Geographical differences of zinc, cadmium, mercury and selenium in polar bears (Ursus maritimus) from Greenland

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Abstract

Muscle, liver, and kidney tissues from 100 polar bears (Ursus maritimus) caught in the Avanersuaq area, north-west Greenland, and Ittoqqortoormiit area, central-east Greenland, were analysed for zinc, cadmium, mercury and selenium. The zinc concentrations in muscle and liver were higher than in kidney. Mean zinc concentrations ranged from 19.7 to 76.0 μg/g (all data are presented as geometric means on a wet wt. basis). The presented cadmium concentrations by area and age groups were all low in muscle and in many cases below the detection limit (range: <0.015–0.048 μg/g). Cadmium concentrations were intermediate in liver (range: 0.120–1.98 μg/g) and highest in kidney tissue (range: 2.16–28.9 μg/g). Mercury was likewise lowest in muscle tissue (range: 0.034–0.191 μg/g). Mercury concentration ranged quite similarly in liver and kidney tissue (liver range: 2.13–22.0 μg/g; kidney range: 2.87–32.0 μg/g). The selenium concentration increased from muscle (range:<0.2–0.452 μg/g) over liver (range: 1.20–9.80 μg/g) to kidney (range: 2.34–13.9 μg/g). No age accumulation was found for zinc. A weak increase was found for selenium, whereas cadmium and mercury clearly accumulated with age. An exception was mercury concentrations in muscle tissue, where no clear pattern was observed. Polar bears had significantly lower cadmium concentrations than ringed seals from the same area in all three tissues. Likewise mercury was significantly lower in the muscle tissue of polar bears than in ringed seals, whereas liver and kidney concentrations were higher. Biomagnification factors are provided for different tissues and age groups. Tissue ratios are given for different age groups and metals to enable a rough extrapolation from one tissue to another. Tissue ratios for cadmium, selenium and for mercury vary up to a factor of 6 with age. No significant differences could be detected between the elements analysed in bears from two management zones in north-west Greenland. This finding is in agreement with the genetic pattern in the two areas. In central-east Greenland, however, cadmium, selenium, and some of the mercury concentrations in polar bears from the southern area were higher than from the northern area, indicating that the east Greenland area represents two different ecological regions with different polar bear populations. Geographical differences between polar bears from north-west and east Greenland were only found for mercury and cadmium in liver tissue, where the concentrations were highest in bears from north-west Greenland. The geographical trend of increasing cadmium concentrations in polar bear liver tissue from west to east, which has been found previously in Canada, could be extended to cover north-west Greenland as well. East of this region a decrease was found. Mercury concentrations in polar bear liver tissue showed an increase from Svalbard over east and north-west Greenland, peaking in bears from SW Melville Island. A marked decrease was found west of Melville Island, and the lowest concentrations were found in the Chukchi Sea.

Introduction

Substantial efforts have been made in recent years to collect data from Arctic biota to obtain a large scale overview of geographical trends in heavy metals and persistent organic pollutants (POPs), and to look for potential anthropogenic patterns (e.g. de March et al., 1998, Dietz et al., 1998a). Geographical differences have often been revealed, such as north–south or east–west gradients, however, some trends may be directed in opposite directions, such as mercury vs. cadmium and mercury vs. POPs for example, which does not facilitate the interpretation. In the case of heavy metals both a natural and an anthropogenic component contribute to regional differences, however, differences in feeding habits are also likely to influence the observed trends. Studies from other areas have revealed considerable year-to-year variation, and local geographical differences may in some cases be in the same order of magnitude as regional differences (e.g. Bignert et al., 1993, Bignert et al., 1994, Olsson, 1995, Dietz et al., 1996, Dietz et al., 1998a; Riget and Dietz, 1998).

Because polar bears (Ursus maritimus) have a circumpolar distribution and occur in a relatively discrete subpopulation within their range (Paetkau et al., 1999) they are suitable study objects for detecting regional trends in pollution. The stock delineation information obtained from satellite telemetry and genetic studies have previously been used in the design of the studies of regional differences of heavy metals and POPs in the Canadian Arctic (Norstrom et al., 1986, Norstrom et al., 1998, Braune et al., 1991). The polar bear has a top position in the Arctic food chain, and is therefore exposed to high concentrations of a number of bioaccumulating contaminants, which makes it important to study.

Geographical differences in heavy metal concentrations have been reported in a number of studies from various polar bear matrices. Polar bear hair has been used to monitor mercury concentrations (Eaton and Farant, 1982, Renzoni and Norstrom, 1990, Born et al., 1991). Internal tissues have also been monitored from polar bears. Lentfer and Galster (1987) analysed mercury concentrations in muscle and liver tissue of polar bears from Alaska. Norstrom et al. (1986) and Braune et al. (1991) analysed 22 elements in Canadian polar bear livers of which geographical trends were only found for cadmium, mercury and selenium. Dietz et al., 1995, Dietz et al., 1996 previously presented zinc, cadmium, mercury and selenium concentrations in muscle, liver and kidney from polar bears from one area in east Greenland. Norheim et al. (1992) presented data on heavy metals and some essential elements present in liver and kidney tissue from a smaller number of polar bears shot or found dead at Svalbard.

In the present article, new data obtained from analysis of a more comprehensive material consisting of 100 Greenland polar bears is added to previously published results. Heavy metal data are presented for different polar bear management zones in an attempt to evaluate whether differences can be detected among these, and if so, to evaluate the magnitude of local geographical differences relative to regional differences. Finally, additional information is added with respect to the bioaccumulation and tissue ratio discussion.

Section snippets

Specimen composition

Muscle, liver and kidney samples from 45 polar bears were collected from the Inuit’ subsistence catch by local hunters in the Avanersuaq/Thule area in north-western Greenland between 1988 and 1990 (year/sample size: 1988/7; 1989/21; 1990/17). These samples comprised 6 juvenile (1 year: 3 M, 3 F), 31 subadult (2–6 years: 18 M, 13 F) and 8 adult (≥7 years: 5 M, 3 F) polar bears. Based on the assumption that different subpopulations of polar bears had been sampled, the material was divided into

Sample composition

The total sample consisted of 56 male and 44 female polar bears ranging between 1 and 18 years of age.

Based on the movement data derived from satellite-telemetry, the polar bears in Kane Basin and Baffin Bay (currently) belong to two separate management zones which are assumed to represent functionally separate groups of bears (cf. Lunn et al., 1998). For the statistical analyses of influence of area the samples from the Avanersuaq/Thule area were separated according to this information

Subdivision of samples by sub-populations

Although it is known that some polar bears immigrate to eastern Greenland from other Arctic areas (e.g. Born, 1995, Durner and Amstrup, 1995), the exchange of polar bears between north-western and eastern Greenland is assumed to be negligible. The two areas are separated by unproductive ice covered oceans rimming the Arctic Ocean, open water areas and the Inland Ice-Cap, which serve as an effective barrier (Born and Rosing-Asvid, 1989, Born, 1995). Accordingly, the samples from north-western

Acknowledgements

The financial support given by the Commission for Scientific Research in Greenland, the Danish Natural Science Research Council and Aage V. Jensen Foundation is gratefully acknowledged. Aqqalu Rosing-Asvid, Kurt Thomsen, Jonas Brønlund, Arne Øland and the late Jens Thygesen did a great job in helping to organise the collection of samples from the Inuit hunters, who are likewise acknowledged. Wendy Calvert (Canadian Wildlife Service, Edmonton), Malcolm A. Ramsay (University of Saskatchewan,

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