Ecotoxicology and Environmental Safety 97 (2013) 17–25

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Ecotoxicology and Environmental Safety

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Inverse mercury and selenium concentration patterns betweenherbivorous and piscivorous fish in the Tapajos River, Brazilian Amazon

D. Sampaio da Silva a,n, M. Lucotte b, S. Paquet b, G. Brux b, M. Lemire c

a CFI, Universidade Federal do Oeste do Pará, Brazilb GEOTOP, Université du Québec à Montréal, Canadac CINBIOSE, Université du Québec à Montréal, Canada

a r t i c l e i n f o

Article history:Received 11 May 2012Received in revised form23 June 2013Accepted 24 June 2013Available online 6 August 2013

Keywords:AmazonTapajósFishMercurySelenium

13/$ - see front matter & 2013 Elsevier Inc. Alx.doi.org/10.1016/j.ecoenv.2013.06.025

espondence to: Universidade Federal do Oça Furtado, no. 2946, 68040-470 Santarém, Pail address: sampaio.delaine@gmail.com (D. S

a b s t r a c t

We evaluated the levels of selenium and mercury in five fish species commonly eaten by localpopulations of the Brazilian Amazon. Fish specimens were sampled in two lotic and three lentic areasat two different phases of the hydrological cycle. Analyses of Carbon and Nitrogen stable isotopesallowed us to confirm the trophic levels of the fish species (one herbivorous, two omnivorous and twopiscivorous) and verify that these levels remained unchanged with the habitats and the season. Thelevels of selenium and mercury in fish varied from 50 ng/g to 1006 ng/g and from 17 ng/g to 3502 ng/grespectively. For both seasons, fish from lotic ecosystems presented higher selenium concentrations. Aninverse pattern was observed between selenium and mercury concentrations within the trophic chain,and this in both seasons. Indeed, the highest mean concentrations of selenium and lowest meanconcentrations of mercury were measured in the herbivorous species and the opposite in the piscivorousspecies. Our results unequivocally demonstrate that local riverside populations will maximize theselenium health benefits of eating fish while minimizing their risk of being chronically exposed tomercury by preferentially consuming herbivorous species and to some extent omnivorous species, whileavoiding piscivorous species.

& 2013 Elsevier Inc. All rights reserved.

1. Introduction

Several studies suggest that ingestion of selenium (Se) by aquaticorganisms up to a certain toxic threshold may counteract the toxiceffects of mercury (Hg) (Cuvin–Aralar and Furness, 1991; Yang et al.,2008; Berry and Ralston, 2008; Khan and Wang, 2009). In theindustrialized region of Sudbury, Canada, Chen et al. (2001) showedthat total mercury content (THg) was inversely related to Se contentin muscle tissue of yellow perch (Perca flavescens) and pike (SanderVitreus). The results obtained by the same research group led to theconclusion that Se plays an important role in limiting the assimila-tion of Hg at the low end of aquatic food chains (Belzile et al., 2006).Possible mechanisms of fish protection (or non-protection) to Hgexposure by Se have been reviewed by Cuvin-Aralar and Furness(1991), while recent laboratory and field works have been reviewedby Yang et al. (2008). Interaction between Se and Hg in aquaticorganisms is real but the true antagonism between these twoelements has not yet been clearly shown (Pelletier, 1986). Morerecently Belzile et al. (2009) proposed that Se concentrations in lake

l rights reserved.

este do Pará, CFI, Avenidaará, Brazil.ampaio da Silva).

ecosystems could be considered as an indicator of the susceptibilityof aquatic organisms to Hg toxicity.

The dynamics of Se in aquatic food chains have been docu-mented by several authors (Skorupa, 1994; Lemly, 1999; Sorensen,1988; Hamilton, 1999). It has been suggested that microorganismsassociated with the detritus of sediments can be important for theincorporation of Se in aquatic food chains (Bender et al., 1991;Alaimo et al., 1994; Bowie et al., 1996; Canton and Van Derveer,1997; De Souza et al., 1999; Hamilton and Lemly, 1999; Hamiltonand Buhl, 2003). Additionally, it has been shown that differencesin the structure of the base of the food chain of lentic and lotichabitats are the origin of different entry points of Se into thoseecosystems and further influence Se levels in the aquatic food web.Orr et al. (2006) demonstrated that higher-level organisms presentin lentic habitats accumulated much higher concentrations of Sethan organisms in lotic environments. In that particular study,concentrations of Se in aquatic primary producers and the lengthof the food chain in the two types of habitats were comparable.The cycle of Se in the dentric sediment, where Se is transformedinto selenomethionine and is subsequently transferred to superiortrophic levels, probably explains the higher concentrations of Se inthe tissue of fish species from lentic habitats compared to those inlotic habitats (Orr et al., 2006). Indeed, several studies have shownthat Se can be more easily assimilated in the food chain of lentic

Fig. 1. Localisation of the studied sites and delimitation of their respective watersheds(RES¼Restinga; CUP¼Cupari; ITA¼ Itapacurazinho; CAP¼Capituã; JAC¼ Jacaré).

D. Sampaio da Silva et al. / Ecotoxicology and Environmental Safety 97 (2013) 17–2518

habitats than that of lotic ones (Lillebo et al., 1988; Canton and VanDerveer, 1997; Lemly, 1999). In complement, seasonality alsoappears to influence the toxicity of dietary intakes of Se by aquaticorganisms (Lemly, 1999). For example, Heinz and Fitzgerald(1993a, b) also showed higher harmful effects of Se in adultmallards along with the stress of winter conditions.

Mercury exposure through fish consumption constitutes amajor health concern for riparian populations of the Amazon,particularly those of the Tapajós region (Passos and Mergler,2008). It has been suggested that Se ingestion within a safe doseinterval may constitute an antidote to Hg toxicity for thesepopulations (Raymond and Ralston, 2009; Lemire et al., 2010a,b). The objective of this study is to evaluate the levels of Se and Hgin five of the most abundant fish species, also frequently con-sumed by local populations. Up to now, very few studies haveaddressed the combined cycle of Se and Hg in tropical aquaticsystems, in particular in the Amazon. In this study, we consideredfive locations representative of the ecosystem of the Tapajós River(lentic and lotic sites), as well as a seasonal dimension (fallingwater season and low water season). We also took into considera-tion the complexity of the Amazonian aquatic food web, depend-ing upon the habitat, fluctuating with the seasons and evolvingwith the aging process of certain fish species as previouslydemonstrated by Sampaio da Silva et al. (2005, 2009) throughstable carbon and nitrogen analyses.

2. Methodology

2.1. Study area

This study was carried out in the lower Tapajós basin, located in the State ofPará, Brazilian Amazon. Studies showed that Hg dissemination and contaminationare not only related to gold mining activities, but also to the erosion of soilscontaining high levels of natural Hg (Roulet and Lucotte, 1995). Roulet et al. (1999)argued that more than 97% of the Hg accumulated in soils of the Tapajós basin isdue to natural sources. The Tapajós basin is an active colonization zone subjected toimportant demographic, cultural and environmental transformations over the last50 years (Farella, 2005). Today, family agriculture is the main form of farming inrural areas in the region, and is one of the principal agents of deforestation and soilerosion (Farella, 2005). For the present study, sampling took place along a 150 Kmsegment on the river, between the municipalities of Aveiro and Itaituba.

2.2. Sampling

Fish were captured with gillnets in two lotic and three lentic locations of theTapajós River using fishing methods detailed in Sampaio da Silva (2008). Themanipulations of the collected fish species were: species identification, measure-ments of total length (cm) and weight (g) and removal of a sample of the dorsalmuscle tissue without skin and bones. Samples were kept frozen at�20 1C on theresearch boat and were later sent to the GEOTOP laboratory of the University ofQuébec in Montréal. The samples were collected during two distinct periods of theyear (falling water season/July–August 2003 and low water season/October–November 2004). The lotic habitats are locally called: Itapacurazinho (ITA) andCupari (CUP) and the lentic habitats: Jacaré (JAC), Restinga (RES) and Capituã (CAP)(Fig. 1). During fishing activities at both lentic and lotic study sites, fish specimensrepresenting five orders, 21 families and 96 species were identified (Sampaio daSilva, 2008). From the compilation and the integration of these data with thosecharacterizing fishing activities and yields obtained by local fishermen (Sampaio daSilva, 2008), it seems that the most abundant fish species of this study are alsothose that are the most consumed by riparian populations. The fish species kept forour study were selected regarding their position in the trophic web as determinedby Ferreira (1998), their importance in the Tapajós ecosystems and their highconsumption frequency by the local residents. The numbers of fish specimens foreach targeted species (Geophagus proximus, Anostomoides laticeps, Hemiodus unim-aculatus, Plagioscion squamosissimus and Acestrorhynchus falcirostris) are given inTable 1.

2.3. Analysis of total Se content

The total Se contents were analyzed by hydride-generation atomic absorptionspectrometry. The samples were digested and reduced following an adaptation of

the analytical protocol outlined by Campos et al. (2002). According to this method,the reduced Se is transformed into a volatile hydride by reacting the sample withsodium tetrahydroborate sodium (NaBH4 0.2% and NaOH 0.05% w/v) in an acidicenvironment (HCl 10% v/v). The hydride was then oxidized into elementary Se in aquartz cell heated to 900 1C before it was measured. Precision and accuracy of theanalytical quality control of Se determination was ensured by the use of certifiedreference material DORM-2 (dogfish muscles) provided by the National ResearchCouncil of Canada. The accuracy of the results was checked daily by running threereplicates of the reference material. Measured Se concentrations on referencematerial did not exceed 5% difference with respect to the certified value. The resultswere expressed in wet weight. The limit of detection of the apparatus is 0.2 ppb insolution.

2.4. Analysis of total Hg content

The Hg content of the lyophilized muscle samples was measured using coldvapour atomic fluorescence spectrometry (CV-AAS), following a modificationmethod developed by Bloom and Fitzgerald (1988). The technique is described ingreater details in Montgomery et al. (1995) and Pichet et al. (1999). Briefly, thesamples were weighted and digested in an acid mixture. The digestate wasreturned to a final volume of 30 ml with purified water then analyzed by atomicfluorescence for total Hg content. Certified reference materials were analyzed in allsample batches. Measured values were always within the provided reference orcertified values (average certified standards¼272760, average obtained results¼269712). The results are expressed in wet weight. The total Hg detection limit is5 ng/g per 1 mg of dry sample.

2.5. Analysis of isotopic signatures

The analysis of isotopic signatures of the samples was conducted with massspectrometry. Briefly, the lyophilized samples were crushed and weighted in tincups. The isotopic compositions were expressed in relative differences in compar-ison to a reference standard. The unit δ * is referred to in parts per thousands.

*δX ¼ ½ðRsample=RstdÞ �1�X 103 ;with X ¼ 13C or 15N and

R¼ 13C:12C or 15N:14N:

2.6. Statistical analysis

Prior to statistical analysis with a General Linear Model (GLM), log10 transfor-mations were applied to Hg and Se concentrations. Normality and homogeneity ofvariance were tested on the distribution of Hg and Se concentrations between andwithin groups (seasons, locations, trophic status) for the GLM and for thecomparison of the means. Shapiro and Wilk W normality test was done to testnormality whereas homogeneity of variance was tested using Bartlett test. Log10transformations were also applied to linearize the relationships between Hg and Seconcentrations as response variables with respect to δ13C signatures. For the samereason, log10 transformations were applied to Hg concentrations as responsevariable in relation to Se concentrations as independent variable. No data

Table 1Summary of sample number and species captured in the Tapajós River. Carbon and nitrogen isotopic signatures (mean7standard deviation), total length, Hg and Seconcentrations (mean7standard deviation and range) in muscular tissues of 5 key species.

G. proximus (H) A. laticeps (O) H. unimaculatus (O) P. squamosissimus (P) A. falcirostris (P)

nN¼80 N¼51 N¼90 N¼69 N¼61

Mean S.D. Min Max Mean S.D. Min Max Mean S.D. Min Max Mean S.D. Min Max Mean S.D. Min Max

Length (cm) 16.8 4.8 8.0 26.0 19.1 3.4 11.5 25.0 17.7 3.7 10.5 28.2 280 6.4 12.1 40.0 21.1 5.1 15.0 35.0δ15N (per cent) 10.3 1.0 8.5 12.5 10.6 0.9 8.8 13.1 9.4 1.6 6.2 13.2 12.6 0.8 10.7 14.4 12.4 0.7 10.9 13.9δ 13C (per cent) –29.9 3.2 –36.4 –22.8 –31.7 2.4 –35.9 –25.9 –31.1 3.4 –36.7 –18.9 –29.4 1.6 –33.9 –27.0 –30.8 1.3 –33.8 –28.5Hg (ng/g wetweight)

152 86 41 390 205 158 55 703 109 105 17 709 573 398 130 2945 1000 701 48 3502

Se (ng/g wetweight)

323 173.7 76 1006 176 91.6 70 510 145 63.4 50 390 190 89.7 84 496 110 37 50 214

H¼herbivorous, O¼omnivorous, P¼piscivorous (feeding strategy according to Ferreira et al., 1998).n Number of specimens analyzed.

D. Sampaio da Silva et al. / Ecotoxicology and Environmental Safety 97 (2013) 17–25 19

transformation was done on isotopic data as they were normally distributed andthe variances were homogenous between tested groups.

Following the GLM analysis and when there were no more than two means tocompare, T-test was used in order to test the difference between the groups. Thedifference in Se contents of the different trophic groups was tested using Tukey-Kramer multiple means comparison test preceded by GLM analysis to highlight thesignificant effects of season (falling water and low water) as well as the geographiclocations of the study sites (5 locations) and possible interactions between factors.GLM and Tukey–Kramer tests were also used to verify the variation of Hg and Seconcentrations (log10) of the different trophic groups of fish species (piscivorous,omnivorous and herbivorous). Additionally, linear regression analyses were con-ducted in order to illustrate the relationships between Hg and Se concentrations infish muscles for the different trophic levels. All the statistic results were defined asstatistically significant if the probability Po0.05. Analyses were performed usingJMP 5.1 software (SAS Institute Inc.).

3. Results

3.1. Isotopic signatures

3.1.1. Influence of season, sampling site and fish trophic level onisotopic signatures

The isotopic signatures δ15N were generally higher in predatoryP. squamosissimus and A. falcirostris and lower in the omnivorousH. unimaculatus (Table 1). The most important variation of δ15Nwithin one fish species was observed in H. unimaculatus (Δ¼7%).Low δ 13C values were observed for all fish species of the studywithout any significant difference from one species to the next.Large variations of δ 13C values within each fish species wereobserved (H. unimaculatus: Δ¼18%; G. proximus: Δ¼14% andA. laticeps: Δ¼10%).

On the whole, a significant enrichment in 15N in all fish speciescaptured in lotic habitats was observed in comparison with thosecaught in lentic habitats (lotic fish: n¼131; lentic fish: n¼198,po0.0001), with the exception of A. laticeps (p¼0.0899, n¼26).No difference in the δ 13C isotopic signature was noted for eitherA. falcirostris or A. laticeps captured in either habitat. In contrast, asignificant enrichment in 13C for G. proximus and H. unimaculatuscaptured in lentic habitats compared to lotic habitats wasobserved (p¼0.0007, n¼35; p¼0.0280, n¼26 respectively). Addi-tionally, lower δ13C signatures were observed for P. squamosissimuscaptured in lotic habitats (po0.0001, n¼21) (Table 1).

An enrichment of 15N in A. falcirostris from ITA was observedduring the low water season compared with the falling water season(p¼0.0253, n¼18). During the falling water season a decrease of 15Nin A. falcirostris from JAC was observed in comparison to A. falcirostrisfrom CUP and ITA (p¼0.0021, n¼16). A highly significant spatialvariation was observed in the δ15N isotopic signature for theomnivorous H. unimaculatus. Specifically, higher δ15N values were

identified for fish samples coming from ITA and CUP compared withthose from RES, CAP and JAC (Table 1).

During the low water season, an enrichment of 15N in thepiscivorous A. falcirostris from ITA was identified in comparisonwith A. falcirostris from CAP and RES. The same situation wasfound in the case of the δ15N signature for G. proximus from ITAcompared with G. proximus from RES, CUP and CAP. δ15N signatureof H. unimaculatus from ITA and CUP was also significantly higherthan that of H. unimaculatus from RES and CAP. For P. squamosissi-mus from CAP a decrease of 15N was observed compared toP. squamosissimus captured in ITA and CUP. During the fallingwater season, G. proximus captured at CAP, CUP and ITA presentedvalues of δ13C that were clearly lower compared to the measuredvalues in fish collected from RES and JAC. This same speciescaptured in ITA, CAP and CUP presented lower values of δ13Ccompared with fish gathered at RES during the low water seasons.During the falling water season, H. unimaculatus collected in ITA,CAP and JAC presented low values of 13C compared to thoseobserved in fish from CUP and RES. Additionally, we observedsignificantly lower values of 13C in the same species during thefalling water season for fish from ITA compared with those fromRES. During the low water season, A. falcirostris from CAP and ITApresented significantly lower δ13C signatures than those of fishfrom RES. For A. laticeps from CAP the δ13C signatures were lowerthan those observed for fish collected from RES and CUP. ForP. squamosissimus, δ13C signatures in muscle tissue of fish from CAPwere clearly lower than those observed in fish from CUP and ITA.The isotopic signatures δ15N and δ13C observed in H. unimaculatus,G. proximus and A. laticeps were highly variable during the entirelength of their life cycles (Fig. 2).

3.1.2. Influence of fish trophic level on the relationship between Hgconcentrations and carbon isotopic signatures

A negative relationship between Hg concentrations and 13Cvalues was observed for herbivorous and omnivorous species, butnot for piscivorous species (p¼0.0657; n¼98). The relationshipbetween Hg content and Carbon isotopic signatures of the fishmuscles remained statistically significant for herbivorous fish(Fig. 3). The equations representing this relationship are as followsfor omnivorous and piscivorous fish captured during the fallingwater season and the low water season: Log(Hg(ng/g))¼1.013–0.065δ13C (R2¼0.07; p¼ 0. 0373; n¼67); Log(Hg(ng/g))¼0.893–0.038 δ13C(R2¼0.10; p¼ 0.0223; n¼50) and Log(Hg(ng/g))¼1.346–0.047 δ13C(R2¼0.09; p¼ 0. 0189; n¼58) and Log(Hg(ng/g))¼3.632–0.024 δ13C(R2¼0.01; p¼0.4873; n¼40) respectively.

Fig. 2. Carbon and nitrogen isotopic signatures of 5 fish species collected in theTapajós River during falling waters (2003) and low waters (2004). Bars representthe full range of the measured values isotopes for given species at the differentlocations and seasons.

D. Sampaio da Silva et al. / Ecotoxicology and Environmental Safety 97 (2013) 17–2520

3.2. Influence of seasons, sampling sites and fish trophic levels on Seand Hg levels in most abundant fish species from the Tapajós riverbasin

For both sampling seasons, a highly significant difference wasobserved between Hg and Se levels among the studied species. Theherbivorous species G. proximus presented the highest average Seconcentrations whereas A. falcirostris presented the lowest averageSe concentrations (Table 2). Predatory species A. falcirostris andP. squamosissimus presented higher average Hg concentrations,where as the omnivorous species H. unimaculatus presented loweraverage Hg concentrations (Table 2). The equations that representthe biomagnification of Hg during the falling water season and thelow water season are: Log(Hg(ng/g))¼0.227+0.191 δ15N (R2¼0.59;po0. 0001; n¼158) and Log(Hg(ng/g))¼-0.271+0.240 δ15N (R2¼0.62;po0.001; n¼126) respectively.

The concentrations of Se measured for P. squamosissimus fromCUP and JAC were more elevated during the low water seasoncompared to the falling water season (p¼0.0396, n¼13 andp¼0.0026, n¼26). The same situation was found for G. proximusfrom CUP (p¼0.0414, n¼10) and for H. unimaculatus from JAC(p¼0.0479, n¼24). Otherwise, higher Se concentrations during thefalling water season compared to the low water season wereobserved for H. unimaculatus from CAP (p¼0.0281, n¼10). Duringthe falling water season, H. unimaculatus gathered from ITA pre-sented higher Se concentrations than those measured in fish fromRES. During the low water season, H. unimaculatus collected in ITAand JAC presented higher concentrations compared to thoseobserved in fish from CAP. Specimens of P. squamosissimus collectedduring the falling water season at ITA presented higher Se con-centrations than those identified in fish from JAC. During the low

water season, G. proximus from CUP presented higher concentra-tions than did specimens from CAP and RES. A. falcirostris capturedin ITA and RES during the same season also presented superior Selevels than those measured from the same fish species from CAPand JAC (Table 2).

Fish Hg concentrations varied from 17 ng/g in H. unimaculatusto 3502 ng/g in A. falcirostris. Se concentrations varied from 50 ng/gin A. falcirostris and H. unimaculatus to 1006 ng/g in G. proximus.During the two sampling seasons, the average Se and Hg concen-trations were higher for the herbivorous species G. proximusand the predatory species A. falcirostris respectively (Table 1). Seconcentrations were also higher in herbivorous fish compared tofish from other trophic levels. No differences were observedbetween Se levels measured in omnivorous and piscivorous fish(Fig. 4). For Hg, the two piscivorous fish species were clearly morecontaminated than those from the other trophic groups, but nodifference was observed between herbivorous and omnivorousspecies (Fig. 5).

3.3. Relations between Hg and Se levels in most abundant fishspecies from the Tapajós river

Positive and significant relations were observed between Hgand Se concentrations both for herbivorous fish (R2¼0.10; p¼0.0057; n¼79; log (Hg ng/g)¼1.312+0.330 log (Se ng/g)) and omni-vorous fish (R2¼0.11; po0.0001; n¼113; log (Hg ng/g)¼0.703+0.610 log (Se ng/g)) (Fig. 6). This relationship was stronger forherbivorous fish during the low water season (R2¼0.15;p¼0.0091; n¼44) and was no longer significant during the fallingwater season (p¼0.4464; n¼35). Surprisingly, the positive rela-tionship became stronger with the omnivorous fish during thefalling water season (R2¼0.15; p¼0.0009; n¼70) and weaker, butstill significant, during the low water season (R2¼0.007; p¼0.0336; n¼63). No significant relationship was observed withthe piscivorous fish during either of the two studied seasons(falling water season: p¼0.2189, n¼63 and low water season:p¼0.3578, n¼63) (Fig. 6). However, taking into account the typeof habitat, an inverse relationship was observed between Hg andSe levels in piscivorous fish captured in lentic habitats (R2¼0.16;p¼0.0004; n¼75; log (Hg ng/g)¼4.070–0.6499 log (Se ng/g)).

4. Discussion

In the Amazon, fishing is a vital traditional subsistence activityfor local populations. Residents from the study area consume onaverage between 9 and 13 fish meals per week (Sampaio da Silvaet al., 2008; 2011). Fish diet also constitutes an important dietarysource of Se which is, contrary to Hg, an essential element up to acertain threshold for most of living organisms, including humans(World Health Organization, 1986).

In the current study, highest Se levels were observed for theherbivorous species and lowest ones in predatory species. InCachoeira do Piriá (State of Pará, Eastern Amazon), Lima et al.(2005) did not observe significant differences between Se levelsmeasured in piscivorous and non-piscivorous fish captured at twodifferent sites. Overall, Se levels in fish varied from 100 to 1210 ng/g, about the same range as that observed in the presentstudy, likewise realised in the State of Pará. It should be underlinedthat these authors only considered 95 specimens and that thenumber of samples considered for each species was often very low.Dorea et al. (1998) analyzed 7 fish species, belonging to 7 differenttrophic levels, captured at 12 sites on the Madeira River (State ofRondônia). The average concentrations of Se and Hg, depending onthe species, were from 0.38.7 to 245.6 ng/g and from 82.2 to1335.9 ng/g respectively. Se levels in this region were lower than

Fig. 3. Linear regression between log10 total Hg concentrations (ng/g w.w.) and Carbon isotopic signatures for herbivorous fish from the Tapajós River. The black and whitesymbols respectively represent the low waters and the falling waters. Each point is one individual fish.

D. Sampaio da Silva et al. / Ecotoxicology and Environmental Safety 97 (2013) 17–25 21

those found in the present study while Hg levels were in the samerange. Contrary to our results, Dorea et al., (1998) mentioned thatthe level of Se measured in the muscle tissue of omnivorous andpiscivorous species (Oxydoras niger, Crenicicla sp., Astronus sp.,Cichla ocellaris and Pseudoplatistoma fasciatus) were higher incomparison to the herbivorous species included in their study(Brycon spp. and Colossoma bidens). However, the number ofsamples collected at the 12 sites by these authors was minimal.Since Se content in fish can vary on the spatial scale depending onthe level of Se present in the soils and the bioavailability of Se atthe base of the food chain (Levander, 1987), these conclusionsmust be considered with caution. As in the Tapajós region, Doreaet al. (1998) observed a positive relationship between Se and Hgconcentrations in herbivorous fish (r¼0.716; p¼0.0088) from theMadeira River. However, no association was reported in omnivor-ous and piscivorous species (r¼�0.2032; p¼0.3407). In Canada,

Chen et al. (2001) also observed similar results to the presentstudy in lakes of the Precambrian Shield, where higher levels of Sewere found in a non-predatory species (perch), compared to apredatory species (pike).

Our results show that during the studied seasons, fish fromlotic ecosystems had higher Se levels than fish from lenticecosystems. In the case of Hg, fish from lotic systems alsopresented the highest concentrations of Hg, as reported inSampaio da Silva et al. (2009). Surprisingly, other studies men-tioned that Se could be more easily incorporated in the food chainin lentic areas than lotic ones (Lillebo et al., 1988; Canton and VanDerveer, 1997; Lemly, 1999). Such environments would be morefavourable to the accumulation of Se, to its biotransformation andto its remobilization in the dentric-benthic portion of the foodchain and also in the roots of aquatic plants (Lemly and Smith,1987; Maier and Knight, 1994; Canton and Van Derveer, 1997).

Table 2Summary of all fish specimens captured in the Tapajós River. Total length, carbon and nitrogen isotopic signatures, Hg and Se concentrations (mean7standard deviation) in muscular tissues of 5 key species.

Falling waters nN Low waters N Fallingwaters

N Low waters N Falling waters N Low waters N Falling waters N Low waters N Falling waters N Low waters

Length (cm) G. proximus (H) A. laticeps (O) H. unimaculatus (O) P. squamosissimus (P) A. falcirostris (P)

CAPnn n.d. n.d. n.d. 17.172.8 10 16.873.5 5 12.570.6 2 28.372.4 2 14.970.5 4 n.d. 23.575.4 11CUP 16.574.9 3 18.673.9 7 n.d. 17.673.0 2 21.473.6 8 19.672.4 7 n.d. 7 34.472.2 5 19.871.0 4 n.d.ITA 15.276.1 3 12.373.0 10 21.472.9 10 18.071.2 4 17.073.4 14 15.974.7 10 24.776.3 9 27.974.2 3 18.471.2 3 20.574.8 9JAC 16.074.8 12 19.174.5 n.d. 18.973.5 5 n.d. 17.072.3 12 18.372.8 28.575.8 16 29.376.6 17.170.3 3 21.676.5RES 20.073.5 10 16.075.3 9 20.472.2 5 18.674.5 10 19.174.6 9 18.172.7 6 28.675.0 9 n.d. 17.9 23.376.5 8

δ 15N (per cent) G. proximus A. laticeps H. unimaculatus P. squamosissimus A. falcirostrisCAP 9.670.9 5 10.270.6 9 n.d. 10.270.6 10 8.770.6 5 7.670.7 2 12.770.4 2 10.970.2 4 n.d. 12.070.6 11CUP 10.370.8 3 10.670.7 7 n.d. 10.670.2 2 10.271.7 8 9.870.7 7 12.970.1 7 13.270.7 5 12.470.1 4 n.d.ITA 11.070.6 3 11.870.7 10 11.171.0 10 11.171.2 4 10.470.8 14 11.071.7 10 12.970.7 9 13.671.2 3 12.770.5 3 13.370.5 9JAC 10.171.4 12 n.d. n.d. 9.970.7 5 n.d. 8.170.8 12 n.d. 12.470.7 16 n.d. 11.470.6 3 n.d.RES 9.770.3 10 9.670.3 9 10.671.0 5 10.470.7 10 8.871.3 9 7.770.6 6 12.670.4 9 n.d. n.d. 11.970.4 8

δ 13C (per cent) G. proximus A. laticeps H. unimaculatus P. squamosissimus A. falcirostrisCAP –32.671.1 5 –31.771.6 9 n.d. –32.871.6 10 –33.070.3 5 –30.470.6 2 –29.970.0 2 –33.370.5 4 n.d. n.d. –31.571.6 11CUP –32.370.4 3 –31.371.6 7 n.d. –27.971.4 2 –28.271.8 8 –30.574.3 7 –28.970.7 7 –29.970.8 5 –29.770.6 4 n.d.ITA –31.070.9 3 –33.72.6 10 –32.672.2 10 –32.473.4 4 –33.472.0 14 –33.072.4 10 –29.571.7 9 –30.871.2 3 –31.170.9 3 –31.370.9 9JAC –28.971.6 12 n.d. n.d. –30.872.9 5 n.d. n.d. –31.772.9 12 n.d. n.d. –28.971.0 16 n.d. –30.771.4 3 n.d.vRES –26.171.5 10 –26.071.9 9 –32.471.4 5 –30.172.0 10 –29.772.3 9 –27.275.7 6 –28.270.9 9 n.d. n.d. –29.470.7 8

Hg (ng/g) G. proximus A. laticeps H. unimaculatus P. squamosissimus A. falcirostrisCAP 180793.4 5 183788.5 10 n.d. 2667153.5 10 36717.8 6 79721.0 6 553773.5 2 174755.6 4 n.d. 8937287.1 12CUP 112735.1 3 158770.2 7 n.d. 95755.9 2 101769.7 8 81742.0 7 4537239.5 8 7287431.2 5 7347148.0 4 n.d.ITA 129767.7 3 245777.9 11 1937116.2 12 4397258.8 5 101769.7 15 81742.0 10 6127266.0 10 16157945.3 4 8737544.2 11 18937970.5 10JAC 150796.8 13 169781.3 7 2087133.3 7 n.d. n.d. 101768.7 15 61730.8 10 5277191.2 17 5017134.2 10 6727384.2 3 11917661.3 10RES 87726.3 11 84732.2 10 106740.9 5 111740.9 10 1267220.4 9 59730.0 6 4287161.1 9 n.d. 191 1 464795.0 10

Se (ng/g) G. proximus A. laticeps H. unimaculatus P. squamosissimus A. falcirostrisCAP 2607124.0 5 2617120.8 10 n.d. 146733.6 10 145740.4 6 86712.0 6 163738.2 2 212715.7 4 n.d. n.d. 82715.6 12CUP 2457130.9 3 5037179,5 7 n.d. n.d. 2447175,4 2 129733,3 8 152753,0 7 193763,7 8 282768,5 5 112725.1 4 n.d. n.d.ITA 472763.2 3 388793.4 11 243799.4 12 202760.1 5 189795.5 15 175741 10 2157124.4 10 3377130.1 4 138732.7 11 144742 10JAC 3697258.1 13 340757.4 7 1877158.6 7 n.d. n.d. 118733.8 15 164771.4 10 123725.6 17 192783.7 10 91729.7 3 82719.7 10RES 185728.5 11 2677202.5 10 122726.2 5 126731.3 10 104738.1 9 112741.0 6 164742.3 9 n.d. n.d. 88 1 112723v.3 10

n.d.¼data not available.H¼herbivorous, O¼omnivorous, P¼piscivorous (feeding strategy according to Ferreira et al., 1998).

n Number of specimens analyzed.nn CAP¼Capituã, CUP¼Cupari, ITA¼ Itapacurazinho, JAC¼ Jacaré and RES¼Restinga. The five locations appear in Fig. 1.

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22

Fig. 4. Relationship between Se bioaccumulation and the δ15N isotopic signaturein three fish trophic groups from the Tapajós River. Error bars are 95% confidenceinterval, significant differences between trophic groups appear when there is nooverlap between confidence interval for a given axe.

Fig. 5. Relationship between Hg bioaccumulation and the δ15N isotopic signaturein three fish trophic groups from the Tapajós River. Error bars are 95% confidenceinterval, significant differences between trophic groups appear when there is nooverlap between confidence interval for a given axe.

D. Sampaio da Silva et al. / Ecotoxicology and Environmental Safety 97 (2013) 17–25 23

In lotic environments however, fine organic sediments, bothderived from deposited particulate matter and from plant andanimal remains, are rare because they are continuously returnedinto suspension and washed away downstream (Lemly, 1999).

Because food chains in lentic ecosystems are often longer, weexpect a more important accumulation of Se in higher order con-sumers, such as in fish and amphibians (Cabana and Rasmussen, 1994;Kidd et al., 1995). By studying the dynamic of Se, Orr et al. (2006)confirmed this hypothesis. First, they showed that Se content inaquatic primary producers and the length of the food chain in lentichabitats were similar to lotic habitats located in a region of Se richsoils in British Columbia. Second, they observed that fish tissues fromlentic environments contained higher Se levels compared to fishtissues from lotic environments. This would be explained by thefavourable hydrologic characteristics to the biotransformation of Seoxyanions to organoselenium by intermediate macrophytes andmicrobiota associated with surface substrates in lentic ecosystems.In addition, the period of hydric retention, that is longer in lenticenvironments, could favour the provision and recycling across thedetritus-sediment route (Orr et al., 2006).

The inverse pattern between Hg and Se concentrations in fishobserved in our study goes in the same line as in previous research(Frost and Lish, 1976; Rudd and Turner, 1983; Turner and Swick, 1983;Curvin–Aralar and Furness, 1991). Chen et al. (2001) suggested theexistence of a strong effect of Se on the accumulation of Hg for twotypes of fish (Perca flavescens and Stizostedion vitreum). Additionally,previous studies realised in vivo showed a shift in the distribution ofHg in different animal organs in the presence of Se (Chen et al., 1974;Brzeznicka and Chmielnicka, 1985). In the same sense as previousstudies, a negative relationship between Hg measured in fish tissueand the source of carbon in the food (δ13C) comes forth again in ourstudy. Power et al. (2002), for example, found that the weight, thetrophic level and the δ13C signatures were the most importantvariables that explained the variation of Hg in fish from a sub-arcticlake. The same results were obtained for a tropical lake where 74% ofthe variability of Hg was attributed to these same variables (Kidd et al.,2003). The regression analysis in this study between the concentra-tions of Hg and δ13C of fish suggested that fish that have a principalsource of carbon enriched with 13C can have inferior concentrations ofHg compared to other fish that have a source of carbon low in 13C.

If we group the 5 sampling sites in relation to the geographiclocation on the Tapajós River basin, no difference in Se content inherbivorous and piscivorous fish species were observed betweenthe south (CAP, JAC and ITA) and the north (CUP and RES) samplingsites. In contrast, higher Se levels were found for omnivorous fishcaptured in southern sites compared to northern sites. Interest-ingly, the significant geographical variations in the distribution ofblood Se levels did not follow the south/north distribution of Se inomnivorous species, which is probably explained by the multipledietary source of Se in the region (Lemire et al., 2010a, b).

Our findings are of significant importance for the riverside fish-eating populations living in study area and chronically exposed toHg (close to JAC, ITA, RES and CUP), while their blood Se levels fallin the upper-normal range, varying from normal to elevated(median 284.3 μg/L, interval 142.1–2029.3 μg/L) (Lemire et al.,2006). The calculated selenium health benefit value, Se-HBV(Raymond and Ralston, 2009), appears highly positive for theherbivorous species considered (average value above 1838 ng/g forG. proximus), slightly positive for both omnivorous species (aver-age of 459 ng/g and 290 ng/g for H. unimaculatus and A. laticepsrespectively), and clearly negative for both piscivorous species(average of �519 ng/g and �3314 ng/g for P. squamosissimus andA. falcirostris respectively). It thus appears that the regular con-sumption of herbivorous fish species and to some extent omnivor-ous fish species should be promoted rather than that ofpiscivorous fish species as part of a healthy diet for riversidepopulations of the Tapajos River.

5. Conclusion

Our results highlight that, independently of the samplingseasons, Hg and Se concentrations vary in function of the fishspecies considered, exhibiting an inverse pattern between thosetwo elements along the aquatic trophic chain. Independently ofthe phase of the hydrological cycle, fish coming from lotic habitatspresented higher levels of Se compared to those from lentichabitats. Otherwise, it appears that at the level of the TapajósRiver basin, the bioavailability of Se and Hg can vary from one siteto another. Since riverside populations of the Amazon are chroni-cally exposed to Hg through frequent fish consumption, thepreferential consumption of non-predatory fish rather than pre-datory fish could increase Se intake and lower Hg exposure.

In the Amazon, there is still a lot to examine with regards to thefull understanding of the Se cycle. Studies pertaining to environ-mental, ecological and physiological factors on levels of Se in fish

Fig. 6. Relationship between Hg and Se levels in fish from the Tapajós River: herbivorous (a) omnivorous (b) et piscivorous (c) fish. The black and white symbols respectivelyrepresent the low waters and the falling waters. Each point is one individual fish.

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remain inexistent. This information is necessary in order to obtaina more complete portrait of the presence of this element intropical aquatic ecosystems as well as the potential risks andbenefits that it represents to the health of fish resources. More-over, further studies should address the relations between dietarySe intake and health of fish-eating populations.

Acknowledgement

This work was supported by a grant from the InternationalDevelopment Research Centre (IDRC-Canada). The authors wouldlike to thank the villagers of the Tapajós and the entire CarusoProject team. Thanks to Isabelle Rheault and Gabrielle Normandfor their laboratory assistance.

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