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Article
The importance of open marine waters to the enrichment of total mercury and monomethylmercury in lichens in the Canadian High Arctic Kyra St. Pierre, Vincent L. St.Louis, Jane Kirk, Igor Lehnherr, Sunny Wang, and Catherine La Farge Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b00347 • Publication Date (Web): 16 Apr 2015 Downloaded from http://pubs.acs.org on April 19, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Subscriber access provided by NEW YORK UNIV
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 31
Environmental Science & Technology
ACS Paragon Plus Environment
Environmental Science & Technology
1
The importance of open marine waters to the enrichment of total
2
mercury and monomethylmercury in lichens in the Canadian High
3
Arctic
4 5 6 7
Page 2 of 31
St. Pierre, K.A.1, *; St.Louis, V.L.1; Kirk, J.L.2; Lehnherr, I.3; Wang, S.1; La Farge, C.1
8 9 10
1
11 12
Canada 2
13 14
Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9,
Aquatic Ecosystem Protection Research Division, Environment Canada, Burlington, Ontario L7R 4A6, Canada
3
15
Department of Geography, University of Toronto Mississauga, Mississauga, Ontario L5L 1C6, Canada
16
* Corresponding author: Phone: 1-780-492-0900; fax: 1-780-492-9234; e-mail:
17
[email protected]
18 19 20 21 22 23 24
ACS Paragon Plus Environment
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Page 3 of 31
25
Environmental Science & Technology
ABSTRACT
26
Caribou, which rely on lichens as forage, are the most important dietary source of
27
neurotoxic monomethylmercury (MMHg) to many of Canada’s Arctic Aboriginal people.
28
However, little is understood about the sources of MMHg to lichens in the High Arctic.
29
We quantified MMHg, total mercury (THg) and other chemical parameters (e.g., marine
30
and crustal elements, δ13C, δ15N, organic carbon, calcium carbonate) in lichen and soil
31
samples collected along transects extending from the coast on Bathurst and Devon
32
islands, Nunavut, to determine factors driving lichen MMHg and THg concentrations in
33
the High Arctic. Lichen MMHg and THg concentrations ranged from 1.41 to 17.1 ng g-1,
34
and from 36.0 to 361 ng g-1, respectively. Both were highly enriched over concentrations
35
in underlying soils, indicating a predominately atmospheric source of Hg in lichens.
36
However, MMHg and THg enrichment at coastal sites on Bathurst Island was far greater
37
than on Devon Island. We suggest that this variability can be explained by the proximity
38
of the Bathurst transect to several polynyas, which promote enhanced Hg deposition to
39
adjacent landscapes through various biogeochemical processes. This study is the first to
40
clearly show a strong marine influence on MMHg inputs to coastal terrestrial food webs
41
with implications for MMHg accumulation in caribou and the health of the people who
42
depend on them as part of a traditional diet.
43 44 45 46 47
ACS Paragon Plus Environment
2
Environmental Science & Technology
48
Page 4 of 31
INTRODUCTION
49
In the Canadian High Arctic, Aboriginal peoples rely heavily on high trophic level
50
organisms such as marine mammals, Arctic char (Salvelinus alpinus) and caribou
51
(Rangifer tarandus) as part of their traditional country diet 1. While hunting and eating
52
these foods provide many social and nutritional benefits 2, they are also sources of certain
53
pollutants that undergo long-range transport to the Arctic, and then bioaccumulate in
54
organisms and biomagnify through food chains 3. One of these pollutants is the
55
neurotoxin monomethylmercury (MMHg). Though lower in MMHg concentrations than
56
marine mammals, caribou represent the most important dietary source of MMHg to most
57
Canadian Arctic Aboriginal people with the exception of the Baffin Inuit, based on
58
estimates of the frequency with which caribou is consumed 4. Lichens are the most
59
important forage for Arctic caribou in winter, when they can make up 77% of the caribou
60
diet 5, 6.
61
Lichens are unique terrestrial organisms that form a symbiotic relationship
62
between fungi, algae and/or cyanobacteria 7. Lichens grow on diverse substrates and are
63
often the dominant autotroph in polar ecosystems. They are typically slow growing and
64
lack cuticle or stomata, which permit direct nutrient and pollutant ad- or absorption to the
65
lichen thalli 8, 9. Unfortunately, lichens have the highest MMHg and total Hg (THg; all
66
forms of Hg in a sample) concentrations of all caribou forage types 5. Potential sources of
67
inorganic Hg to lichens include wet 9 deposition, dry deposition of aerosols and larger
68
particulate matter 10, and direct gaseous elemental Hg (Hg0 ) uptake 11. Though never
69
investigated, possible sources of MMHg to lichens include wet deposition and dry
70
deposition of aerosols and larger particulate matter, but also methylation of inorganic Hg
ACS Paragon Plus Environment
3
Page 5 of 31
Environmental Science & Technology
71
to MMHg on or within lichen, as well as adsorption of MMHg from soils. Another
72
potential source of MMHg to lichens in coastal terrestrial environments is the
73
atmospheric deposition of MMHg originating from the photodemethylation of DMHg 12.
74
DMHg is often the dominant form of organic Hg in seawater 13, but is extremely volatile
75
and readily evades to the atmosphere where it can be rapidly photodegraded to MMHg
76
then deposited to nearby landscapes 14, 15. In Alaska, Norway and Hudson Bay 16-18,
77
higher concentrations of total Hg (THg) in coastal lichens were attributed to higher
78
gaseous oxidized Hg (GOM) and particulate bound Hg (PBM) deposition during
79
springtime atmospheric Hg depletion events (AMDEs) 18. However, trends in MMHg
80
were not examined in these studies.
81
To assess the role of marine waters as the source of MMHg to terrestrial
82
ecosystems, we collected lichens along transects starting at the coast and moving inland
83
on Bathurst and Devon islands in the Canadian High Arctic, Nunavut. The eastern coast
84
of Bathurst Island is located in proximity to several polynyas (Figure 1), unlike the
85
western coast of Devon Island. It was hypothesized that concentrations of MMHg and
86
THg would be highest in lichens on Bathurst Island near the coast due to the presence of
87
polynyas and the unique biogeochemical processes associated with these perennially
88
open water areas. Lichens and their underlying soils were analyzed for concentrations of
89
lithogenic metals and elements associated with marine sources (Na, K, Ca, Sr) to
90
determine the role of oceans, soils and the atmosphere in lichen Hg burdens. Stable
91
carbon (δ13C) and nitrogen (δ15N) isotope ratios in lichens were also quantified to assess
92
whether lichen physiology influenced Hg accumulation. This study is the first to show the
ACS Paragon Plus Environment
4
Environmental Science & Technology
93
transfer of marine-derived MMHg to coastal terrestrial Arctic food webs, with
94
implications for biomagnification into higher trophic levels.
Page 6 of 31
95 96
MATERIALS AND METHODS
97
Sample Collection and Preparation. Lichen and soil samples were collected in July
98
2007 along transects extending from the ocean-inland on Bathurst (5 sites) and Devon (4
99
sites) islands, Nunavut, Canada (Figure 1, Table S1). Both islands are sparsely vegetated
100
over the majority of their landmasses 19 and home to populations of endangered Peary
101
caribou (R. tarandus pearyi). Soils along both transects were turbic cryosols with
102
sporadic organic zones overlying continuous permafrost 20. The location of our transects
103
on these two islands were selected as a natural control-impact experiment to determine
104
whether the influence of marine waters had an impact on terrestrial MMHg accumulation.
105
The start of the transect on the eastern coast Bathurst Island was located in proximity to
106
several open-water polynyas, whereas there were no open water regions near our transect
107
on the western coast of Devon Island (Figure 1). Polynyas provide perennial access to
108
open water areas and are one of the few areas for constant ocean-atmosphere exchange in
109
a region otherwise locked in by sea ice for the majority of the year.
110
Three bulk lichen samples were collected at each site from soil surfaces. The top
111
2-5 cm of soil underlying lichens were then sampled using a stainless steel soil corer. At
112
one site where lichens on soil were not abundant, lichens were also collected from the
113
surface of a rock. All samples were collected using “clean hands-dirty hands” sampling
114
protocol for trace metals (EPA Method 1669) and stored in sterile polyethylene Whirl-
ACS Paragon Plus Environment
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Environmental Science & Technology
115
Paks®. Samples were frozen within hours of collection and subsequently stored at -20°C
116
until processing and analysis.
117
Because dust particles can be important components of overall contaminant loads
118
10, 21, 22
119
samples were not washed prior to analyses. Lichens were identified and separated by
120
species. In total, 9 species representing 3 families were identified, including one unknown
121
species (Table S2). Thamnolia vermicularis was found at all sites on both islands, while
122
Flavocetraria cucullata and Vulpiceda tilesii were found at all sites on Bathurst Island.
123
and are unlikely to be selectively removed by caribou during foraging, lichen
Lichen and soil samples were freeze-dried and homogenized with either an acid-
124
washed glass mortar and pestle (lichens) or a stainless steel coffee grinder (soils).
125
Samples were then sub-sampled for the analyses described below. However, due to
126
insufficient mass, not all analyses could be performed on all lichen samples. As a result,
127
analyses were prioritized as follows: Hg (MMHg then THg), δ13C and δ15N, and
128
lithogenic and marine elements. Soils were also analyzed for % organic carbon (OC) and
129
% calcium carbonate (CaCO3) content.
130 131
Mercury Analyses. MMHg was first extracted from lichen and soils using distillation.
132
MMHg concentrations were then determined using isotope-dilution gas chromatography
133
inductively-coupled plasma mass spectrometry 23 (ID-GC-ICP/MS; Tekran 2700
134
Methylmercury Analyzer coupled with a Perkin Elmer Elan DRC-e ICP-Mass
135
Spectrometer) at the accredited and internationally intercalibrated University of Alberta
136
Biogeochemical Analytical Service Laboratory (BASL). MM201Hg was added to samples
137
as a species-specific internal standard prior to the distillation to correct for procedural
ACS Paragon Plus Environment
6
Environmental Science & Technology
Page 8 of 31
138
recoveries. Standard reference material (SRM) IAEA-405 (estuarine sediment,
139
International Atomic Energy Agency) was used to assess method accuracy.
140
All samples were analyzed for THg concentrations using thermal decomposition,
141
pre-concentration and atomic absorbance spectrophotometry (Milestone DMA-80 direct
142
Hg analyzer) at the Canada Centre for Inland Waters (CCIW; Burlington, ON, Canada).
143
SRMs TORT-2 (lobster hepatopancreas, National Research Council (NRC)), MESS-3
144
(marine sediment, NRC), SRM-2976 (mussel, National Institute of Standards and
145
Technology (NIST)) and SRM1556b (oyster, NIST) were used as quality controls.
146
Additional quality control and protocol details are described in the Supplementary
147
Information.
148 149
δ 13C and δ 15N: Both lichens and soils were analyzed for δ13C and δ15N at the BASL
150
using a EuroVector EuroEA3028-HT elemental analyzer coupled to a GV Instruments
151
IsoPrime continuous-flow isotope ratio mass spectrometer. 1.2 mg and 6-7 mg of lichen
152
sample were used to quantify δ13C and δ15N, respectively. 2.5 mg or 25 mg of soils were
153
analyzed depending on whether they were organic or mineral. δ13C and δ15N ratios were
154
calculated according to equation 1, where R is the ratio of 13C/12C or 15N/14N, and X is the
155
stable isotope signature of C or N. Rsample is measured relative to Rstandard of Pee Dee
156
Belemnite for C and ambient air for N:
157
X (‰) =
!!"#$%& !!"#$%#&%
− 1 × 1000
(1)
158
SRM NIST 8415 (whole egg powder) was used for quality assurance and control, with
159
standard deviations of 0.1 and 0.2% for δ13C and δ15N, respectively.
160
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Lithogenic and marine elemental analyses. Following HF-HNO3 digestion (see
162
Supplementary Information), lichen and soil samples were analyzed for concentrations of
163
46 elements by ICP/MS at the Canadian Centre for Isotopic Microanalysis (University of
164
Alberta), and for concentrations of Ca, Mg, Fe, Na, and Al by inductively coupled plasma
165
optical emission spectroscopy (ICP/OES) at the BASL.
166 167
Soil %OC and %CaCO3 content: 100 mg of lichen or 200 mg of soil were heated in a
168
muffle furnace at 550°C for 4 h to determine %OC content, then subsequently at 950°C
169
for 2 h to determine %CaCO3 24.
170 171
Data Analysis. All statistical analyses were completed in R 25. Principal component
172
analyses (PCA) were performed on soil and lichen lithogenic and marine element
173
concentrations to elucidate possible factors driving soil composition at a given sub-site
174
(e.g., marine aerosols versus local geology). Soil PC scores were then used in a
175
subsequent multiple regression analysis as measures of soil elemental composition.
176
Separate multiple linear regression analyses were completed to assess the
177
importance of each factor (PC scores, δ13C, δ15N, %OC and %CaCO3) in determining
178
soil MMHg and THg concentrations and the percentage of THg as MMHg (%MMHg).
179
Models were compared and the most parsimonious model was selected using Akaike
180
Information Criteria (AIC). In cases where assumptions of linearity, normality,
181
homoscedasticity and/or auto-correlation were violated, regression coefficients were
182
obtained by bootstrap analysis.
183
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RESULTS AND DISCUSSION
185
Soil composition. Like other Arctic locales 26, 27, the soils on both Bathurst and Devon
186
islands were generally low in %OC content (median: 6.20%; range: 0.84% to 59.8%).
187
%CaCO3 ranged between 0.45% and 36.4% (median: 12.1%). %OC and %CaCO3 were
188
significantly higher in soils collected on Devon Island (medians 13.8% and 23.3%) than
189
in those collected on Bathurst Island (medians 4.70% and 8.71%) (%OC: t =-3.59, p
Article
The importance of open marine waters to the enrichment of total mercury and monomethylmercury in lichens in the Canadian High Arctic Kyra St. Pierre, Vincent L. St.Louis, Jane Kirk, Igor Lehnherr, Sunny Wang, and Catherine La Farge Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b00347 • Publication Date (Web): 16 Apr 2015 Downloaded from http://pubs.acs.org on April 19, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Subscriber access provided by NEW YORK UNIV
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 31
Environmental Science & Technology
ACS Paragon Plus Environment
Environmental Science & Technology
1
The importance of open marine waters to the enrichment of total
2
mercury and monomethylmercury in lichens in the Canadian High
3
Arctic
4 5 6 7
Page 2 of 31
St. Pierre, K.A.1, *; St.Louis, V.L.1; Kirk, J.L.2; Lehnherr, I.3; Wang, S.1; La Farge, C.1
8 9 10
1
11 12
Canada 2
13 14
Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9,
Aquatic Ecosystem Protection Research Division, Environment Canada, Burlington, Ontario L7R 4A6, Canada
3
15
Department of Geography, University of Toronto Mississauga, Mississauga, Ontario L5L 1C6, Canada
16
* Corresponding author: Phone: 1-780-492-0900; fax: 1-780-492-9234; e-mail:
17
[email protected]
18 19 20 21 22 23 24
ACS Paragon Plus Environment
1
Page 3 of 31
25
Environmental Science & Technology
ABSTRACT
26
Caribou, which rely on lichens as forage, are the most important dietary source of
27
neurotoxic monomethylmercury (MMHg) to many of Canada’s Arctic Aboriginal people.
28
However, little is understood about the sources of MMHg to lichens in the High Arctic.
29
We quantified MMHg, total mercury (THg) and other chemical parameters (e.g., marine
30
and crustal elements, δ13C, δ15N, organic carbon, calcium carbonate) in lichen and soil
31
samples collected along transects extending from the coast on Bathurst and Devon
32
islands, Nunavut, to determine factors driving lichen MMHg and THg concentrations in
33
the High Arctic. Lichen MMHg and THg concentrations ranged from 1.41 to 17.1 ng g-1,
34
and from 36.0 to 361 ng g-1, respectively. Both were highly enriched over concentrations
35
in underlying soils, indicating a predominately atmospheric source of Hg in lichens.
36
However, MMHg and THg enrichment at coastal sites on Bathurst Island was far greater
37
than on Devon Island. We suggest that this variability can be explained by the proximity
38
of the Bathurst transect to several polynyas, which promote enhanced Hg deposition to
39
adjacent landscapes through various biogeochemical processes. This study is the first to
40
clearly show a strong marine influence on MMHg inputs to coastal terrestrial food webs
41
with implications for MMHg accumulation in caribou and the health of the people who
42
depend on them as part of a traditional diet.
43 44 45 46 47
ACS Paragon Plus Environment
2
Environmental Science & Technology
48
Page 4 of 31
INTRODUCTION
49
In the Canadian High Arctic, Aboriginal peoples rely heavily on high trophic level
50
organisms such as marine mammals, Arctic char (Salvelinus alpinus) and caribou
51
(Rangifer tarandus) as part of their traditional country diet 1. While hunting and eating
52
these foods provide many social and nutritional benefits 2, they are also sources of certain
53
pollutants that undergo long-range transport to the Arctic, and then bioaccumulate in
54
organisms and biomagnify through food chains 3. One of these pollutants is the
55
neurotoxin monomethylmercury (MMHg). Though lower in MMHg concentrations than
56
marine mammals, caribou represent the most important dietary source of MMHg to most
57
Canadian Arctic Aboriginal people with the exception of the Baffin Inuit, based on
58
estimates of the frequency with which caribou is consumed 4. Lichens are the most
59
important forage for Arctic caribou in winter, when they can make up 77% of the caribou
60
diet 5, 6.
61
Lichens are unique terrestrial organisms that form a symbiotic relationship
62
between fungi, algae and/or cyanobacteria 7. Lichens grow on diverse substrates and are
63
often the dominant autotroph in polar ecosystems. They are typically slow growing and
64
lack cuticle or stomata, which permit direct nutrient and pollutant ad- or absorption to the
65
lichen thalli 8, 9. Unfortunately, lichens have the highest MMHg and total Hg (THg; all
66
forms of Hg in a sample) concentrations of all caribou forage types 5. Potential sources of
67
inorganic Hg to lichens include wet 9 deposition, dry deposition of aerosols and larger
68
particulate matter 10, and direct gaseous elemental Hg (Hg0 ) uptake 11. Though never
69
investigated, possible sources of MMHg to lichens include wet deposition and dry
70
deposition of aerosols and larger particulate matter, but also methylation of inorganic Hg
ACS Paragon Plus Environment
3
Page 5 of 31
Environmental Science & Technology
71
to MMHg on or within lichen, as well as adsorption of MMHg from soils. Another
72
potential source of MMHg to lichens in coastal terrestrial environments is the
73
atmospheric deposition of MMHg originating from the photodemethylation of DMHg 12.
74
DMHg is often the dominant form of organic Hg in seawater 13, but is extremely volatile
75
and readily evades to the atmosphere where it can be rapidly photodegraded to MMHg
76
then deposited to nearby landscapes 14, 15. In Alaska, Norway and Hudson Bay 16-18,
77
higher concentrations of total Hg (THg) in coastal lichens were attributed to higher
78
gaseous oxidized Hg (GOM) and particulate bound Hg (PBM) deposition during
79
springtime atmospheric Hg depletion events (AMDEs) 18. However, trends in MMHg
80
were not examined in these studies.
81
To assess the role of marine waters as the source of MMHg to terrestrial
82
ecosystems, we collected lichens along transects starting at the coast and moving inland
83
on Bathurst and Devon islands in the Canadian High Arctic, Nunavut. The eastern coast
84
of Bathurst Island is located in proximity to several polynyas (Figure 1), unlike the
85
western coast of Devon Island. It was hypothesized that concentrations of MMHg and
86
THg would be highest in lichens on Bathurst Island near the coast due to the presence of
87
polynyas and the unique biogeochemical processes associated with these perennially
88
open water areas. Lichens and their underlying soils were analyzed for concentrations of
89
lithogenic metals and elements associated with marine sources (Na, K, Ca, Sr) to
90
determine the role of oceans, soils and the atmosphere in lichen Hg burdens. Stable
91
carbon (δ13C) and nitrogen (δ15N) isotope ratios in lichens were also quantified to assess
92
whether lichen physiology influenced Hg accumulation. This study is the first to show the
ACS Paragon Plus Environment
4
Environmental Science & Technology
93
transfer of marine-derived MMHg to coastal terrestrial Arctic food webs, with
94
implications for biomagnification into higher trophic levels.
Page 6 of 31
95 96
MATERIALS AND METHODS
97
Sample Collection and Preparation. Lichen and soil samples were collected in July
98
2007 along transects extending from the ocean-inland on Bathurst (5 sites) and Devon (4
99
sites) islands, Nunavut, Canada (Figure 1, Table S1). Both islands are sparsely vegetated
100
over the majority of their landmasses 19 and home to populations of endangered Peary
101
caribou (R. tarandus pearyi). Soils along both transects were turbic cryosols with
102
sporadic organic zones overlying continuous permafrost 20. The location of our transects
103
on these two islands were selected as a natural control-impact experiment to determine
104
whether the influence of marine waters had an impact on terrestrial MMHg accumulation.
105
The start of the transect on the eastern coast Bathurst Island was located in proximity to
106
several open-water polynyas, whereas there were no open water regions near our transect
107
on the western coast of Devon Island (Figure 1). Polynyas provide perennial access to
108
open water areas and are one of the few areas for constant ocean-atmosphere exchange in
109
a region otherwise locked in by sea ice for the majority of the year.
110
Three bulk lichen samples were collected at each site from soil surfaces. The top
111
2-5 cm of soil underlying lichens were then sampled using a stainless steel soil corer. At
112
one site where lichens on soil were not abundant, lichens were also collected from the
113
surface of a rock. All samples were collected using “clean hands-dirty hands” sampling
114
protocol for trace metals (EPA Method 1669) and stored in sterile polyethylene Whirl-
ACS Paragon Plus Environment
5
Page 7 of 31
Environmental Science & Technology
115
Paks®. Samples were frozen within hours of collection and subsequently stored at -20°C
116
until processing and analysis.
117
Because dust particles can be important components of overall contaminant loads
118
10, 21, 22
119
samples were not washed prior to analyses. Lichens were identified and separated by
120
species. In total, 9 species representing 3 families were identified, including one unknown
121
species (Table S2). Thamnolia vermicularis was found at all sites on both islands, while
122
Flavocetraria cucullata and Vulpiceda tilesii were found at all sites on Bathurst Island.
123
and are unlikely to be selectively removed by caribou during foraging, lichen
Lichen and soil samples were freeze-dried and homogenized with either an acid-
124
washed glass mortar and pestle (lichens) or a stainless steel coffee grinder (soils).
125
Samples were then sub-sampled for the analyses described below. However, due to
126
insufficient mass, not all analyses could be performed on all lichen samples. As a result,
127
analyses were prioritized as follows: Hg (MMHg then THg), δ13C and δ15N, and
128
lithogenic and marine elements. Soils were also analyzed for % organic carbon (OC) and
129
% calcium carbonate (CaCO3) content.
130 131
Mercury Analyses. MMHg was first extracted from lichen and soils using distillation.
132
MMHg concentrations were then determined using isotope-dilution gas chromatography
133
inductively-coupled plasma mass spectrometry 23 (ID-GC-ICP/MS; Tekran 2700
134
Methylmercury Analyzer coupled with a Perkin Elmer Elan DRC-e ICP-Mass
135
Spectrometer) at the accredited and internationally intercalibrated University of Alberta
136
Biogeochemical Analytical Service Laboratory (BASL). MM201Hg was added to samples
137
as a species-specific internal standard prior to the distillation to correct for procedural
ACS Paragon Plus Environment
6
Environmental Science & Technology
Page 8 of 31
138
recoveries. Standard reference material (SRM) IAEA-405 (estuarine sediment,
139
International Atomic Energy Agency) was used to assess method accuracy.
140
All samples were analyzed for THg concentrations using thermal decomposition,
141
pre-concentration and atomic absorbance spectrophotometry (Milestone DMA-80 direct
142
Hg analyzer) at the Canada Centre for Inland Waters (CCIW; Burlington, ON, Canada).
143
SRMs TORT-2 (lobster hepatopancreas, National Research Council (NRC)), MESS-3
144
(marine sediment, NRC), SRM-2976 (mussel, National Institute of Standards and
145
Technology (NIST)) and SRM1556b (oyster, NIST) were used as quality controls.
146
Additional quality control and protocol details are described in the Supplementary
147
Information.
148 149
δ 13C and δ 15N: Both lichens and soils were analyzed for δ13C and δ15N at the BASL
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using a EuroVector EuroEA3028-HT elemental analyzer coupled to a GV Instruments
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IsoPrime continuous-flow isotope ratio mass spectrometer. 1.2 mg and 6-7 mg of lichen
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sample were used to quantify δ13C and δ15N, respectively. 2.5 mg or 25 mg of soils were
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analyzed depending on whether they were organic or mineral. δ13C and δ15N ratios were
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calculated according to equation 1, where R is the ratio of 13C/12C or 15N/14N, and X is the
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stable isotope signature of C or N. Rsample is measured relative to Rstandard of Pee Dee
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Belemnite for C and ambient air for N:
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X (‰) =
!!"#$%& !!"#$%#&%
− 1 × 1000
(1)
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SRM NIST 8415 (whole egg powder) was used for quality assurance and control, with
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standard deviations of 0.1 and 0.2% for δ13C and δ15N, respectively.
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Lithogenic and marine elemental analyses. Following HF-HNO3 digestion (see
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Supplementary Information), lichen and soil samples were analyzed for concentrations of
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46 elements by ICP/MS at the Canadian Centre for Isotopic Microanalysis (University of
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Alberta), and for concentrations of Ca, Mg, Fe, Na, and Al by inductively coupled plasma
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optical emission spectroscopy (ICP/OES) at the BASL.
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Soil %OC and %CaCO3 content: 100 mg of lichen or 200 mg of soil were heated in a
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muffle furnace at 550°C for 4 h to determine %OC content, then subsequently at 950°C
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for 2 h to determine %CaCO3 24.
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Data Analysis. All statistical analyses were completed in R 25. Principal component
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analyses (PCA) were performed on soil and lichen lithogenic and marine element
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concentrations to elucidate possible factors driving soil composition at a given sub-site
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(e.g., marine aerosols versus local geology). Soil PC scores were then used in a
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subsequent multiple regression analysis as measures of soil elemental composition.
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Separate multiple linear regression analyses were completed to assess the
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importance of each factor (PC scores, δ13C, δ15N, %OC and %CaCO3) in determining
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soil MMHg and THg concentrations and the percentage of THg as MMHg (%MMHg).
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Models were compared and the most parsimonious model was selected using Akaike
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Information Criteria (AIC). In cases where assumptions of linearity, normality,
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homoscedasticity and/or auto-correlation were violated, regression coefficients were
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obtained by bootstrap analysis.
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RESULTS AND DISCUSSION
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Soil composition. Like other Arctic locales 26, 27, the soils on both Bathurst and Devon
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islands were generally low in %OC content (median: 6.20%; range: 0.84% to 59.8%).
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%CaCO3 ranged between 0.45% and 36.4% (median: 12.1%). %OC and %CaCO3 were
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significantly higher in soils collected on Devon Island (medians 13.8% and 23.3%) than
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in those collected on Bathurst Island (medians 4.70% and 8.71%) (%OC: t =-3.59, p
