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Total mercury, organic mercury and selenium in liver andkidney of a South American coastal dolphin

Tercia G. Seixas a,b,*, Helena do A. Kehrig b, Monica Costa c, Gilberto Fillmann d,Ana Paula M. Di Beneditto e, Eduardo R. Secchi d,f, Cristina M.M. Souza e,

Olaf Malm b, Isabel Moreira a

a Departamento de Quımica, PUC-Rio, 22453-900 Rio de Janeiro, RJ, Brazilb Laboratorio de Radioisotopos Eduardo Penna Franca, IBCCF-UFRJ, 21949-900 Rio de Janeiro, RJ, Brazil

c Departamento de Oceanografia, CTG-UFPE, 50740-550 Recife, PE, Brazild Departamento de Oceanografia, FURG, C.P. 474, 96201-900 Rio Grande, RS, Brazil

e Laboratorio de Ciencias Ambientais, CBB-UENF, 28013-602 Campos dos Goytacazes, RJ, Brazilf Laboratorio de Tartarugas e Mamıferos Marinhos, Departamento de Oceanografia e Museu Oceanografico ‘‘Prof. E.C. Rios’’,

FURG, 96201-900 Rio Grande, RS, Brazil

Received 1 December 2007; accepted 24 January 2008

This study presents some useful data on the bioaccumulation of Hg, organic Hg and Se, and theirinter-element relationships in a Brazilian small cetacean.

Abstract

Selenium and total and organic mercury were determined in the liver and kidney of franciscana dolphin (Pontoporia blainvillei) incidentallycaught in fishing nets along two Brazilian coastal areas (southeast and south). Regional differences in the concentrations of these contaminantswere observed in P. blainvillei. Liver showed the highest organic and total mercury. In general, samples of individuals collected at the southernof Brazil had the highest concentrations of selenium and total and organic mercury. No significant gender differences were observed. Growthstage influenced the accumulation of these contaminants in both organs, and hepatic concentrations increased with the body length, according tothe sampling area. Molar mercury and selenium concentrations in liver were significantly correlated, with a Se:Hg ratio close to 4. The among-site differences we found may be related to differences in preferred prey, bioavailability in the marine environment, environmental conditions, orthese individuals may belong to distinct populations.� 2008 Elsevier Ltd. All rights reserved.

Keywords: Brazilian coast; Franciscana dolphin; Trace elements; Organic mercury; Ecological and biological factors

1. Introduction

Mercury is an exogenous and harmful metal, which accu-mulates in the tissues of higher food web organisms (such asmarine mammals) as they grow (Caurant et al., 1994; Haragu-chi et al., 2000; Kunito et al., 2004; Feroci et al., 2005).

Conversely, selenium is recognized as an essential element(WHO, 1987) for metabolic activity of aquatic mammals,acting as a protective agent against the toxicity of exogenousmetals such as mercury (USA/EPA, 1998; Feroci et al.,2005). Studies have shown that selenium may reduce the avail-ability of mercury, as methylmercury, blocking it in insolublecompounds (Feroci et al., 1997; Sasakura and Suzuki, 1998).

In the southwestern Atlantic some studies have documentedtrace element concentrations in the tissues of franciscana, Pon-toporia blainvillei (Marcovecchio et al., 1994; Gerpe et al.,2002; Lailson-Brito et al., 2002; Kunito et al., 2004; Seixas

* Corresponding author. Laboratorio de Radioisotopos Eduardo Penna

Franca, IBCCF-UFRJ, 21949-900 Rio de Janeiro, RJ, Brazil. Tel./fax: þ55

21 2561 5339.

E-mail address: tercia@rdc.puc-rio.br (T.G. Seixas).

0269-7491/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.envpol.2008.01.030

Available online at www.sciencedirect.com

Environmental Pollution 154 (2008) 98e106www.elsevier.com/locate/envpol

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et al., 2007b), and the present research adds new insight to andcomplements the existing studies.

Pontoporia blainvillei (Gervais & D’Orbigny, 1844) isa coastal small cetacean endemic to the western South Atlanticfrom Gulf San Matias (42�100S), Penınsula Valdes, Argentina(Crespo et al., 1998) up to Itaunas (18�250S), southeastern Bra-zil (Siciliano, 1994). It is a dolphin species which inhabitsmainly shallow waters (up to around 30 m or a little further)(Secchi et al., 2003b), making it particularly vulnerable toanthropogenic activities, especially fishing activities (Secchiet al., 2001, 2003a).

This study assesses: (1) total (Hg), organic mercury(OrgHg) and selenium (Se) concentrations in the liver and kid-ney of this small cetacean, the franciscana (P. blainvillei); (2)regional differences in accumulation of these elements; (3) theinfluence of ecological and biological factors, such as bodylength, gender and geographical area on these concentrations;and (4) inter-element relationships in liver and kidney of thefranciscana dolphin and its possible influence on trace elementbio-accumulation.

2. Materials and methods

2.1. Sampling

Liver and kidney samples of 31 individuals of franciscana incidentally

caught in fishing nets were freeze dried and analyzed for selenium (Se), total

(Hg) and (OrgHg) organic mercury. Eighteen individuals were caught off the

Brazilian southeastern coast (SE) (21�180Se25�250S) from 1998 to 2005, and

thirteen off the Brazilian southern coast (S) (29�300Se33�300S) between 2003

and 2004 (Fig. 1). The samples were supplied by the Biological Specimen

Banking from the Bioscience and Biotechology Center (UENF) and the

Department of Oceanography (FURG).

Samples covered mature and immature individuals with body lengths vary-

ing from 68 to 147 cm (southeast) and 99.5 to 153.5 cm (south). Immature

individuals (71%) and males (65%) predominated (Table 1).

2.2. Chemical analysis

Total mercury (Hg) concentrations in the liver and kidney samples were

determined by cold vapor atomic absorption spectrometry with a Flow Injec-

tion Mercury System (FIMS) e FIAS 400 (Perkin Elmer, USA) equipped with

an auto sampler AS90 (Perkin Elmer, USA) and using sodium borohydride as

a reducing agent (Kehrig et al., 2006). Analysis of organic mercury (OrgHg)

was conducted according to Wagemann et al. (2000). Samples were mixed

with an aqueous solution of acid sodium bromide and cupric sulfate to release

all the organic forms of mercury (Uthe et al., 1972), and then extracted into

dichloromethane (DCM) e hexane phase. An aliquot (1 mL) of the organic

phase was removed and then heated with a mixture of acids (HNO3-H2SO4;

1:4 v/v) in water bath for 30 min at 60 �C. The organic solvent mixture was

evaporated without detectable loss of OrgHg. The remaining phase was

digested and analyzed for total mercury.

Selenium concentrations in the tissues were determined by graphite fur-

nace atomic absorption spectrometry, using an Analytic Jena Model ZEEnit

60 spectrometer (Analytik Jena, Germany) with Zeeman Effect background

correction equipped with MPE-52 auto sampler. Palladium nitrate was used

as a chemical modifier (Seixas et al., 2007a).

Precision and accuracy of the analytical methods were determined and

monitored using certified material from the National Research Council-Can-

ada (DORM-2, Dogfish muscle sample and TORT-2, Lobster hepatopancreas).

Results for total mercury DORM-2 (N ¼ 12) were 4.60 � 0.45 mg kg�1, and

the CRM has a certified T-Hg value of 4.64 � 0.26 mg kg�1. For TORT-2

(N ¼ 10), total mercury found was 0.29 � 0.03 mg kg�1 and the CRM has

a certified T-Hg value of 0.27 � 0.06 mg kg�1. Our results for OrgHg were

4.39 � 0.46 mg kg�1 (N ¼ 7). In the muscle sample, DORM-2, around

100% of the organic mercury is methylmercury (MeHg) that CRM value is

4.47 � 0.32 mg kg�1. The mean of our OrgHg results corresponded to 98%

of the CRM MeHg certified value. Our results for OrgHg TORT-2 were

0.145 � 0.022 mg kg�1 (N ¼ 5). The mean of our OrgHg results corresponded

to 95% of the CRM MeHg certified value. Results for selenium DORM-2

(N ¼ 12) were 1.38 � 0.09 mg kg�1, and the CRM has a certified Se value

of 1.40 � 0.09 mg kg�1. For TORT-2 (N ¼ 6), selenium found was

5.38 � 0.51 mg kg�1 and the CRM has a certified selenium value of

5.63 � 0.67 mg kg�1. The coefficient of variation (SD/mean) for the duplicate

samples was less than 10%.

2.3. Statistical analyses

Statistical analyses were performed using STATISTICA� 6.0 for Windows

(StatSoft, Inc. 1984e2001, USA). Data were tested for normal distributions

and non-parametric tests were then applied. The analysis of variance was

done by KruskaleWallis ANOVA followed by a post-hoc test (ManneWhit-

ney test). Multiple regressions (R2) were performed to determine the relation-

ships between body length and concentrations of individual trace elements in

the two organs, inter-element relationships (on a molar basis) in both organs

and relationships of individual trace element concentrations within the two tis-

sues, liver and kidney. A p value of less than 0.05 was chosen to indicate sta-

tistical significance. Values are presented as median and range (minemax) of

dry weights.

3. Results and discussion

The concentrations of selenium (Se), total mercury (Hg)and organic mercury (OrgHg) found in the tissues of fran-ciscana were of the same order of magnitude as thosereported in earlier studies with the same species from Braziland Argentina (Table 2).

Among sites, no significant difference ( p > 0.05) in bodylengths was found, making it possible to directly compareamong-site differences in concentrations of these elements(Monaci et al., 1998).

Selenium and total and organic mercury in female tissueswere not significantly different from concentrations found inmales ( p > 0.05) from both populations. Normally, marinemammals present no gender differences in the accumulationof trace elements (O’Shea, 1999), as seen for the franciscanafrom the southeastern (Lailson-Brito et al., 2002a; Seixaset al., 2007b) and southern-southeastern coast of Brazil(Kunito et al., 2004; Seixas et al., 2007b) and also the BuenosAires Province coast, Argentina (Gerpe et al., 2002).

In this study, regional differences in the accumulation ofselenium and total and organic mercury were also observedin franciscana. In general, samples of individuals collectedin southern Brazil had the highest concentrations of all threeelements. This may be because the individuals belong to twodistinct genetic and demographic populations (e.g. Secchiet al., 2003b), which have different nutritional, developmentaland reproductive characteristics (e.g. Ramos et al., 2000;Barreto and Rosas, 2006; Danilewicz, 2003; Secchi et al.,2003b). Despite the fact that animals collected from both areashad similar body lengths, they were not necessarily the sameage. At a similar size, individuals from the southeastern coastwere likely older than those from the southern coast.

99T.G. Seixas et al. / Environmental Pollution 154 (2008) 98e106

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Franciscanas from Rio de Janeiro (southeast) reach asymptoticlength when they are approximately 2 years old and 115 cm(male) or 3 years old and 130 cm long (female) (Di Benedittoand Ramos, 2001). On the other hand, individuals from RioGrande do Sul (south) attain asymptotic length when theyare 4 years old and about 130 cm long (male) or 6 years oldand 146e152 cm long (female) (Barreto and Rosas, 2006).These differences are not only clinal since individuals fromintermediate areas (i.e. Santa Catarina, Parana and S~ao Paulostates) are smaller than those from both Rio Grande do Suland Rio de Janeiro (e.g. Secchi et al., 2003b). Genetic dataindicate strong differences between individuals from thesetwo areas (Secchi et al., 1998; Ott, 2002; Lazaro et al.,2004). This may be the reason that individuals from the south-eastern coast had the lowest concentrations of these traceelements.

The lower trace element concentrations found in francis-cana tissues from the southeastern Brazilian coast might be

because they feed mainly on teleost fish (Stellifer sp., Anchoafilifera, Pellona harroweri and Isopisthus parvipinnis)(Di Beneditto and Ramos, 2001); in contrast southern francis-canas feed both on teleost fish (Cynoscion guatucupa, Micro-pogonias furnieri, Urophycis brasiliensis) and cephalopods(Loligo sanpaulensis) (Rodriguez et al., 2002). Althougha number of physiological and environmental factors mayalso have some influence, the feeding behavior (based onfish and cephalopods rich in Hg and Se) and the complexityof the marine food chain are the major factors responsiblefor the bioaccumulation of trace elements in long-lived marineanimals (Shibata et al., 1992; Bustamante et al., 1998; Monaciet al., 1998).

3.1. Mercury

Liver had higher mercury concentrations than kidney andthis finding was consistent with previous studies on

Brazil

Atlan

tic O

cean

Pacific O

cean

N

Argentina

Chi

le

Rio de Janeiro

São Paulo

SantaCatarina

Rio Grandedo Sul

Uruguay

Capture areas

SE

S

Fig. 1. Areas along the Brazilian coast where the individuals of Pontoporia blainvillei were captured between 1998e2005 (SE) and 2003e2004 (S).

100 T.G. Seixas et al. / Environmental Pollution 154 (2008) 98e106

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franciscana from Argentina (Gerpe et al., 2002) and Brazil(Lailson-Brito et al., 2002; Seixas et al., 2007b). Individualsof the southeastern coast had mercury concentrations in liverabout two times as high as those found in their kidney,whereas franciscana from the southern coast had hepatic mer-cury concentrations that were about nine times as high as thosefound in the kidney. Probably, the higher hepatic mercury con-centrations observed in these individuals are related to the roleplayed by the liver of marine mammals in terms of storage andbiotransformation (Thompson, 1990). Studies have beenshown that organic mercury, especially methylmercury,obtained from the diet is transformed into less toxic inorganicforms, such as inert mercuric selenide (HgSe), in the liver ofmarine mammals (Ikemoto et al., 2004; Kunito et al., 2004).Moreover, detoxification of mercury may also result frominteractions with metallothioneins and formation of granuleswithin hepatocytes (Andre et al., 1990).

There was a significant positive correlation (R2 ¼ 0.32;p < 0.01) between the hepatic and renal concentrations of

mercury in franciscana from both sampling areas, indicatingproportional accumulation of mercury in these two organs.Other studies have also reported significant inter-tissuerelationships for the concentrations of mercury in marinemammals (Meador et al., 1999; Roditi-Elasar et al., 2003;Ikemoto et al., 2004; Seixas et al., 2007b).

The effect of body length on the accumulation of mercurywas also investigated. A significant positive correlation( p < 0.05) was observed between the length and hepatic mer-cury in individuals from the southeast (Fig. 2a) but not thesouth (Fig. 2b), whereas individuals from the south (but notsoutheast) had a significant positive correlation ( p < 0.05)between length and concentrations of mercury in kidney(Fig. 3b).

The relationship between body length and Hg concentra-tions in liver and kidney of different species of marinemammal has been extensively examined (Leonzio et al.,1992; Caurant et al., 1994; Dietz et al., 1996; Monaciet al., 1998; O’Shea, 1999; Gerpe et al., 2002; Lailson-Brito

Table 1

Biological data of the Pontoporia blainvillei individuals accidentally caught along the coast of Rio de Janeiro State (SE) and Rio Grande do Sul State (S)

Area Growth stage Sex N� individuals Body length

(minemax in cm)

SE Immature Male 8 82.0e113.0

Female 4 68.0e107.0

Mature Male 3 116.0e123.0

Female 2 135.0e147.0

Not identified 1 103.0

Total number

of individuals

18

S Immature Male 6 101.0e126.0

Female 4 99.5e132.2

Mature Male 2 130.0e133.0

Female 1 153.5

Total number

of individuals

13

Table 2

The range of concentrations of total mercury (Hg), organic mercury (OrgHg) and selenium (Se) (mg g�1 dry wt.) in the tissues of Pontoporia blainvillei collected

from some regions of the South America

Tissues Hg OrgHg Se Location Reference

Liver (0.66e9.65)a (0.12e2.36)a (0.84e9.05)a Southeast Brazil This study

Kidney (0.45e5.11)a (0.16e1.13)a (1.49e11.58)a

Liver (1.23e51.65)a (0.05e4.21)a (3.76e54.33)a South Brazil This study

Kidney (0.62e4.70)a (0.26e1.82)a (6.79e12.33)a

Liver 12.7b e e Argentina Marcovecchio et al., 1994

Liver 29.01b e e Argentina Gerpe et al., 2002

Kidney 7.62b e eLiver (2.97e24.42)a e e Southeast Brazil Lailson-Brito et al., 2002

Kidney (1.85e18.06)a e

Liver 3.5b 2.0b 9.1b South Brazil Kunito et al., 2004

Liver 2.6b e 3.2b Southeast Brazil Seixas et al., 2007b

Kidney 1.4b e 7.0b Southeast Brazil Seixas et al., 2007b

Liver 10.7b e 11.1b South Brazil Seixas et al., 2007b

Kidney 1.7b e 8.8b South Brazil Seixas et al., 2007b

Wet weight basis concentration was converted to dry weight basis concentration assuming that moisture content was 69.7% (Yang and Miyazaki, 2003).a Minemax.b Mean.

101T.G. Seixas et al. / Environmental Pollution 154 (2008) 98e106

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et al., 2002; Szefer et al., 2002; Monteiro-Neto et al., 2003;Ikemoto et al., 2004; Kunito et al., 2004; Seixas et al.,2007b). In general in long-lived marine mammals, thehepatic Hg concentration increases with age (Ikemotoet al., 2004) because the biological half-life is long in theseanimals and due to the strong affinity of mercury to the SHgroup in cysteine.

The growth stage seems to influence mercury accumulationin both tissues of franciscana. This pattern was also observedin previous studies with franciscana from Brazil (Kunito et al.,2004).

A significant regional difference ( p < 0.05) was observedbetween mercury concentrations in the liver of mature individ-uals from both areas. In southeast, the median hepatic concen-tration was 4.38 mg g�1 dry wt. (ranging from 0.66 to9.65 mg g�1 dry wt.), whereas in the south was 31.13 mg g�1

dry wt. (ranging from 13.52 to 51.65 mg g�1 dry wt.). Imma-ture individuals also presented a significant regional difference( p < 0.05) in their hepatic mercury levels. In southeast, themedian hepatic concentration was 1.54 mg g�1 dry wt. (rang-ing from 0.83 to 2.59 mg g�1 dry wt.), whereas in the southwas 2.89 mg g�1 dry wt. (ranging from 1.23 to 13.81 mg g�1

dry wt.). However, no significant regional difference( p > 0.05) was observed between the concentrations of mer-cury in kidney of mature and immature individuals.

3.2. Organic mercury

We found higher organic mercury concentrations (OrgHg)in liver than kidney in both areas. In southeast, the median he-patic concentration was 0.75 mg g�1 dry wt. (ranging from0.15 to 2.22 mg g�1 dry wt.), whereas the renal was0.67 mg g�1 dry wt. In the south the median hepatic concentra-tion was 1.25 mg g�1 dry wt. (ranging from 0.05 to4.21 mg g�1 dry wt.), whereas the renal was 0.52 mg g�1 drywt. (ranging from 0.26 to 1.82 mg g�1 dry wt.). However, inboth areas, approximately half of the total mercury in the kid-ney of franciscana was present as organic mercury (51%),whereas in the liver 38% (from 7% to 81%) of the total mer-cury was present as organic mercury. The same pattern wasobserved by Kunito et al. (2004) with this dolphin speciesfrom the Brazilian coast, indicating that liver plays effectiveprocesses of mercury detoxification and storage of inorganicforms.

In this study, OrgHg concentrations in liver were not signif-icantly related ( p > 0.05) to the concentrations found in kid-ney of franciscana within the same sampling area.

Kunito et al. (2004) has reported that concentrations ofOrgHg in the liver increase with body length of franciscana.In the present study, a similar pattern was observed only inthe liver of individuals from the south area (Fig. 2b), whereasno significant correlation ( p > 0.05) was found between bodylength and concentrations of OrgHg in kidney of individualsfrom the southeast (Fig. 3a,b).

In the present study, we observed that in both internalorgans the proportion of organic mercury to total mercury(% OrgHg) showed pronounced decreases with the growthstage. Mature adults had the lowest % OrgHg, ranging from5 to 30%. These low proportions suggest more efficient deme-thylation/elimination processes of the most toxic forms ofmercury in these organs.

3.3. Selenium

Information on selenium concentration in tissues of francis-cana is still very limited in the literature, and few data abouthepatic (Kunito et al., 2004; Seixas et al., 2007b) and renal(Seixas et al., 2007b) selenium in this coastal dolphin arereported.

In this study, individuals from the southeastern area hadhigher hepatic selenium concentrations than those in their kid-ney; selenium concentrations in liver were about two timeshigher than concentrations measured in the kidney. It is well

Southeast

HgSeOrgHg

60 70 80 90 100 110 120 130 140 150 160L (cm)

L (cm)

-2

0

2

4

6

8

10

12

(

g.g

-1 d

ry w

t.)

(

g.g

-1 d

ry w

t.)

L x Hg: R2 = 0.25; p = 0.03; y = -3.82 + 0.06*xL x Se: R2 = 0.33; p = 0.01; y = -3.09 + 0.06*xL x OrgHg: R2 = 0.17; p = 0.09; y = -0.72 + 0.02*x

South

HgSeOrgHg

90 100 10 120 130 140 150 160-10

0

10

20

30

40

50

60L x Hg: R2 = 0.23; p = 0.10; y = -46.86 + 0.48*xL x Se: R2 = 0.17; p = 0.17; y = -34.51 + 0.38*xL x OrgHg: R2 = 0.84; p = 1 x 10-5; y = -6.89 + 0.07*x

a

b

Fig. 2. Relationship between trace element concentrations in the liver of the

Pontoporia blainvillei and body length e L (cm).

102 T.G. Seixas et al. / Environmental Pollution 154 (2008) 98e106

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known that marine mammals tend to exhibit high concentra-tions of selenium in their liver and kidney because this traceelement is involved in detoxification mechanism of heavymetals, such as mercury, lead and cadmium, in both organs(Arai et al., 2004).

No significant correlation ( p > 0.05) was found betweenliver and kidney selenium concentrations for franciscanawithin the same sampling area. Previous studies (Ikemotoet al., 2004; Seixas et al., 2007b) also reported no significantinter-tissues relationship for selenium in different species ofmarine mammals.

Data analysis from the two sampling populations showedthat hepatic and renal selenium concentrations increasedwith body length for the southeastern individuals, whereasno significant correlations ( p > 0.05) were found betweenbody length of individuals from the southern coast and their

hepatic or renal selenium concentrations. In the literature,the relationship between body length and selenium concentra-tions in the tissues of franciscana is still scarce, and few stud-ies have reported an increase in hepatic (Kunito et al., 2004;Seixas et al., 2007b) and renal (Seixas et al., 2007b) seleniumwith body length of franciscana.

The growth stage seems to influence selenium accumula-tion in both tissues of the franciscana collected in our study.This is consistent with previous studies with franciscana(Kunito et al., 2004).

A significant regional difference was observed betweenselenium concentrations in the livers of mature individualsfrom both areas. In southeast, the median hepatic concentra-tion was 4.90 mg g�1 dry wt. (ranging from 0.84 to9.05 mg g�1 dry wt.), whereas in the south was 19.38 mg g�1

dry wt. (ranging from 12.77 to 54.33 mg g�1 dry wt.). Imma-ture individuals also had a significant regional difference( p < 0.05) in their hepatic selenium levels. In southeast, themedian hepatic concentration found was 2.48 mg g�1 dry wt.(ranging from 1.16 to 4.26 mg g�1 dry wt.), whereas in thesouth this median was 4.75 mg g�1 dry wt. (ranging from3.69 to 10.70 mg g�1 dry wt.). However, no significant regionaldifference ( p > 0.05) was observed between the concentra-tions of selenium in kidney of mature or immature individuals.

3.4. Relationships

3.4.1. Organic mercury to mercury relationshipAn inverse and significant linear relationship was found

between the total mercury (Hg) and the percentage of organicmercury to total mercury (% OrgHg) in the liver of franciscanafrom the southern part of the country (R2 ¼ 0.52; p < 0.001),i.e. the highest hepatic Hg corresponded to the lowest %OrgHg. Individuals from the southeastern area showed thesame trend. These inverse correlations could be associatedwith the efficient processes of demethylation and/or elimina-tion of the most toxic organic form of mercury, methylmercuryin these internal organs. In contrast, no significant relation-ships were observed between Hg and the % OrgHg in the kid-ney of franciscana from both sampling areas.

3.4.2. Selenium to mercury relationshipsA significant positive linear relationship was found between

the molar concentrations of selenium and mercury in the liverof individuals from both areas (Fig. 4a,b). However, only indi-viduals from the southern area had a significant correlationbetween molar concentrations of selenium and mercury in kid-ney (Fig. 4b). This high positive correlation between mercuryand selenium in these organs of marine mammals is wellknown (Caurant et al., 1994; Wagemann et al., 1998; Meadoret al., 1999; Capelli et al., 2000; Das et al., 2000; Dietz et al.,2000; Endo et al., 2002; Brunborg et al., 2006). It could bereflecting a direct association between these two elements inthe liver and kidney, which could be explained by the essentialroles of selenium besides its detoxification function againstmercury mainly in the liver (Arai et al., 2004).

Southeast

HgSeOrgHg

60 70 80 90 100 110 120 130 140 150 160L (cm)

L (cm)

-2

0

2

4

6

8

10

12

14

b

a

L x Hg: R2 = 0.13; p = 0.21; y = -0.77 + 0.02*xL x Se: R2 = 0.31; p = 0.04; y = -3.45 + 0.10*xL x OrgHg: R2 = 0.01; p = 0.72; y = 0.38 + 0.002*x

South

HgSeOrgHg

90 100 110 120 130 140 150 160-2

0

2

4

6

8

10

12

14L x Hg: R2 = 0.53; p = 0.005; y = -6.71 + 0.07*xL x Se: R2 = 0.24; p = 0.09; y = 1.42 + 0.06*xL x OrgHg: R2 = 0.46; p = 0.01; y = -1.83 + 0.02*x

(

g.g

-1 d

ry w

t.)

(

g.g

-1 d

ry w

t.)

Fig. 3. Relationship between trace element concentrations in the kidney of the

Pontoporia blainvillei and body length e L (cm).

103T.G. Seixas et al. / Environmental Pollution 154 (2008) 98e106

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In the present study, selenium concentrations were higher thanthose of mercury in the liver, on a molar basis (Se/Hg molar ra-tio ¼ 4:1). On the other hand, in the kidney of franciscana, sele-nium concentrations were much higher than those of mercury onmolar basis (Se/Hg molar ratio ¼ 16:1). Some studies have re-ported a 1:1 molar ratio of selenium and mercury in the liver ofdifferent species of marine mammals (Leonzio et al., 1992; Palm-isano et al., 1995; Meador et al., 1999; Capelli et al., 2000; Dietzet al., 2000; Wagemann et al., 2000; Cardellicchio et al., 2002;Endo et al., 2002; Bustamante et al., 2003; Law et al., 2003;Kehrig et al., 2004), as was also seen for other dolphin species(Sotalia guianensis) from the Brazilian coast (Kehrig et al.,2004; Kunito et al., 2004). However, in previous studies with fran-ciscana from the Brazilian coast, selenium concentrations wereeven higher than those of mercury (Kunito et al., 2004; Seixas

et al., 2007b). According to Ikemoto (2004), the Se/Hg molar ra-tio is close to unity only in the marine mammals with hepatic mer-cury concentrations higher than 200 mg g�1 dry wt. The resultsfound with franciscana were lower (max. 51.65 mg g�1 dry wt.)than those reported by Ikemoto et al. (2004).

3.4.3. Selenium to organic mercury relationshipsIn the present study, significant positive linear relationships

were found between the molar concentrations of selenium andorganic mercury in the liver of franciscana from the southeast-ern and southern populations (R2 ¼ 0.69; p < 0.0001 andR2 ¼ 0.31; p < 0.05, respectively). The high correlationscould be reflecting the existence of detoxification mechanismsused for both selenium and organic mercury, as methylmer-cury (Arai et al., 2004).

Selenium concentrations were higher than those of organicmercury in the liver, on a molar basis (Se/OrgHg molarratio ¼ 23:1). The same pattern was observed in the kidneyof franciscana, where selenium concentrations were alsohigher than those of organic mercury on molar basis (Se/Hgmolar ratio ¼ 44:1).

4. Conclusions

Based on the results presented here, we can conclude thatthe life span and growth stage significantly affected the accu-mulation of selenium, total mercury and organic mercury inthe internal organs of franciscana from the Brazilian coast.

The differences found in the hepatic and renal concentra-tions of selenium, total mercury and organic mercury betweenthe individuals from the southern and southeastern Braziliancoast may be attributed to differences in these distinct popula-tions, the predominant environmental conditions (such as wa-ter temperature and primary production), and also to otherfactors, such as the geological formations and upwelling,that also influence trace element concentrations in food web.But differences in prey choices and availability across loca-tions is likely more important. Probably, the preys availablefor individuals from the southern Brazilian coast are richerin mercury and selenium than those ones available for individ-uals from the southeastern coast.

Acknowledgements

The authors would like to thank the Conselho Nacional deDesenvolvimento Cientıfico e Tecnologico (CNPq) for finan-cial support through the course of this work. H.A. Kehrigwas sponsored by FAPERJ (E-26/150187/2006) and CNPq(151634/2006-8). G. Fillmann was sponsored by CNPq (PQ303218/2003-7), and A.P.M. Di Beneditto by CNPq (PQ305160/2006-0). The authors thank Dr. Karen Kidd for the in-sightful comments on the final version of the manuscript.

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0 20 40 60 80 100 120

Se (nmol.g-1

)

0

10

20

30

40

50a

b

Hg

(n

mo

l.g

-1)

Se:Hg (Liver): R2 = 0,82; p = 2 x 10-7; y = -4.06 + 0.42*x2Se:Hg (Kidney): R2 = 0,05; p = 0.42; y = 4.41 + 0.05*x

South

0 100 200 300 400 500 600 700 800

Se (nmol.g-1

)

0

50

100

150

200

250

300

Hg

(n

mo

l.g

-1)

Se:Hg (Liver): R2 = 0.92; p = 2 x 10-7; y = -4.02 + 0.41*xSe:Hg (Kidney): R2 = 0.54; p = 4 x 10-3; y = 4.26 + 0.03*x

LiverKidney

LiverKidney

Fig. 4. Relationship between molar concentrations of selenium and mercury in

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