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Temporal trends of mercury in marine biotaof west and northwest Greenland

Frank Riget a,*, Rune Dietz a, Erik W. Born b, Christian Sonne a, Keith A. Hobson c

a National Environmental Research Institute, Frederiksborgvej 399, P.O. Box 358, DK-4000 Roskilde, Denmarkb Greenland Institute of Natural Resources, P.O. Box 570, DK-3900 Nuuk, Greenland

c Environment Canada, 11 Innovation Blvd., Saskatoon, Saskatchewan, Canada S7N 3H5

Abstract

Temporal trends in mercury concentrations ([Hg]) during the last two to three decades were determined in liver of shorthorn sculpin,ringed seal and Atlantic walrus from northwest Greenland (NWG, 77� N) and in liver of shorthorn sculpin and ringed seal from centralwest Greenland (CWG, 69� N) during the last decade. Stable-nitrogen (d15N) and carbon (d13C) isotope values were determined in mus-cle of ringed seals to provide insight into potential trophic level changes through time. Log-linear regressions on annual median [Hg] didnot reveal any temporal trend in shorthorn sculpin from CWG and NWG and walrus from NWG. In ringed seals from NWG, anincrease in [Hg] of 7.8% per year was observed. When based on d15N-adjusted [Hg] this rate increased to 8.5% but was still non-signif-icant. In ringed seal from CWG no trend was found in [Hg] during the period 1994–2004. However, during the last part of the period(1999–2004) the [Hg] increased significantly. Including tissue d15N values as a covariate had a marked effect on these results. The annualchanges in d15N-adjusted [Hg] was estimated to �5.0% for the whole period and 2.2% during the last 5 years compared to �1.3% and12.4%, respectively, for the non-adjusted [Hg].� 2006 Elsevier Ltd. All rights reserved.

Keywords: Temporal trend; Mercury; Greenland; Marine biota; Stable isotopes; Nitrogen d15N

1. Introduction

In recent years, mercury (Hg) in the arctic environmenthas become an issue of great interest. The primary reasonfor this is because Hg constitutes a health threat for localhuman populations who are exposed to Hg through theirtraditional diet (Johansen et al., 2004). Another reason isthe discovery of atmospheric Hg depletion during spring-time in the arctic indicating that the arctic may act as a sinkfor the global Hg cycle (Schroeder et al., 1998; AMAP,2005). Furthermore, the global Hg emission to the atmo-sphere may be increasing despite the recent emission reduc-tions in North America and Western Europe (AMAP,2005).

Monitoring of temporal trends of contaminants is essen-tial to any programme concerned with the possible effectsof contaminants on human health and wildlife. Monitoringalso provides a tool to evaluate the effects of any regulatoryaction to reduce contaminant emissions (Bignert et al.,2004). However, it is important to realize that concentra-tion levels in the biota are integrated results of several pro-cesses (e.g. climate change, food change and availability,growth change), which can change from year to year orgradually with time.

Generally, Hg has increased in the arctic and Greenlandenvironment when compared to pre-industrial times (Nils-son and Huntington, 2002; Braune et al., 2005; Dietz et al.,2006). However, temporal trends in Hg over the last two orthree decades are more uncertain. One of the main reasonsfor this uncertainty is the brief Hg time-series available andthe lack of statistical power to detect changes (Bignertet al., 2004).

0025-326X/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.marpolbul.2006.08.046

* Corresponding author. Tel.: +45 46301948; fax: +45 46301914.E-mail address: ffr@dmu.dk (F. Riget).

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Marine Pollution Bulletin 54 (2007) 72–80

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The Arctic Monitoring and Assessment Programme(AMAP) concluded in their 2002 assessment that ‘‘thereis a trend of increasing mercury levels in marine birdsand mammals in the Canadian Arctic, and some indica-tions of increases in West Greenland’’ (AMAP, 2005).The Canadian Hg time-series have been updated andin the recent review by Braune et al. (2005), it was statedthat the temporal trends of Hg over the past 20–30 yearsare inconsistent because some populations showed increaseof Hg whereas others did not.

AMAP’s conclusion with regard to temporal trends inWest Greenland was based on data sets up to 2000 (Rigetet al., 2004). Since then, new years of data of Hg in short-horn sculpin (Myoxocephalus scorpius) and ringed seal(Phoca hispida) have been added to the time-series and atime trend study of Hg in Atlantic walrus (Odobenus rosm-

arus rosmarus) have been made based on archived samples.Shorthorn sculpin and ringed seal were originally selectedfor monitoring by AMAP (2003) because they are widelydistributed in the arctic and represent different animalgroups and trophic levels. Furthermore, both species areconsidered to show a high degree of site fidelity. Althoughlong-range movement of some ringed seal individuals hasbeen documented, studies involving tagging and satellitetelemetry indicate a large degree of site fidelity in ringedseals in the Baffin Bay-Davis Strait area (Heide-Jørgensenet al., 1992; Kapel et al., 1998; Teilmann et al., 1999; Bornet al., 2004).

Shorthorn sculpin feeds benthically on a variety ofinvertebrates whereas ringed seals feed in pelagic anddemersal habitats (Smith, 1987; Weslawski et al., 1994).Ringed seals are euryphagous (McLaren, 1958; Weslawskiet al., 1994; Siegstad et al., 1998; Holst et al., 2001), eatinga variety of crustaceans (mainly the hyperiid amphipodThemisto [Parathemisto] – libellula) and fish (mainly polarcod, Boreogadus saida, and arctic cod, Arctogadus glacialis)(Wathne et al., 2000). There are, however, indications thatringed seal food preferences may differ regionally and alsocan change seasonally and between years (Holst et al.,2001) which may influence their dietary exposure to con-taminants. Marine animals that feed at relatively high tro-phic levels have higher [Hg] compared with organisms atlower trophic levels (Dietz et al., 2000). To explore theeffects of potential change of diet in marine mammals weanalyzed the stenophageous walrus sampled in northwestGreenland (seven sampling years during 1977–2003).Although they occasionally may eat vertebrates like seals,birds and fish, walruses are benthic feeders that foragealmost exclusively on bivalves (Vibe, 1950; Fay, 1982; Bornet al., 2003). Hence, for walrus, the potential effect ofannual change in diet on Hg exposure is likely minimal.

Stable nitrogen isotope values (d15N) in tissues of mar-ine biota is related to trophic position (Michener andSchell, 1994) and several researchers have used data ond15N to study the transmission of contaminants in foodwebs (Hobson et al., 2002; Dehn et al., 2006) including pat-terns of [Hg] in ringed seals (Atwell et al., 1998). In the case

of ringed seal, we performed the temporal Hg trend analy-sis both with and without d15N seal tissue values as acovariate.

In this paper we present temporal trend analyses of [Hg]in liver of shorthorn sculpin, ringed seal and Atlantic wal-rus from northwest and central west Greenland. We use tis-sue d15N as a proxy of trophic position in case of ringedseal. We also make recommendations of a continued mon-itoring of Hg in the arctic.

2. Material and methods

2.1. Sampling and laboratory analyses

Shorthorn sculpins from the Qaanaaq municipality(77�30’ N, 69�19’ W; Northwest Greenland) and the Qeqer-tarsuaq/Disko municipality (69�15’ N, 53�32’ W; CentralWest Greenland) (Fig. 1) were collected by nets or jigged.Samples of ringed seals from Northwest and Central WestGreenland were obtained from local hunters mainly duringMay and June. The animals were either shot or caught innets. The treatment of the liver samples from the sculpinsand the ringed seals and age determination of seals fol-lowed methods in Dietz et al. (1991) and Riget et al.(2000). Liver samples of walruses were obtained from theInuits’ subsistence hunts in the Qaanaaq municipality(Northwest Greenland) in the period March–July during

Fig. 1. Study area and the two sampling locations in West Greenland. Thedistance between the two locations is approximately 1000 km.

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1977–2003. Samples from 1977 and 1978 were collected byscientists whereas those sampled during the other years(1987, 1988, 1989, 1990, 2003) were collected by the huntersthemselves (Born, 2001, 2003). Ages were determined fromreading of incremental layers in the cementum of molari-form teeth (Mansfield, 1958). The age of the walruses fromthe years 1977, 1978, 1988, 1989, and 1999 ranged from 9to 16 years. Exact individual age (e.g. based on incrementallayering in tooth cementum) was not available for 7 (allfemales) walruses collected in 1987 and 10 (4 females, 5males, 1 unid. sex) collected in 2003. Hence, for these sam-ples an individual minimum age was estimated from stan-dard body length and the length of the crown of thetusks (measured along the frontal curvature) and their cir-cumference (measured at the gum line). Measurements ofbody and tusk dimensions were compared with bodylength-at-age growth curves (Knutsen and Born, 1994)and information on age-specific growth of tusks in walrusesfrom Northwest Greenland (Born unpublished data). Theseven female walruses sampled in 1987 were all estimatedto be adults (6 were 10+ and 1 was 7+ years old). Of thewalruses sampled in 2003, eight were estimated to be 2–5years old and 2 females were judged to be 10+ years old.All samples had been archived at �20 �C until chemicalanalyses. Number of samples by area and year that wereincluded in the analyses is presented in Table 1.

In the laboratory the samples were further dissectedwith a stainless steel scalpel in a clean room with air filteredthrough cotton filter bags. All surfaces were freshly cutleaving an uncontaminated subsample for analyses. A sep-arate sub-sample was taken for the determination ofthe dry weight % by weighing before and after drying at

105 �C for 12 h. Approximately 500 mg sample were trans-ferred to the tarred Teflon liner of Anton Paar MicrowaveSample Preparation System ‘‘Multiwave 3000’’. Then 4 mlof 65% HNO3 (Merck Suprapur�) and 4 ml deionizedwater (Millipore�) were added before the liners were closedand microwaved according to the method recommended bythe manufacturer. Following a cooling period the decom-posed and completely clear digests were transferred to 50ml screw-cap polyethylene bottles and adjusted to 25 gweight using metal-free, deionized water (Millipore�).Approximately 8% HNO3 was used dilution of sampleswith high mercury concentration and for preparation ofstandards. The Teflon liners were precleaned with thedetergent RBS followed by nitric acid. The polyethylenecontainers were cleaned with 5% nitric acid over night.Blind values have shown that this cleaning was satisfactory.

All Hg analyses were carried out by use of atomicabsorption spectrometry (AAS) (hydride generation andthe flow injection analyses) described by Asmund et al.(2004). All data are presented on a dry weight basis andthe detection limit was 0.005 mg/kg dw. Analytical qualitywas ensured by repeated analyses and by frequent analysisof various certified reference materials (TORT-2 (lob-ster hepatopancreas), DORM-2 and Dolt-3) supplied bythe National Research Council of Canada (MarineAnalytical Chemistry Standards Program). The NationalEnvironmental Research Institute, Department of ArcticEnvironment laboratory participated in the internationalinter-comparison exercises conducted by the InternationalCouncil for the Exploration of the Sea (ICES), EEC(QUASIMEME), and the Department of Fisheries andOceans, Winnipeg, Canada (Asmund and Cleemann,

Table 1Mean (SD) [Hg] (mg/kg wet weight) and number of liver samples by area and year of Greenlandic shorthorn sculpin, ringed seal and walrus

Year Northwest Greenland Central West Greenland

Ringed seala

64 years oldWalrus9–16 years oldd

Shorthornsculpin >27 cm

Ringed seala

6 4 years oldShorthornsculpin 6 27 cm

Shorthornsculpin >27 cm

1977 1.72 (0.70) 101978 3.45 (2.41) 101984 2.17 (2.11) 261985 0.99 (1.28) 111987 2.35 (2.56) 7 0.054 (0.023) 71988 3.64 (1.57) 71989 2.54 (1.54) 61990 1.27 (0.48) 91994 2.45 (3.79) 15 1.64 (0.97) 29 0.029 (0.026) 12 0.026 (0.012) 131995 0.056 (0.036) 71998 3.17 (2.29) 91999 1.67(4.00) 14 0.010 (0.004) 12 0.017 (0.006) 82000 0.76 (0.47) 15 0.011 (0.004) 10 0.015 (0.006) 102001 0.90 (0.35) 18 0.022 (0.012) 7 0.061 (0.039) 122002 0.093 (0.031) 3 1.36 (0.83) 20b 0.015 (0.007) 9 0.016 (0.010) 122003 3.30 (2.45) 10 1.45 (1.26) 18c 0.044 (0.035) 192004 3.66 (1.49) 19 0.065 (0.033) 18 1.70 (1.12) 20 0.016 (0.006) 20

a Analysed for stable C and N isotopic composition.b Nineteen samples analysed for stable C and N isotopic composition.c Seventeen samples analysed for stable C and N isotopic composition.d Except in 2004 where four individuals were between 2 and 5 years old (see Section 2).

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2000; Asmund et al., 2004). The coefficient of variation wasdetermined by participation in laboratory inter compari-sons to 6% with a detection limit of 0.0025 mg/kg (Asmundet al., 2004).

It is generally accepted that tissue d15N (15N/14N) valuesare related to trophic position in marine food webs (e.g.Post, 2002) and d13C (13C/12C) values can also be relatedto benthic vs. pelagic feeding (Hobson and Welch, 1992;France, 1995). Values of d13C and d15N in muscle sampleswere determined by continuous-flow isotope-ratio massspectrometry (CF-IRMS) according to Hobson et al.(2002). The presence of lipids in marine mammal tissuescan alter their d13C values (Hobson and Welch, 1992). Lip-ids were extracted to avoid effects of differential lipid con-tent among individuals since this can affect comparisons ofd13C values. So, prior to isotopic analysis, samples weredried, ground to a fine powder and, with the exception ofbaleen (since this material does not contain lipids), lipidswere extracted using successive rinses in a 2:1 chloro-form:methanol solution. Results are presented in the usuald notation relative to PeeDee belemnite and atmosphericnitrogen (AIR) for d13C and d15N measurements, respec-tively. Based on thousands of measurements of our labora-tory standard (albumin and whale baleen), analytical erroris estimated to be +0.1o/oo for d13C values and +0.3o/oo ford15N.

2.2. Statistical analyses

Mercury levels increase with fish size and age of ringedseals (Riget et al., 1997; Atwell et al., 1998; Dietz et al.,1998). In order to control for size in temporal trend analy-ses, shorthorn sculpins were divided into small (total lengthbelow or equal to 27 cm) and large (total length above27 cm). In the case of ringed seal only individuals at 4 yearsold or younger were included in the temporal trend analy-ses. The seven female walruses sampled in 1987 were allestimated from body and tusk dimensions to be adults (6were 10+ and 1 was 7+ years old) whereas of those sam-pled in 2003, eight were estimated to be 2–5 years oldand 2 females were judged to be 10+ years old. Hence,most of the walrus were between 9 and 16 years old. Pear-son’s correlation analyses were applied to test for correla-tions between log-transformed mercury concentrationsand d15N and d13C values. In the case of ringed seal fromNorthwest Greenland, an analysis of covariance(ANCOVA) of log-transformed [Hg] including year as afactor, d15N as a covariate and the interaction betweenthe year factor and the covariate d15N showed that theinteraction term was not significant (F = 0.94, p = 0.444,df = 4). This means that the slope of the relationshipbetween log-transformed [Hg] and d15N did not differ sig-nificantly among sampling years. Therefore, log-trans-formed [Hg] were normalized to an overall mean d15Nvalue of 16.4& assuming a common slope between log-transformed [Hg] and d15N for all years. A similarANCOVA performed for ringed seal from Central West

Greenland showed that the slope of the relationshipbetween log-transformed [Hg] and d15N differed signifi-cantly among sampling years (F = 2.52, p = 0.025,df = 6). However, this was due to one seal having the high-est d15N value and the lowest [Hg] among all seals. We con-sidered this data point as an outlier and when removing itfrom the ANCOVA, the slope of the relationship betweenlog-transformed [Hg] and d15N no longer differed signifi-cantly among sampling years (F = 1.31, p = 0.259,df = 6). Therefore, log-transformed [Hg] were normalizedto an overall mean d15N value of 14.6& assuming a com-mon slope for all years. Temporal trend analyses ofd15N-adjusted and non-adjusted [Hg] and tissue d15N andd13C values were performed by a log-linear regression anal-ysis using annual median values. Median [Hg] values werechosen in order to minimise influence of outliers. TheNorth Atlantic Oscillation (NAO) winter (Hurrell, 1995)index was chosen as an large-scale climate index. TheNAO index is highly correlated with the local weather inWest Greenland (Forchlammer and Post, 2004). Pearson’scorrelation analyses were applied to test for correlationsbetween log-transformed mercury concentrations and theNAO index in the same year and in the year before.

3. Results

In ringed seals from Northwest Greenland (1984–2004;Fig. 2a) the annual change in [Hg] was estimated at 7.8%by linear regression on log-transformed median concentra-tions. Although this rate of increase was not statisticallysignificant (R2 = 0.46, p = 0.210), it is high and is of con-cern in terms of the risk of making a Type II error becauseof lack of power of the test. The log-transformed [Hg] werefound to be significantly positively correlated with d15N(r = 0.338, p = 0.002, df = 78) but not significantly corre-lated with d13C (r = �0.095, p = 0.404, df = 78). The posi-tive correlation between log-transformed [Hg] and d15Nindicates that individuals with relatively high [Hg] alsofed at relatively higher trophic levels. Tissue d15N valuesshowed no significantly temporal trend by a log-linearregression analyses using annual medians (R2 = 0.11,p = 0.579) indicating that no changes with time in feedingbehaviour had occurred. Mercury concentrations were nor-malized to a common d15N of 16.4& assuming a commonslope for all years (see Section 2). The temporal trend anal-yses of d15N normalized [Hg] estimated an annual increaseof 8.5%, which was higher than the rate estimated using thenon-adjusted concentrations but still non-significant(R2 = 0.53, p = 0.165) (Fig. 2b).

The annual change of [Hg] in ringed seal from CentralWest Greenland (Fig. 2c) was estimated to be �1.3%(R2 = 0.02, p = 0.750) for the total period (1994–2004).However, during 1999–2004, [Hg] increased significantlyby 12.4% per year (R2 = 0.96, p < 0.001). The log-trans-formed [Hg] values were found to be significantly positivelycorrelated with d15N (r = 0.357, p < 0.001, df = 129) andd13C (r = 0.324, p < 0.001, df = 129). Tissue d15N values

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pyshowed no significantly temporal trend for the total periodby a log-linear regression analyses using annual medians(R2 = 0.36, p = 0.155). During 1999–2004, the period withsignificantly increasing [Hg] values, the tissue d15N alsoincreased significantly (R2 = 0.81, p = 0.015) indicatingthat changes in feeding behaviour had occurred. Mercuryconcentrations were normalized to the overall mean valueof d15N of 14.6&. Adjusting [Hg] with seal d15N valueshad a considerable effect on the estimated annual [Hg]changes (Fig. 2d). For the whole period, the adjustedchange of the [Hg] was estimated to be �5.0% per year(R2 = 0.42, p = 0.114) indicating a higher but still non-sig-nificant rate of decrease compared with the non-adjustedvalues. For the period 1999–2004 the adjustment led to achange from a statistically significant 12.4% increase to anon-significant increase of 2.2% per year (R2 = 0.08,p = 0.587).

In the stenophagous walrus from Northwest Greenland(1977–2003) the annual change of [Hg] was very low (0.3%per year, R2 = 0.004, p = 0.888) (Fig. 3), and therefore didnot support a general increase in [Hg] in marine biota fromNorthwest Greenland. This was also the case in large short-horn sculpins from the same area (1987–2004) (Fig. 4a). Inthis benthically feeding fish, the annual rate of change in[Hg] of 0.6% per year was non-significant (R2 = 0.01,

p = 0.872). Similar to the situation in Northwest Green-land, the annual changes in [Hg] in shorthorn sculpin(small and large) from Central West Greenland were notstatistically significant (Fig. 4b and c). The annual changein small (1994–2002) and large (1994–2004) shorthorn

1985 1990 1995 2000

0.2

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1.0

2.0

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Hg

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Fig. 2. Temporal trend in [Hg] (mg/kg wet weight (ww)) in livers of ringed seal from Northwest Greenland no adjustment (a), NWG adjusted for d15Nvalues in muscle (b), Central West Greenland no adjustment (c), CWG adjusted for d15N values in muscle (d). Open circles are individual [Hg] values, solidcircles are the median value and the solid line represents the results of log-linear regression analyses of median values. The stippled line in 2c and 2d is theregression line for the period 1999–2004. For number of samples see Table 1.

1980 1985 1990 1995 2000

24

68

Year

Hg

mg/

kg w

w

1977 2003

Fig. 3. Temporal trends of [Hg] (mg/kg wet weight (ww)) in liver ofAtlantic walrus from Northwest Greenland. Open circles are individual[Hg] values, solid circles are the median value and the line represents theresults of log-linear regression analyses of median values. For number ofsamples see Table 1.

76 F. Riget et al. / Marine Pollution Bulletin 54 (2007) 72–80

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sculpins were estimated to be �6.2% (R2 = 0.21, p = 0.433)and �0.6% (R2 = 0.001, p = 0.943), respectively. In bothtime series a considerable year-to-year variation in median[Hg] was found and the relatively high annual decreasefound in small shorthorn sculpin was driven by the rela-tively high median concentration in the first year (1994)of the time-series (Fig. 4b).

No of the annual median [Hg] time-series showed signif-icantly (p > 0.05) correlation with the NAO index in thesame year or the NAO index in the year before.

4. Discussion

The present study gives no consistent picture of the [Hg]trend in the marine biota in Northwest and Central WestGreenland. In ringed seal from Northwest Greenland, theestimated annual increase in [Hg] was relatively high, butstill statistically insignificant. The temporal changes in[Hg] in walrus and shorthorn sculpin from NorthwestGreenland were rather small and statistically insignificant.In ringed seal and small and large shorthorn sculpin fromCentral West Greenland from the period 1994 to 2004,the Hg time-series all showed a decreasing non-significanttrend. The increasing trend observed in ringed seal duringthe period 1999–2004 may likely be explained by changesin feeding behaviour.

The temporal trend of Hg in marine biota in the Cana-dian Arctic has recently been reviewed by Braune et al.(2005). The trend studies covering the last two to three dec-ades included seabird eggs, ringed seal and beluga (Del-

phinapterus leucas). The concentration of Hg in eggs ofthick-billed murre (Uria lomvia), northern fulmar (Fulma-

rus glacialis) and black-legged-kittiwake (Rissa tridactyla)increased in the period 1975–2005. The rate of increase dif-fered among the three species and probably depended ontheir different main winter range (Braune et al., 2001; Bra-une et al., 2005). Mercury in ringed seal liver from severallocations in the Canadian Arctic showed both increasingand decreasing trends with time and large year-to-year var-iation was seen at several locations presumably due to die-tary shifts (Braune et al., 2005). Mercury concentrations(age-adjusted) in liver of belugas from Mackenzie Rivershowed an increasing trend over the period 1981–2002(Lockhart et al., 2005).

In our study, the increasing trend in ringed seals fromNorthwest Greenland during the period 1984–2004 couldnot be explained by changing trophic position with time,whereas this was a likely explanation with the increasingtrend found in ringed seals from Central West Greenlandduring the period 1999–2004. Previously, Riget and Dietz(2000) pointed out that changing feeding behaviour withtime might be the cause of opposite time trends of Hgand Cd observed in seals in Greenland. The euryphagousringed seals feed both pelagically and benthically. Theirfood consists of a variety of crustaceans and fish (Siegstadet al., 1998; Holst et al., 2001). There are indications thatprey selection and feeding strata in the water column differregionally and among age and sex categories. Immatureringed seals forage at different depths than older (Bornet al., 2004) and immatures generally take more crusta-ceans than adults (Holst et al., 2001). We do not haveany specific information about the diet of ringed seals inCentral West Greenland during 1999–2004. During thisperiod temperatures has changed markedly in this area(Greenland Institute of Natural Resources, unpublisheddata) and the ice cover in eastern Davis Strait hasdiminished (L. Toudal, Technical University of Den-mark, unpublished data). However, we did not observed

1990 1995 2000

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Fig. 4. Temporal trend of [Hg] (mg/kg wet weight (ww)) in liver of largeshorthorn sculpin from Northwest Greenland (a) and small (b) and largeshorthorn sculpin from Central West Greenland (c). Open circles areindividual concentrations, solid circles are the median values and the solidlines represent the results of log-linear regression analyses of medianvalues. For number of samples see Table 1.

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a correlation between [Hg] and climate represented by theNAO index.

The use of d15N values in time trends studies of contam-inants has increased in recent years (Outridge et al., 2002;Braune et al., 2001, 2002). In these studies, trend in stableisotope values has helped the interpretation of the timetrend of contaminants. However, in the present studyd15N value was used as a covariate and was therefore inte-grated in the statistical trend analyses. Of course, a funda-mental assumption in the application of this approach isthat baseline food web d15N values did not change throughtime and that changes in tissue d15N values are relatedexclusively to changes in trophic level. This could bechecked using isotopic measurements of several organisms,including those of lower trophic-level feeders that may bet-ter reflect any isotopic changes in primary productivity(Michener and Schell, 1994).

In our case, the slope of the relationship between log-transformed [Hg] and d15N values did not differ signifi-cantly among sampling years; therefore we used a commonrelationship in the adjustment of [Hg] values. This may notalways be the case when analysing temporal trends of con-taminants and if contaminant-covariate relationshipchanges over time the results can be difficult to interpret(Fryer and Nicholson, 2002). Another statistical approachmay be needed such as splitting the contaminant data intotwo groups according to whether the corresponding covar-iates are smaller or larger than some specified value as pro-posed by Fryer and Nicholson (2002). This approach hasbeen applied in the present study regarding the size orage covariate.

Including d15N values as a covariate in the temporaltrend analyses of Hg has a risk of also including someunnecessary ‘‘noise’’ derived from analytical error intothe analyses. In our study the within-year variation (on alog scale) was reduced by about 9% (from 0.351 to 0.318)when adjusting [Hg] with d15N values in the case of ringedseals from Central West Greenland. The correspondingreduction was about 7% for the time-series from NorthwestGreenland. This indicates the potential for reducing thewithin-year variation in [Hg] by adjusting with the d15Nvalue. Furthermore, the effect of the adjustment on thelog-linear regression analyses of the median [Hg] value inringed seals was a reduction of the residual standard errorby about 30% (from 0.302 to 0.211) in case of seals fromCentral West Greenland and 4% (from 0.836 to 0.802)for Northwest Greenland seals, indicating an increasedpower of the temporal trend analysis to detect changes.

Most of the temporal trend analyses made in the presentstudy have resulted in non-significant results. The degreesof freedom of log-linear regression analyses of the annualmedian concentrations is small when the number of yearsincluded is small. In the present study the number of yearsranged from 4 to 7 and the power of the test was expectedto be poor. Bignert et al. (2004) analysed the statisticalpower of 42 time-series of Hg in arctic biota and found thatabove 10 years of sampling is required to obtain sufficient

power to detect a time trend. Similar results have beenobtained in other contaminant monitoring programmesas in the Baltic (Bignert et al., 1997) and the InternationalCouncil for Exploitation of the Sea’s Co-operative Moni-toring Programme (Fryer and Nicholson, 1993).

This study brings no evidence of a general increasingtrend of Hg in marine biota from West Greenland, how-ever, it is recommended to continue monitoring in orderto increase the power of the trend analyses. Furthermore,the study demonstrates that adjustment of [Hg] in consum-ers by including their tissue d15N values may have a majorimpact on the temporal trend analyses and will alsoincrease the power of the analyses.

Acknowledgements

The study was funded by the Danish EnvironmentalProtection Agency as part of the environmental supportprogramme Dancea – Danish Cooperation for Environ-ment in the arctic and an operating grant to KAH fromEnvironment Canada. Sampling of seals and sculpins from1985 to 1988 were financially supported by, the Commis-sion of Scientific Research in Greenland and the DanishNational Science Foundation and later by the Dancea pro-gramme. Aage V. Jensen’s Foundation financially sup-ported the sampling of walruses during 1977–1990. Theauthors are solely responsible for the results and conclu-sions presented, which do not necessary reflects the posi-tion of the Danish Environmental Protection Agency orEnvironment Canada. We are grateful to the hunterswho secured the seals and walrus samples. The late Chris-tian Overgaard Nielsen, Jonas Teilmann, Maja Kirkegaardand Hardy Larsen assisted two of the authors on the agedeterminations. Blanca Mora Alvarez assisted with samplepreparation for stable isotope analyses which were per-formed by Myles Srocki at the University of SaskatchewanDepartment of Soil Science.

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