until processing. The Wardenaar cores were wrapped in plastic film, then aluminum foil, and taken back to the laboratory intact and stored frozen (18 °C). We went back to the mire in autumn 2009 and collected two complete replicate profiles from the locations of S1 and F1, i.e., the two sites straddling the boundary between pinecovered fen and the open Sphagnum area. The profiles were collected approximately one meter from the original sampling locations and were labeled S1b and F1b. The F1b pro-

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Fig. 1. Location of Umea˚ and the mire Ro¨dmossamyran (a). Aerial photograph covering the mire and its surroundings (b, www.lantmateriet.se), the thin black line indicates the extent of the pine-covered fen whereas hatched area corresponds to the open Sphagnum area. Map showing the spatial variation in peat depth over the mire area (c), the thick black lines mark the three transects along which peat (A) and plant (A–C) samples were collected.

nation of a PVC-monolith corer for the upper part and an overlapping series of Russian peat cores (0.75-m-long, 5-cm diameter) for the lower parts; all cores were cut in the field in 10 cm samples. The F1b and S1b profiles were 275 and 290 cm long, respectively. On three occasions during August 2009 we also determined the distance to the groundwater table in four groundwater wells at sites S1, F1, F2 and F3. 2.2. Plants

Fig. 2. Picture showing the well-defined border between the open Sphagnum area and the pine-covered fen and the approximate locations of the F1- and S1-sites.

file was collected in the same manner as mentioned above except that the Wardenaar core was cut in the field in 5 cm samples. The S1b profile was collected with a combi-

In September 2008 we collected plant samples along two transects (Fig. 1), one on the central open Sphagnum area (3 sample points; transect B) and one from the open Sphagnum-area northwest into the pine-covered area (6 sample points; transect C). Each sample plot consisted of four 1m2 sub-plots, separated by 1 m. Several individuals of each of the dominant plants species were collected from each sub-plot and combined into composite samples (1–5 composite samples per plant species per sub-plot). The plants were: green/white Sphagnum centrale (samples were dominated by Sphagnum centrale with minor contribution

Importance of vegetation type for mercury sequestration

of S. palu´stre and S. subsecundum), red/purple S. subsecundum (samples had a minor contribution of S. magella´nicum), and Eriophorum vaginatum on the open Sphagnum area; and in the pine-covered fen green/white S. centrale (also here with minor contribution of S. palu´stre and S. subsecundum), heather (Calluna vulgaris (L.) Hull), Labrador tea (Ledum palustre L.) and pine (Pinus sylvestris L.). Only the uppermost 2–3 cm of mosses were sampled and analyzed, as it represents the most recent atmospheric deposition (ca. 1–2 years). For the other field-layer plants and pine only leaves or needles were collected. Pine needles were collected from the lower 2 m from all sides of the tree. Samples were gathered by cutting the vegetation with a stainless steel scissor onto clean weighing boats and then transferred into zip-lock bags. Polyvinyl gloves were used at all times during sampling. All told, 163 composite samples were analyzed. When we revisited the mire in 2009 we also complemented the plant sampling by adding sample plots along transect A (at the same sites as the peat profiles). Because of small differences between C. vulgaris and L. palustre (see Results) we only included C. vulgaris in this sampling campaign, E. vaginatum was also excluded because of poor representation in the transect A plots. Hence, the 2009plant sampling included P. sylvestris, C. vulgaris and the two Sphagnum species. The sampling was done in a similar way as that of 2008 with each sample plot divided into 4 sub-plots and samples taken from each sub-plot. A total of 56 composite plant samples were collected and analyzed. 3. ANALYTICAL METHODS For the 2005-peat profiles sectioning of the Wardenaar cores was done in a freezer room (18 °C). We removed the outermost 1 cm on a band saw with a stainless steel blade, cleaned the surface using a woodworkers hand plane, and then cut each prepared core into 2-cm-thick slices (for the 2009-peat profiles all sectioning was done in the field in 5- or 10-cm sections). The 2005-peat samples and most of the 2008-plant samples were freeze dried (30 plant samples were dried at 30 °C), while the 2009-samples (both peat and plants) were dried at 30 °C. All samples were weighed for calculating dry bulk density and ground using an agate ball mill. Carbon (C) and nitrogen (N) were determined using a Perkin–Elmer 2400 series II CHNS/O-analyzer operated in CHN-mode. Analytical quality of the analyses was con-

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trolled by the inclusion of replicates and reference materials along with every 10th sample. Replicate analyses were within ±5 and ±10%, and reference materials were consistently within ±3 and ±7% for carbon and nitrogen, respectively. C:N-ratios are calculated as atomic ratios. To determine the mercury content of the samples we used automated thermal decomposition atomic absorption spectrophotometers (TD-AAS), which eliminates the need for sample pretreatment (US EPA Method 7473). The 2005-samples (and about 30 of the 2008-plant samples) were analyzed on a Leco AMA 254, for the remaining 130 2008-plant samples we used a Milestone DMA-80 and all 2009-samples were analyzed on a Perkin–Elmer SMS-100. We included standard reference material (SRMs) and replicate samples in the analyses along with approximately every 10th sample. For measured concentrations above 20 ng Hg g1 replicate analyses were within ±5%, and for concentrations below 20 ng Hg g1 replicate analyses were within ±1 ng Hg g1. The SRMs (NIST – 1515, Apple leaves; CANMET – LKSD-4, lake sediment; NCS – ZC73002, soil; Low ash peat (Yafa et al., 2004)) were all within certified ranges and showed very good consistency between the three Hg-analyzers (Table 1). All mercury concentrations are reported on a dry mass basis, ng Hg g1. Statistical significance between sub-sets of the plant data were evaluated using independent sample t-tests (between Sphagnum and vascular plants, and between the different sampling years for vascular plants) and one-way ANOVA (between six sub-sets of Sphagnum mosses defined by sampling year, vegetation type and species). For all analysis the significance level (p-value) was set to 0.01. In order to obtain normally distributed sub-sets with equal variance all data were log-transformed prior to analysis. A Shapiro–Wilk test was used to determine that the data were normally distributed and equality of variance was validated using Levene-statistics; in both cases the p-value was set to 0.05. All statistical analyses were done with the aid of the SPSS PASW statistics 18.0 software-package (www.spss.com). To be able to assess whether any differences between the vegetation types are due to differences in bulk deposition or re-emission of volatile Hg-species, we analyzed the top parts, including the living vegetation, of cores S1b and F1b for lead-210 (210Pb; www.flettresearch.ca). Lead-210 is considered to be a good analogue for mercury deposition (Lamborg et al., 2000) but it does not have any volatile spe-

Table 1 Obtained mercury concentrations for the standard reference materials (SRMs). Both for all analysis and with each of the three Hg-analyzers reported separately. Certified range (ng g1)

All (ng g1)

Leco AMA-254 (ng g1)

Milestone DMA-80 (ng g1)

Perkin–Elmer SMS-100 (ng g1)

NIST-1515, Apple leaves

44 ± 4

43 ± 3 (n = 17)

42 ± 3 (n = 4)

43 ± 1 (n = 30)

CANMET-LKSD-4, sediment NCS-ZC73002, soil

190

43 ± 3 (n = 52) 182 ± 3 (n = 3) 60 ± 6 (n = 10) 166 ± 6 (n = 15)

182 ± 5 (n = 2)

182 (n = 1)

–

59 ± 4 (n = 6)

62 ±9 (n = 4)

–

166 ± 6 (n = 13)

168 ±1 (n = 2)

–

Low ash peat (Yafa et al., 2004)

61 ± 9 169 ± 7

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Table 2 Summary of some of the data presented in the results.

Peat thickness (cm) Weighted average Hg (ng g1) Weighted average background Hg (ng g1) Peak Hg (ng g1) Weighted average C:N-ratio Weighted average bulk density (g cm3) Distance to water table (cm) 210Pb-inventory (kBq m2)

S1

F1

F2

F3

F4

270 20 11

260 26 16

245 37 26

150 42 29

30 179 n.d.

84 99 0.06

169 100 0.07

190 58 0.08

263 69 0.1

335 39 0.09

3 2.3

25 3.2

42 n.d.

60 n.d.

n.d. n.d.

cies, and hence any differences in unsupported 210Pb inventories between sites S1 and F1 would be due to a difference in deposition of 210Pb (and mercury). Note that only the entire unsupported 210Pb inventories are used, no dating has been done for either of the profiles. 4. RESULTS 4.1. Peat When moving along transect A, from the open Sphagnum area to the pine-covered fen, there is an increase in the weighted average peat bulk density from around 0.06 g cm3

at the S1-site to 0.1 g cm3 in the F3 and F4 sites (Table 2). There is also a decrease in the weighted average C:N-ratio along the same transect, from 100 at sites S1 and F1, 60–70 for sites F2 and F3 and down to 40 for the F4-site (Table 2). The measurements of distance to the groundwater table show that in the open Sphagnum area the late summer groundwater almost reaches the peat surface, while in the pine-covered fen the distance to the groundwater table increases towards the mire edge, 25, 42 and 60 cm at sites F1, F2 and F3, respectively (Table 2). The weighted average peat mercury concentrations along transect A increased from the central parts of the mire towards the mire edge, 20, 26, 37, 42 and 178 ng Hg g1 for S1, F1, F2, F3 and F4, respectively (Fig. 3). In all peat profiles the general temporal trend is similar (except in F4 that is not long enough to reach a stable background). The background is low and relatively stable (12, 16, 26 and 29 ng Hg g1 for S1, F1, F2 and F3, respectively), the slight increase in mercury concentration in the bottommost samples can be attributed to the time period when the mire transformed through isostatic rebound from a coastal bay to a mire. Between 25 and 50 cm peat depth there is a pronounced increase in mercury concentration, with peak mercury concentrations of 84, 169, 190, 263 and 335 ng Hg g1 (S1, F1, F2, F3 and F4, respectively), followed by a decline towards the peat surface (Fig. 3). If we compare the replicate profiles, S1 with S1b and F1 with F1b, there is very good reproducibility between

Fig. 3. Cross-section of the mire along transect A with the coring sites (S1-2, F1-4) marked (note the difference in length and height scales). The diagrams show the mercury profiles for the complete peat profiles. Different core sections are indicated with different symbols and the F1b and S1b cores are represented by a thin black line (see also Fig. EA-1-1 for F1–F1b profiles).

Importance of vegetation type for mercury sequestration

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the pairs. For the F1-site there are some minor depth discrepancies between the two profiles in the topmost part (Wardenaar-cores), which can be related to differences in peat compaction introduced during coring. When mercury concentrations are plotted against cumulative peat mass instead of peat depth these discrepancies disappear (Fig. EA1-1). The total unsupported 210Pb inventories for S1b and F1b were 2.2 ± 0.12 and 3.2 ± 0.14 kBq m2, respectively (error is given as cumulative counting errors; Table 2). Lead-210 was analyzed down to 50 and 45 cm for S1b and F1b, respectively, which should capture the entire unsupported 210Pb inventories even if the possibility of downward transport is considered. However, due to coarse sample resolution (15 cm) in the deeper part of the S1b profile we suspect that some of the 12 kBq kg1, which corresponds to 5% of the maximum activity, found in the deepest sample of this profile contains some of the unsupported 210Pb inventory. The S1b and F1b cores are relatively similar in ash content (i.e., 1–2% for S1b and 2–3% for F1b) and can be assumed to have relatively similar supported 210Pb activities. Hence, we have used the deepest sample in the more highly resolved (10 cm) F1b profile (4 kBq kg1) to subtract the supported 210Pb activity from both profiles in order to calculate the atmospherically supplied (unsupported) component.

ence between the 2008 and 2009 sampling occasions (t-test p-value = 0.106), however, for the P. sylvestris the 2009 sampling gave significantly higher mercury concentrations, 6 and 14 ng g1 for 2008 and 2009, respectively (t-test p-value <0.01). In the open Sphagnum area where both moss species occur, mercury concentrations are significantly higher in S. subsecundum (24 ± 3 ng Hg g1, n = 28) than in S. centrale (18 ± 2 ng Hg g1, n = 23). Spatially there were significant differences between mosses in the open Sphagnum area and the pine-covered fen, both when all mosses were considered (21 and 30 ng Hg g1, respectively) and for S. centrale exclusively (18 and 30 ng Hg g1, respectively). According to the ANOVA-analysis there were no statistical differences between the 2008 and 2009 sampling occasions for the Sphagnum samples, but in the 2009 sampling the S. centrale in the pine-covered fen could not be statistically separated from the S. subsecundum in the open Sphagnum area, possibly due to the low number of S. subsecundum samples (n = 9) in 2009 (Table 3). Also worth noting is that the mercury concentrations in the plants are lower compared to those in the surface interval (0–2 cm) of the peat cores (30–50 ng Hg g1), which consists of a mixture of both fresh plant material and plant litter/peat (Fig. 3).

4.2. Plants

From the peat and plant data from Ro¨dmossamyran a few patterns can be identified. For the peat profiles there is an increasing trend in mercury concentration towards the mire edge (from S1 to F4), with considerably higher mercury concentrations in the pine-covered fen (i.e., F1– F4) than in the open Sphagnum area (i.e., S1 and an incomplete profile from site S2). The same pattern can be seen in the plant mercury data where mercury concentrations in Sphagnum mosses are significantly higher in the pine-covered fen (30 and 20 ng Hg g1 for the pine-covered and open parts, respectively). In addition to this there are significant differences in mercury concentrations between the two dominating Sphagnum species when they grow side-by-side in the central part of the mire, 24 ng Hg g1 for S. subsecundum and 18 ng Hg g1 for S. centrale. The explanation for the increasing trend in mercury concentrations from F1 to F4 is likely differences in decomposition rates between the sites, as indicated by increasing bulk density and decreasing C:N-ratios (Table 2). All four sites have rather similar plant mercury concentrations, but there are differences in, for example, the distance to the groundwater table (Table 2). A greater distance to the groundwater table would, if we assume constant peat growth rates, increase the time for peat degradation in the acrotelm (i.e., the upper oxic zone of the peat where most of the degradation occurs; Ingram, 1978; Clymo, 1984). For the F1 and F2-sites the conditions are rather similar with a summer groundwater table 20–30 cm below the peat surface, whereas the F3-site has the groundwater table 50–60 cm below the surface. The increased rate of peat decomposition can be observed in less total vertical accumulation of peat (peak mercury concentrations at 21, 17, 11 and 5 cm depth for S1, F1, F2 and F3, respectively),

Our sampling strategy was designed to make it possible to assess how large the spatial variations in plant mercury concentration are; within the 1 m2 sub-plots, between the four sub-plots at each site, between sites and finally between vegetation types. For all sites the within sub-plot and between sub-plot variation is very similar, and this holds true for all analyzed plant species (Fig. EA-1-2). Generally the variation is within 10%, which is slightly higher than the analytical uncertainty. For each species the between site variation is about the same as the within and between sub-plot variation when comparing within the same vegetation type (i.e., just comparing sites either within the open Sphagnum area or the pine-covered fen; Fig. 4). Hence, we can conclude that plant mercury concentrations are relatively homogenous, and any differences between plant species or vegetation types exceeding the within/between subplot variation can be considered as real, and not due to spatial heterogeneity. For the plant samples the average mercury concentration of all samples (n = 219) is 17 ± 9 ng Hg g1, with a range from 5 to 48 ng Hg g1 (Fig. 4 and Table EA-1-1). As expected, the concentration of mercury is significantly higher in the two Sphagnum species as compared to the vascular plants (t-test p-value = 0.01). The average concentration for the two mosses is 25 ± 7 ng Hg g1 (n = 100), which is consistent with reported values for forest mosses in this region (8–38 ng Hg g1; www.ivl.se), and for the vascular plants the average concentrations are lower; 9 ± 1 for E. vaginatum (n = 12), 11 ± 3 for C. vulgaris (n = 51), 10 ± 3 for L. palustre (n = 34) and 8 ± 4 ng Hg g1 for pine needles (n = 16). For C. vulgaris there is no statistical differ-

5. DISCUSSION

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Hg

(ng g -1 )

40 30 20 10

S. centrale S. subsecundum

0

Hg

(ng g -1 )

40 30 20 10 0

E. vaginatum

Hg

(ng g -1 )

40 30 20 10 0

P. sylvestris

Hg

(ng g -1 )

40 30 20 10 0

C. vulgaris

(ng g -1 )

Hg

Open Sphagnum area

Pine-covered fen

40

Pine-covered fen

30 20 10

L. pallustre 0 F4

F3

F2

F1

S1

S2

1B

2B

3B

2C

3C

4C

5C

6C

7C

Fig. 4. Variation in plant mercury concentration depending on sampling location and plant species. Boxes show the 25th and 75th-percentile with a line marking the median value, the whiskers show the 10th and 90th-percentiles and outliers are marked with circles. In the top-most panel the S. centrale is represented by grey boxes and filled circles, while white boxes and open circles represent S. subsecundum. The two dashed lines mark the division between pine-covered fen and open Sphagnum area, and the arrows indicate sampling year.

higher bulk densities and lower C:N-ratios as we move towards the mire’s edge. The loss of organic matter during decomposition results in higher mercury concentrations and peak mercury concentrations closer to the peat surface (Biester et al., 2003; Franzen et al., 2004). A higher degree of peat decomposition is also connected with the slight increase in mercury concentration observed between 120

and 150 cm peat depth, which corresponds to samples with a higher bulk density and lower C:N-ratios. Alternative explanations for the higher mercury concentrations in F3 and F4 would be a slower plant biomass production or differences in mercury net deposition (i.e., deposition minus re-emission). However, there are no indications of higher plant mercury concentrations in sites F3/F4 as compared

Importance of vegetation type for mercury sequestration

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Table 3 p-Values for the ANOVA analysis of the moss mercury data, grouped according to species, sampling location and sampling occasion. The significance level was set to 0.01 and p-values for which the analysis indicates a statistically significant difference between the samples are indicated with bold figures. 2008

2009

Pine-covered

Open

Pine-covered

Open

S. centrale

S. centrale

S. subsecundum

S. centrale

S. centrale

S. subsecundum

2008

Pine-covered Open

S. centrale S. centrale S. subsecundum

– 0.000 0.000

0.000 – 0.000

0.000 0.000 –

0.573 0.000 0.009

0.000 1.000 0.006

0.002 0.003 1.000

2009

Pine-covered Open

S. centrale S. centrale S. subsecundum

0.573 0.000 0.002

0.000 1.000 0.003

0.009 0.006 1.000

– 0.000 0.099

0.000 – 0.009

0.099 0.009 –

to F1/F2, and the mercury accumulation per gram of peat accumulated is similar between sites F1–F3 (Fig. 5), and therefore we suggest peat decomposition is the main factor responsible for the differences in mercury concentration between sites F1–F4. The differences in peat mercury concentrations between the open Sphagnum area (S1-site) and the pine-covered fen (F1–F4) can partly be found in the spatial differences in moss mercury concentrations (because only a minor part of the peat in both vegetation types consists of vascular plants we focus our discussion on the peat-forming Sphagnum mosses). However, these initial differences are also enhanced further due to a higher decomposition rate in the pine-covered fen (as indicated by higher bulk density and lower C:N-ratios). If we compare the fresh moss samples to the topmost sample of the peat profiles (i.e., a mixture of plant material, litter and peat) we find that in the open Sphagnum area there is an increase in mercury concentration of 40% from plant to peat whereas the increase is 55% on average for the four sites from the pine-covered fen (even if there are large variations between F-sites). This leaves us with the question of why mercury concentrations and net accumulation (Fig. 5) are lower in the open Sphagnum area as compared to the pine-covered fen. For volatile substances, like mercury, the net deposition is determined by both bulk deposition (wet and dry) and

Cum. peat mass (kg m-2)

0

20

F1b F2

40

F1

F3 S1

60

0

1

S1b 2

3

4

Cum. Hg inventory (mg m-2) Fig. 5. The cumulative inventory of mercury (mg m2) plotted against cumulative peat mass (g m2) for the S1, S1b, F1, F1b, F2 and F3 profiles.

any re-emission. In order to detect and try to quantify any difference in the bulk deposition between the F1- and S1-sites, the 210Pb inventories were measured. The approach of simply comparing the unsupported 210Pb inventories between the two sites circumvents any problems associated with downward transport of 210Pb that may complicate 210Pb-dating of peat records (Biester et al., 2007). The 210Pb inventory at site F1 is 33% higher in the F1-site as compared to the S1-site just 2 m away. Considering the presence of ericaceous shrubs, but no overhanging pine, at site F1, this is in good agreement with a number of studies reporting that an increased interception area, i.e., a canopy, will increase mercury deposition (Lindberg et al., 1998; Zhang et al., 2009). It has also been reported that a higher plant growth rate can increase the interception of metals (Kempter et al., 2010), and because the vertical peat accumulation for site S1 and F1 is similar, but the decomposition slightly higher in the latter (as indicated by the bulk density), the F1-site likely has a slightly higher plant growth rate, which can contribute to the difference in deposition between the two sites. However, mercury concentrations between the two sites differ by 42–61% for living mosses, 63% for the topmost peat and 77–96% for mercury inventories, which are all greater than the 33% difference indicated by the unsupported 210Pb inventories. This discrepancy between bulk deposition, as indicated by the 210Pb inventories, and the mercury sequestration in plants and ultimately peat can either be due to the deposition of 210Pb not being comparable to mercury deposition, or that there is a higher re-emission of mercury in the open Sphagnum area. We have no data to suggest which of these possibilities might be more realistic; however, the re-emission hypothesis would be in agreement with other studies where photo-reduction of Hg2+ to Hg0 has been studied in relation to different physical properties. The volatilization of Hg0 has been shown to increase with light intensity and temperature (Amyot et al., 1994; Gustin et al., 2002; Gustin et al., 2006), which would both be influenced by the presence or absence of ericaceous shrubs due to shading effects. There is also a clear difference in distance to the water table between the S1 and F1-sites, where S1 is considerably wetter compared to F1. Higher soil moisture has also been shown to increase re-emission of mercury (Gustin et al., 2006), and re-emissions from lake surfaces, which

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has some similar characteristics to saturated Sphagnum peat, can be as high as 33–59% of the deposited mercury (Poulain et al., 2006). A higher net mercury accumulation in drier parts as compared to wetter, has previously also been observed in other mires, both between hummocks and hollows (Norton et al., 1997; Klaminder et al., 2008) and within hummocks of a raised bog (Bindler et al., 2004). In addition to the spatial differences between vegetation types, there are some species specific differences in the mercury sequestration between different Sphagnum mosses. For the open area where S. subsecundum and S. centrale grow under the same conditions (i.e., no difference in wetness, microtopography etc.), the former has 34% higher mercury concentrations than the latter. Why this is so we can only speculate, but it has been shown that different moss species exposed to similar conditions can have different concentrations of nutrients (e.g., nitrogen, phosphorus, calcium; (Malmer, 1988; Jauhiainen et al., 1998)) and metals (Berg and Steinnes, 1997; Fernandez et al., 2002; Kempter and Frenzel, 2008). This indicates that species specific physiological traits or some type of regulatory mechanisms can affect the incorporation of these elements in Sphagnum mosses (Malmer, 1988; Jauhiainen et al., 1998), and it is possible that similar species specific mechanisms or traits also affect mercury. However, regardless of the cause, these differences in initial mercury concentrations imply that, if preserved during peat formation, a change in species composition, even between different Sphagnum species, has the potential to affect both mercury concentration and longterm accumulation in the peat record. 6. CONCLUSION Our data show that differences in vegetation type can affect the net deposition and sequestration of mercury in mosses, and that there is a difference in mercury sequestration between different moss species growing in the same vegetation type. We also show that, at least spatially, the initial differences in mercury concentration are incorporated into the peat, and that differences in peat mercury concentration between sites within a single vegetation type can be related to differences in terms of decomposition. Mires are neither static environments nor are they similar from one location to another; with time both physical properties (e.g., surface wetness) as well as plant composition can change (cf. Svensson, 1988; Gunnarsson et al., 2002). There are also large differences in appearance, and possibly also ecosystem function, depending on in what region of the world the mire is situated. Our data suggest that changes, or differences, in vegetation type, peat decomposition or plant species composition have the potential to modify the conditions for mercury sequestration. This stresses the need to consider mire development and differences in mire structure when interpreting peat mercury records or when comparing different mires. ACKNOWLEDGEMENTS Financial support was provided from the Faculty of Science and Technology at Umea˚ University and The Swedish Research

Council. We also thank the students taking the course “Field methods in Geoecology” at Umea˚ University in autumn 2009 for help with field work, and Antonio Martinez Cortizas for comments on an earlier draft of the manuscript.

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