Archives of Environmental Contamination and Toxicology

, Volume 53, Issue 3, pp 473–482

Trace Element Concentrations in Livers of Polar Bears from Two Populations in Northern and Western Alaska

Authors

    • Wadsworth Center, New York State Department of Health, and Department of Environmental Health SciencesState University of New York at Albany
  • Tetsuro Agusa
    • Center for Marine Environmental Studies (CMES)Ehime University
  • Thomas J. Evans
    • United States Fish and Wildlife Service
  • Shinsuke Tanabe
    • Center for Marine Environmental Studies (CMES)Ehime University
Article

DOI: 10.1007/s00244-007-0018-x

Cite this article as:
Kannan, K., Agusa, T., Evans, T.J. et al. Arch Environ Contam Toxicol (2007) 53: 473. doi:10.1007/s00244-007-0018-x

Abstract

Concentrations of 20 trace elements (V, Cr, Mn, Co, Cu, Zn, Rb, Sr, Mo, Ag, Cd, In, Sn, Sb, Cs, Ba, Hg, Tl, Pb, and Bi) were measured in livers of polar bears (Ursus maritimus) collected from Northern and Western Alaska from 1993 to 2002 to examine differences in the profiles of trace metals between the Beaufort Sea (Northern Alaska) and the Chukchi Sea (Western Alaska) subpopulations in Alaska. Among the trace elements analyzed, concentrations of Cu (50–290 µg/g, dry wt) in polar bear livers were in the higher range of values that have been reported for marine mammals. Concentrations of Hg in polar bears varied widely, from 3.5 to 99 µg/g dry wt, and the mean concentrations in polar bears were comparable to concentrations reported previously for several other species of marine mammals. Mean concentrations of Pb and Cd were 0.67 and 1.0 µg/g dry wt, respectively; these concentrations were lower than levels reported elsewhere for polar bears from Greenland and Canada. Age- and gender-related variations in the concentrations of trace elements in our polar bears were minimal. Concentrations of Hg decreased slowly in samples collected during 1993–2002, whereas Cd and Pb concentrations were found to be stable or slowly increasing, in the livers of Alaskan polar bears. Concentrations of Ag, Bi, Ba, Cu, and Sn were significantly higher in the Chukchi Sea subpopulation than in the Beaufort Sea subpopulation. Concentrations of Hg were significantly higher in the Beaufort Sea subpopulation than in the Chukchi Sea subpopulation. Differences in the profiles and concentrations of Hg, Ag, Bi, Ba, Cu, and Sn suggest that the sources of exposure to these trace elements between Western and Northern Alaskan polar bears are different, in agreement with findings reported earlier for several organic contaminants.

Keywords

Polar bearTrace elementsMetalsArcticMercuryAlaskaSeals

Considerable attention has been paid to assessing the state of the Arctic environment, especially with respect to the presence and biological effects of anthropogenic contaminants (AMAP 2005). The Alaskan Arctic marine region, like the other Arctic regions, is affected by contaminants released by human and industrial activities in other regions of the world. Among several contaminants of concern, trace elements, particularly heavy metals such as Hg, Cd, and Pb, have been studied in the Arctic ecosystem. In contrast to the number of studies describing heavy metals in the Canadian and Norwegian Arctic, few studies have examined the status of heavy metals in biota from the Alaskan Arctic (AMAP 2005; Becker et al. 1995; Dietz et al. 2000a, 2000b; Gerlach et al. 2006; Woshner et al. 2001). Little is known about trace metal contamination in polar bears from Alaska (Woshner et al. 2001). The small number of studies available from other Arctic regions suggests the existence of geographical differences in heavy metal concentrations in polar bear tissues (Dietz et al. 2000a, 2000b; Dietz et al. 2006; Muir et al. 1999). There is a trend of temporal increase in Hg levels in marine birds and mammals in the Canadian Arctic and West Greenland (Braune et al. 2006; Wagemann et al. 1996). A few studies also suggest stable or decreasing levels of Hg in other regions, perhaps indicating the importance of local or regional processes in trace metal accumulation and distribution (AMAP 2005).

The Marine Mammal Protection Act of the United States, enacted in 1972, banned hunting of polar bears unless done by Alaskan native hunters for subsistence and creation of handicrafts. For the present study, polar bear liver tissues obtained from subsistence hunters in Alaska were available for analysis. We have determined trace metal concentrations in polar bears from two subpopulations in Alaska, namely the Southern Beaufort Sea subpopulation (Beaufort Sea subpopulation; occurs from approximately Icy Cape, west of Point Barrow to Pearce Point) and the Chukchi/Bering Sea subpopulation (Chukchi Sea subpopulation; occurs in the Northern Bering Sea and southern Chukchi Sea adjacent to Russia and Western Arctic Alaska) (Amstrup et al. 2004; Evans et al. 2003). In an earlier study, we reported the occurrence of chlorinated, brominated, and fluorinated contaminants in livers of the same individuals analyzed in this study (Kannan et al. 2005). The objectives of the present study were to determine concentrations of 20 trace elements (V, Cr, Mn, Co, Cu, Zn, Rb, Sr, Mo, Ag, Cd, In, Sn, Sb, Cs, Ba, Hg, Tl, Pb, and Bi) in livers of polar bears from Alaska and to examine differences in trace element concentrations between the Beaufort Sea and the Chukchi Sea subpopulations. Age- and gender-related differences and temporal variations in the concentrations of trace elements, including Hg, Cd, and Pb, were examined.

Materials and Methods

Samples

Livers were collected from 34 polar bears in Northern and Western Alaska by Alaska native subsistence hunters and the US Fish and Wildlife Service. Samples collected from eight villages in Alaska between 1993 and 2002 included both males (n = 25) and females (n = 9) and all age classes (adults = 26; subadults = 5; cubs = 3) (Table 1). The Beaufort Sea subpopulation consisted of samples from Prudhoe Bay, Nuiqsut, Barrow, and Point Lay, and the Chukchi Sea subpopulation consisted of samples from Shismaref, Little Diomede, Gambell, and Savoonga (Fig. 1). Point Lay, located in northwestern Alaska, is an area of overlap between the two Alaskan populations (Amstrup et al. 2005). In this study, we included the only Point Lay sample from an adult male, in the Beaufort Sea group, for the purpose of data analysis. A premolar tooth was collected from each animal for age determination (Calvert and Ramsay 1998). In addition, harvest information such as age, gender, capture location, and date of collection was recorded. Liver samples were stored at −20°C until chemical analysis.
Table 1

Concentrations (µg/g dry wt) of trace elements in livers of polar bears from the Beaufort Sea and the Chukchi Sea (Alaska), 1993–2002

 

Beaufort Sea (n = 8; 7M, 1F)

Chukchi Sea (n = 26; 18M, 8F)

Overall (n = 34; 25M, 9F)

B-C

Mean

Range

Median

Mean

Range

Median

Mean

Range

Median

V

0.239

0.11–0.44

0.205

0.293

0.11–0.54

0.29

0.280

0.11–0.54

0.27

NS

Cr

0.304

0.22–0.46

0.295

0.365

0.17–0.96

0.29

0.351

0.17–0.96

0.29

NS

Mn

12.6

8.48–15.1

13.1

12.0

6.89–15.1

12.15

12.1

6.89–15.1

12.5

NS

Co

0.021

0.011–0.043

0.017

0.022

0.004–0.036

0.022

0.022

0.004–0.043

0.02

NS

Cu

94.7

50.4–159

88.1

141

55.9–285

126

130

50.4–285

116

p < 0.05

Zn

180

126–269

170

184

117–363

162

183

117–363

162

NS

Rb

12.9

9.34–16.1

13.7

11.4

8.73–14.9

11.3

11.8

8.73–16.1

11.4

NS

Sr

0.136

0.059–0.312

0.103

0.161

0.085–0.298

0.152

0.155

0.059–0.312

0.145

NS

Mo

1.36

1.02–1.83

1.33

1.46

0.787–2.34

1.44

1.44

0.787–2.34

1.44

NS

Ag

0.179

0.1–0.3

0.15

0.381

0.16–0.72

0.415

0.334

0.1–0.72

0.3

p < 0.05

Cd

1.07

0.365–1.65

1.06

0.976

0.308–1.73

1.015

0.998

0.308–1.73

1.03

NS

In

0.002

0.001–0.003

0.001

0.003

0.001–0.01

0.002

0.003

0.001–0.01

0.002

NS

Sn

0.032

0.015–0.071

0.023

0.083

0.022–0.24

0.074

0.071

0.015–0.24

0.051

p < 0.05

Sb

0.020

0.01–0.04

0.02

0.028

0.01–0.09

0.02

0.026

0.01–0.09

0.02

NS

Cs

0.073

0.05–0.1

0.07

0.074

0.03–0.14

0.07

0.074

0.03–0.14

0.07

NS

Ba

0.004

0.001–0.013

0.001

0.046

0.001–0.56

0.012

0.036

0.001–0.56

0.008

p < 0.05

Hg

33.1

14–99

24

10.1

3.5–25

8.5

15.5

3.5–99

10.5

p < 0.05

Tl

0.002

0.001–0.004

0.002

0.003

0.001–0.01

0.001

0.003

0.001–0.01

0.001

NS

Pb

0.288

0.061–1.13

0.181

0.782

0.058–5.17

0.221

0.666

0.058–5.17

0.217

NS

Bi

0.002

0.001–0.005

0.002

0.006

0.002–0.013

0.005

0.005

0.001–0.013

0.0045

p < 0.05

Note: B-C: Comparison between Beaufort Sea and Chukchi Sea samples; NS= not significant at 5% level

All Beaufort Sea samples are adults and Chukchi Sea samples include 3 cubs, 5 subadults, and 18 adults

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Fig. 1

Sampling locations of polar bears from Alaska

Trace Element Analysis

Trace elements were analyzed following the method described elsewhere (Agusa et al. 2005; Anan et al. 2002; Asante et al. 2007; Ichihashi et al. 2001) with slight modification. Approximately 0.2 g of sample (dry wt) was weighed and treated with ultrapure HNO3. A closed-vessel microwave system (Ethos D, Milestone S.r.l., Sorisole BG, Italy) was used for acid digestion. The digestion program was: 2 min at 250 W, 3 min at 0 W, 5 min at 250 W, 5 min at 400 W, 5 min at 500 W, 10 min at 400 W, and 5 min for ventilation. Concentrations of V, Cr, Mn, Co, Cu, Zn, Rb, Sr, Mo, Ag, Cd, In, Sn, Sb, Cs, Ba, Tl, Pb, and Bi were determined by an inductively coupled plasma–mass spectrometer (ICP-MS; Hewlett-Packard model HP-4500; Avondale, PA, USA) using yttrium (Y) as an internal standard. Concentration of Hg was determined by a cold vapor–atomic absorption spectrometer (CV-AAS) using a cold vapor generation system (Model HG-3000; Sanso, Tsukuba, Japan) coupled to an AAS (Shimadzu, Kyoto, Japan). The limit of detection for trace elements was 1 ng/g dry wt, except for Sb and Cs (10 ng/g dry wt) and Hg (50 ng/g dry wt). Accuracy of the analysis was examined by analyzing Certified Reference Materials: dogfish muscle (DORM2; National Research Council, Ottawa, ON, Canada) and bovine liver (SRM1577b; National Institute of Standards and Technology, Gaithersburg, MD, USA) along with the samples. Recoveries of all the elements were in the range of 89–104%. The moisture content of the liver samples ranged from 54% to 70% (mean: 62%). The results are expressed on a dry-weight basis.

Statistical analyses were performed using Statgraphics® 5 (Manugistics, Inc., Rockville, MD, USA). The nonparametric Mann–Whitney W (Wilcoxon) test was applied, to allow the comparison of concentrations between two groups. A comparison of multiple groups was performed using the Kruskal–Wallis analysis of variance (ANOVA) test. Spearman rank correlation was used to determine correlation among trace elements. Values below the limit of detection were assigned a value at the detection limit (0.01 or 0.001, depending on the analyte) for the analysis.

Results and Discussion

Residue Levels and Patterns

The concentrations of 20 trace elements found in the livers of the polar bears are shown in Table 1. The overall distribution of trace elements in the polar bear livers was in the following order: Zn > Cu > Mn > Rb > Hg > Mo > Cd > Ag > Cr > V > Pb > Sr > Cs > Sn > Co > Sb > Ba > Bi > In > Tl. This pattern was similar for the Beaufort Sea and the Chukchi Sea subpopulations, except for a relatively higher proportion of Hg than Rb and Mn in the Beaufort Sea subpopulation and a higher proportion of Ag than Cr, V, and Pb in the Chukchi Sea subpopulation (Fig. 2 and Table 1). Sn and Ba concentrations were also high in polar bears from the Chukchi Sea. Variations in the concentrations of trace metals between the two subpopulations are discussed in detail below. Among the 20 trace elements analyzed, the mean concentrations of Zn and Cu were 180 and 130 µg/g dry wt, respectively; these concentrations varied between threefold and fivefold among the 34 individuals analyzed. Whereas the concentrations of essential elements such as Zn, Cu, and Mn varied within 5-fold, concentrations of nonessential elements such as Hg, Pb, and Sn varied by more than 10-fold among individuals analyzed. The concentrations of essential elements are controlled by homeostasis, whereas the concentrations of nonessential elements in the livers of polar bears reflect local sources of contamination and exposures.
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Fig. 2

Relative distribution of trace elements in livers of polar bears from the Chukchi and Bering Seas. The upper panel represents trace elements with median concentrations >10 µg/g dry wt; the lower panel represents trace elements with median concentrations <10 µg/g dry wt. Others indicate Cs, Sn, Sb, Co, Bi, Tl, Ba, and In

Concentrations of Cu, Mn, Zn, Ag, Cd, Pb, and Hg have been reported in livers of polar bears collected from Barrow, Alaska, during 1995–1997 (Woshner et al. 2001). To enable comparisons with the data in the earlier report, we normalized their data to dry-weight basis, based on an average moisture content of 62% found in the liver of polar bears in our study. The mean concentrations of Cu, Mn, Zn, and Cd in samples from Barrow (n = 5 adult male) analyzed in our study were similar to the values reported earlier (Woshner et al. 2001), whereas Pb concentrations were twofold higher and Ag concentrations were twofold lower in our samples than in those reported earlier. The concentrations of Hg in our Barrow samples were 30% lower than the values reported earlier (Woshner et al. 2001). An adult male collected in 2000 in Barrow contained lower concentrations of Hg (17 µg/g dry wt) than those collected during 1993–1999 (31–38 µg/g dry wt).

The concentrations of Hg in the livers of polar bears from Alaska were lower than concentrations in polar bears from the Canadian Arctic (19–200 µg/g dry wt; Muir et al. 1999), particularly those from Western Canada (100–200 µg/g dry wt) collected in the 1980s. The concentrations of Cd in polar bears from Alaska were approximately threefold lower than in polar bears from Eastern Canadian Arctic (1.8–4.6 µg/g dry wt; Muir et al. 1999). The median concentrations of Cd and Hg in livers of polar bears from northwestern Greenland collected in the late 1980s were 3.8 and 33 µg/g dry wt, respectively (Dietz et al. 2000a; 2000b), each of which were threefold greater than the concentrations found in Alaskan polar bears. Thus, concentrations of Hg and Cd were lower in polar bears from Alaska than those in the Canadian Arctic and Greenland.

The concentrations of Zn, Cu, Hg, and Cd in polar bears were compared with those reported for various other marine mammals (Anan et al. 2001; Bustamante et al. 2003; Das et al. 2004; Law et al. 1992, 2003; Ikemoto et al. 2004; Kannan et al. 2006; Kunito et al. 2002, 2004; Mackey et al. 2003; Meador et al. 1999; Paludan et al. 1993; Ruelas & Paez-Osuna 2002a, 2000b; Szefer et al. 2002) (Fig. 3). General ranges for the concentrations of Zn, Cu, Cd, and Hg in marine mammals were 50–200, 5–50, 0.5–25, and 0.5–25 µg/g dry wt, respectively (Anan et al. 2002; Kunito et al. 2004; Law et al. 2003). The concentrations of Cu in polar bears were in the higher range of values reported for marine mammals (Fig. 3). Elevated concentrations of Cu in polar bears were comparable to those reported for sea otters from coastal California (Kannan et al. 2006). Similarly, earlier studies have reported elevated accumulation of Cu in sea turtles (Lam et al. 2004), which was attributed to induction of metallothionein. Concentrations of Zn fell in the midrange of values that have been reported for marine mammals, whereas Cd levels were relatively lower than the levels reported for marine mammals collected from coastal areas. Concentrations of Hg in polar bears varied widely, from 3.5 to 99 µg/g dry wt; although the mean concentrations in polar bears were comparable to concentrations reported for several other species of marine mammals, they were lower than concentrations in marine mammals from southern Brazil or coastal Japan or concentrations in bottlenose dolphin from the Gulf of Mexico (Fig. 3).
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Fig. 3

Hepatic concentrations of Zn, Cu, Hg, and Cd in Alaskan polar bears compared with concentrations in some other species of marine mammals collected after 1990 from various locations. CS = Chukchi Sea, BeS = Beaufort Sea, CA = California coast, GM = Gulf of Mexico, AU = Australia, FL = Florida, GC = Gulf of California, BS = Baltic Sea, GL = Greenland, BL = Black Sea, IS = Irish coast, SB = South Brazil, NC = New Caledonia, AO = Antarctic Ocean, LB = Lake Baikal, CP = Caspian Sea, JA = Japan coast. References are given in text

Very few studies have reported concentrations of V, Cr, Co, Rb, Sr, Mo, Ag, In, Sn, Sb, Cs, Ba, Tl, and Bi in Arctic biota (AMAP 2005). Our study establishes baseline concentrations of these rarely studied elements in liver tissues of polar bears.

Accumulation Features

Concentrations of trace elements in the livers of polar bears stratified based on the gender are shown in Figure 4. Concentrations of most of the trace elements did not show a significant difference between the genders, except for V, Mo, Sn, and Ba, for which significantly higher concentrations (p < 0.05) were found in livers of female polar bears. Pb concentrations were significantly higher in males than in females. Although the concentrations of Cu, Cs, and Hg were higher in females and Zn and Sb were higher in males, the differences were not statistically significant. Gender-related differences in the accumulation of trace elements in marine mammals vary depending on biological and geological factors, although such differences are generally minor (Anan et al. 2002; Becker et al. 1995; Law 1996). The higher concentrations of a few trace elements in the female bears here might be related to such biological factors as age, location, and nutritive condition.
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Fig. 4

Gender differences in median concentrations (µg/g dry wt) of trace metals in livers of polar bears from Alaska (asterisk indicates significant difference)

Concentrations of trace metals were compared in the livers of cubs (<3 years; n = 3), subadults (3–5 years; n = 5), and adults (>5 years; n = 26). Manganese concentrations in livers of adult and subadult bears were significantly higher (p < 0.05) than the mean concentrations in cubs (Fig. 5). Zn concentrations were also higher in adults than in cubs. This pattern is different from what was found for common dolphins, for which concentrations of Mn, Zn, and Cu were lower in the livers of adults than in juveniles (Zhou et al. 2001). Concentrations of Rb were significantly higher (p < 0.05) in adult bears than in cubs. Concentrations of Sn in cubs were significantly higher than those in adults but did not differ significantly from those in subadults. Concentrations of Hg and Pb did not vary significantly among the three age groups, although concentrations in adults were greater than those in subadults and cubs.
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Fig. 5

Median concentrations (µg/g dry wt) of trace metals in livers of polar bears from Alaska stratified by age-groups (asterisk indicates significant difference)

Age-dependent accumulation of V, Ag, and Hg has been reported earlier for marine mammals (Ikemoto et al. 2004). The age- and gender-related differences in the concentrations of certain trace elements in polar bears appear to deviate from patterns in other marine mammals; potential explanation could relate to numerous physical and biological factors, including specific feeding habits (long winter starvation/hibernation), location, geological conditions, health, reproductive status, body condition, metabolism, and pharmacokinetics in polar bears. For instance, unlike other marine mammals in which Hg is concentrated in the liver, polar bears concentrate Hg in the kidneys (Woshner et al. 2001). Hepatic Hg concentrations were similar to or higher than concentrations found in kidneys of polar bears (Dietz et al. 2000a, 2000b; Woshner et al. 2001). Cd concentrations in kidneys of polar bears were 10-fold greater than those in the livers (Dietz et al. 2000a, 2000b). Pb concentrations were significantly higher in the kidneys than in the livers of polar bears (Woshner et al. 2001). Elevated accumulation of Hg in kidneys is similar to the pattern of tissue distribution found in terrestrial mammals and contrary to what was generally seen for marine mammals. The induction of metallothionein can lead to differences in metal accumulation patterns between males and females as well as juveniles and adults. Significant induction of metallothionein by Hg in the kidney of terrestrial mammals relative to that in marine mammals has been reported (Das et al. 2000). Overall, there appear to be some differences in the tissue distribution of trace elements between polar bears and other marine mammals. Further studies should examine trace elements in other tissues such as the kidney, muscle, spleen, and brain.

Concentrations of several trace elements in the livers of polar bears, particularly essential elements, did not vary considerably during 1993–2002. However, concentrations of toxic metals such as Cd, Hg, and Pb showed some variations over time (Fig. 6). Cd concentrations in the livers of polar bears increased from 1995 to 2002 by 43%, whereas Hg concentrations decreased by 34% during this period. Nevertheless, when the data for 1993 were compared with those for 2002, concentrations of Cd did not change during that decade. Over the past two decades, no trends have been found for Cd levels in the kidney and liver of beluga whales and narwhal in the Canadian Arctic (AMAP 2005). Similarly, mussels in Alaska, Greenland, Iceland, and Norway have not exhibited a consistent temporal trend in Cd concentrations (AMAP 2005). Concentrations of Pb in our samples remained stable from 1994 to 2000, but the samples (n = 6) collected in 2002 showed unusually high concentrations of Pb in polar bear livers. All six of the samples were from adult males from Gambell and Savoonga. Very few studies have reported trends for any trace element levels in polar bears, and this study establishes baseline results for future comparisons.
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Fig. 6

Temporal differences in the mean concentrations (µg/g dry wt) of Cd, Hg, and Pb in livers of polar bears from Alaska (n = 1, 2, 3, 3, 4, 10, 5, and 6 for the years 1993, 1994, 1995, 1997, 1998, 1999, 2000, and 2002, respectively)

Comparison Between the Beaufort and Chukchi Subpopulations

Concentrations of trace elements in livers of polar bears grouped according to the sampling villages in Alaska are shown in Table 2. Whereas the differences in the concentrations of most of the trace elements were not significant among the sampling villages, concentrations of some trace elements did vary significantly. For example, concentrations of Sr and Hg were elevated in the polar bear from Prudhoe Bay. Cu concentrations were high in polar bears collected from Little Diomede, Gambell, and Savoonga. Sn concentrations were relatively low in samples collected from the northern locations such as Barrow and Prudhoe Bay. Pb concentrations were high in Gambell and Savoonga. Because the number of samples analyzed for each sampling village is small and because of the mobility of polar bears, we sought to examine the spatial differences in the concentrations of trace elements by combining samples from Northern Alaska with those from Western Alaska.
Table 2.

Concentrations (µg/g dry wt; mean / median; for samples with n = 1 or 2, only mean is presented) of trace elements in polar bear livers stratified based on sampling villages in Alaska

 

Beaufort Sea

Chukchi Sea

Barrrow (n = 5)

Nuiquist (n = 1)

Prudhoe Bay (n = 1)

Pt.Lay (n = 1)

Gambell (n = 11)

Savoonga (n = 6)

L.Diomede (n = 7)

Shishmaref (n = 2)

V

0.21 / 0.17

0.23

0.27

0.36

0.299 /0.32

0.285 / 0.22

0.304 / 0.3

0.245

Cr

0.274 / 0.27

0.31

0.46

0.29

0.321 / 0.29

0.477 / 0.27

0.29 / 0.25

0.54

Mn

12.1 / 13.1

13.6

11.9

15.1

12.7 / 13.2

11.9 / 11.2

11.2 / 12.1

11.2

Co

0.021 / 0.016

0.024

0.025

0.015

0.025 / 0.028

0.018 / 0.022

0.019 / 0.018

0.022

Cu

106 / 88.4

50.4

119

58.9

142 / 122

156 / 163

143 / 130

81.6

Zn

191 / 185

154

207

126

188 / 163

200 / 174

171 / 163

155

Rb

13.8 / 14.9

10.3

9.51

14.3

11.9 / 12.1

11.5 / 11.2

10.8 / 10.1

10.8

Sr

0.123 / 0.102

0.104

0.312

0.059

0.168 / 0.152

0.157 / 0.165

0.155 / 0.146

0.155

Mo

1.36 / 1.44

1.02

1.83

1.22

1.56 / 1.45

1.28 / 1.35

1.48 / 1.49

1.46

Ag

0.18 / 0.15

0.12

0.3

0.11

0.418 / 0.4

0.448 / 0.455

0.328 / 0.24

0.16

Cd

1.29 / 1.21

0.365

1.03

0.743

0.931 / 1.02

0.935 / 0.929

1.11 / 1.12

0.867

In

0.002 / 0.001

0.001

0.001

0.002

0.004 / 0.003

0.004 / 0.004

0.002 / 0.001

0.002

Sn

0.029 / 0.019

0.044

0.024

0.044

0.095 / 0.091

0.065 / 0.073

0.082 / 0.064

0.074

Sb

0.022 / 0.02

0.02

0.02

0.01

0.032 / 0.02

0.023 / 0.01

0.026 / 0.02

0.03

Cs

0.074 / 0.07

0.09

0.05

0.07

0.065 / 0.07

0.062 / 0.065

0.091 / 0.09

0.1

Ba

0.003 / 0.001

0.001

0.001

0.013

0.013 / 0.008

0.052 / 0.012

0.098 / 0.008

0.024

Hg

27 / 31

15

99

16

9.81 / 9.2

8.37 / 8.5

13.4 / 12

5.55

Tl

0.002 / 0.001

0.002

0.003

0.001

0.003 / 0.001

0.003 / 0.001

0.003 / 0.001

0.003

Pb

0.401 / 0.273

0.158

0.08

0.061

1.06 / 0.225

1.09 / 0.238

0.24 / 0.177

0.256

Bi

0.003 / 0.003

0.001

0.002

0.001

0.006 / 0.006

0.007 / 0.006

0.006 / 0.004

0.004

Concentrations of Ag, Bi, Ba, Cu, and Sn were significantly higher in the Chukchi Sea subpopulation of polar bears than in the Beaufort Sea subpopulation (Wilcoxon test, p < 0.05) (Table 1 and Fig. 7). On the other hand, concentrations of Hg were significantly higher in the Beaufort Sea subpopulation than in the Chukchi Sea subpopulation. The elevated concentration of Hg in the single polar bear from Prudhoe Bay served to up-weight the mean levels in the northern population relative to the western population. Earlier studies have shown increasing concentrations of Hg in polar bears from Eastern to Western Canadian Arctic and have suggested the presence of sources of Hg near the Western Canadian Arctic and the Northeastern Alaskan Arctic (Muir et al. 1999). Elevated concentrations of Hg in polar bear from Prudhoe Bay might be related to local sources of discharges in this area, by the Mackenzie River, which has been reported to contribute to significant sources of Hg in the Beaufort Sea (Leitch et al. 2007). The results for Hg in addition to Cu and Ag suggest the two subpopulations of polar bears are exposed to different sources of contamination. We have reported earlier that the concentration profiles of perfluorinated compounds, organochlorine pesticides, and dioxins differed between the two subpopulations of polar bears in Alaska (Kannan et al. 2005; Senthilkumar et al. 2002). Concentrations of most of the organohalogens, including dioxins/furans, were greater in the Beaufort Sea population than in the Chukchi Sea subpopulation, except for hexachlorocyclohexanes (HCHs) and perfluorononanoic acid, which were higher in polar bears from the Chukchi Sea (Kannan et al. 2005). The present study provides further evidence that the sources of exposure, to both organic and inorganic contaminants (Hg, Cu, and Ag), differ between the Western and Northern Alaskan polar bear populations.
https://static-content.springer.com/image/art%3A10.1007%2Fs00244-007-0018-x/MediaObjects/244_2007_18_f7.jpg
Fig. 7

Box and whisker plots of Ag, Bi, Ba, Cu, Sn, and Hg concentrations (µg/g dry wt) in livers of polar bears from the Beaufort Sea (Northern Alaska) and Chukchi Sea (Western Alaska) subpopulations

Relationships Among Trace Elements

The relationships among trace elements in the livers of polar bears were examined by Spearman’s rank correlation (Table 3). Zn and Co showed significant correlations with several other trace elements analyzed in this study. A strong, positive correlation was found between Zn and Cu, possibly resulting from sequestration of these two metals by metallothionein. A significant association between Zn and Cd can be explained by similar physical/chemical properties of these two metals. Cd might displace Zn from metallothioneins because of the stronger affinity of Cd for this protein involved in metal homeostasis and detoxification. Ag was significantly correlated with Zn and Cu, suggesting common binding to metallothionein. The associations between Zn and Cu, Zn and Cd, Ag and Cu, and Ag and Zn have been reported for marine mammals (Ikemoto et al. 2004).
Table 3

Spearman rank correlation coefficients for interrelationships among trace elements in livers of polar bears from Alaska

 

V

Cr

Mn

Co

Cu

Zn

Rb

Sr

Mo

Ag

Cd

In

Sn

Sb

Cs

Ba

Hg

Tl

Pb

Bi

V

1.00

0.10

0.43

0.62

0.24

0.14

0.30

0.19

0.45

0.31

−0.23

0.34

0.14

0.06

−0.01

0.15

−0.01

0.10

0.06

0.24

Cr

 

1.00

−0.07

0.12

−0.05

0.13

−0.15

0.15

0.06

0.02

0.01

0.12

−0.16

0.58

−0.23

0.01

−0.08

0.52

0.16

0.11

Mn

  

1.00

0.53

0.19

0.20

0.61

0.02

0.43

0.13

−0.26

0.26

−0.01

−0.01

0.13

−0.09

0.23

0.09

−0.24

0.03

Co

   

1.00

0.22

0.34

0.38

0.30

0.51

0.19

-0.14

0.59

−0.06

0.36

0.09

−0.08

0.08

0.44

0.03

0.49

Cu

    

1.00

0.51

0.33

0.17

0.44

0.80

0.30

0.30

−0.10

0.17

−0.13

−0.08

−0.15

0.31

0.19

0.45

Zn

     

1.00

0.21

0.17

0.57

0.38

0.47

0.15

0.46

0.43

−0.07

0.45

−0.05

0.46

0.27

0.24

Rb

      

1.00

−0.31

0.37

0.05

0.12

0.32

−0.27

0.03

0.18

−0.17

0.20

0.06

0.12

0.08

Sr

       

1.00

0.15

0.37

−0.14

0.26

0.16

0.07

0.41

0.20

−0.03

0.19

−0.07

0.34

Mo

        

1.00

0.31

0.16

0.09

−0.15

0.21

0.14

-0.20

0.02

0.36

0.08

0.15

Ag

         

1.00

0.12

0.40

0.18

0.11

−0.30

0.21

−0.24

0.15

0.22

0.47

Cd

          

1.00

0.01

−0.26

0.13

−0.10

−0.24

0.23

0.10

0.29

−0.04

In

           

1.00

0.11

0.36

−0.08

0.31

−0.18

0.30

0.11

0.74

Sn

            

1.00

0.44

−0.03

0.77

−0.22

0.55

−0.10

0.01

Sb

             

1.00

0.03

−0.32

−0.19

0.84

0.24

0.55

Cs

              

1.00

0.01

0.17

−0.03

0.39

−0.11

Ba

               

1.00

−0.20

0.44

−0.08

0.08

Hg

                

1.00

−0.09

0.34

0.39

Tl

                 

1.00

0.10

0.56

Pb

                  

1.00

0.25

Bi

                   

1.00

Note: Bold values indicate significance at 1% level (p < 0.01); underlined values indicate significance at 5% level (p < 0.05)

Among toxic metals, Hg was significantly, but negatively, correlated with Pb. Indeed, concentrations of Hg in polar bear livers were negatively correlated with concentrations of most of the trace elements analyzed in this study. Sn concentrations were positively correlated with Ba concentrations and negatively correlated with Sb and Tl concentrations in the polar bear livers.

In summary, this study provides baseline data for several trace element concentrations in livers of polar bears from Alaska. Variations in the concentrations of certain essential and nonessential elements (Ag, Bi, Ba, Cu, Sn, Hg), between the Beaufort and Chukchi Sea subpopulations, suggest differences in local sources of exposures. The Hg concentration was elevated in a polar bear (99 µg/g dry wt) from Prudhoe Bay, northeastern Alaska; this concentration was above the threshold for adverse effects (60 µg/g dry wt; Law 1996). Furthermore, polar bears appear to have tissue distribution and pharmacokinetics for trace metals (such as Hg) that differ from patterns seen in other marine mammals. Further studies should focus on the analysis of the kidney, brain, and muscle tissues in addition to the liver.

Acknowledgments

A portion of the study was supported by a grant from “21st Century Center for Excellence (COE) Program” from the Ministry of Education, Culture, Sports, Science and Technology, Japan. We thank the native subsistence hunters for providing samples for this study.

Copyright information

© Springer Science+Business Media, LLC 2007