Hydrobiologia 416: 41–49, 1999.© 1999Kluwer Academic Publishers. Printed in the Netherlands.

41

Cd, Cu and Zn concentrations in sediments and the invasivebivalvesLimnoperna fortuneiand Corbicula flumineaat the Rıo de la Plata basin, Argentina

Carlos Villar1,∗, Jorge Stripeikis2, Liliana D’Huicque2, Mabel Tudino2,Osvaldo Troccoli2 & Carlos Bonetto11Instituto de Limnolog´ıa ‘Dr. Raul A. Ringuelet’ CC 712. 1900 La Plata, Buenos Aires, ArgentinaE-mail: cvillar@ilpla.edu.ar2INQUIMAE, FCEyN. Ciudad Universitaria 1428 Buenos Aires, Argentina

Received 30 June 1998; in revised form 26 May 1999; accepted 29 June 1999

Key words:metal content, invasive bivalves,Limnoperna fortunei, Corbicula fluminea, Parana River, Rıo de laPlata Estuary

Abstract

Cd, Cu and Zn content in sediments and bivalves (Limnoperna fortuneiandCorbicula fluminea) were studied inthe lower 500 km stretch of the Paraná River and in the right margin of the Río de la Plata Estuary. Both bivalvesspecies showed higher metal contents at the Paraná River, possibly related to water chemistry. The Paraná wateris circumneutral and soft, while the Río de la Plata coastal water is alkaline, hard, with a higher organic content.Bivalves sampled at the thermal effluent of the Atucha Nuclear Power Plant, at the Paraná River, showed the highestmetal content, either from increased absorption at the higher water temperatures (12◦C higher than the river water)or from metal corrosion of the power plant cooling system.L. fortuneiandC. flumineaappeared to be successfulbiomonitor organisms for Cd pollution in the basin, while Cu and Zn content in bivalve tissues was apparently notrelated to metal load.

Introduction

A wide range of organisms has been employed tomonitor the environmental effect and bioavailability ofpollutants in the aquatic environment. Bivalves havebeen early recognised as useful sentinel organisms ofcontamination in aquatic ecosystems because of theirwide distribution, extensive populations, sedentarism,filtering habits and capacity to accumulate xenobi-otic substances above background levels. The ‘MusselWatch Program’ successfully exploits this ability offiltering mollusks to identify coastal marine areas withelevated toxin levels (Goldberg et al., 1978; Sericanoet al., 1993). In freshwater and brackish ecosystems,a similar approach has been employed, although con-siderably less developed, using other bivalve species(Corbicula fluminea, Foe & Knight, 1987; Bilos et

∗ Author for correspondence

al., 1998;Crassostrea virginica, Roosemburg, 1969;Sanders et al., 1991;Ischadium recurvum, Riedel etal., 1995).

Field measurements of sublethal effects and bioac-cumulation patterns of metals in the Río de laPlata basin were scarcely assessed until recently(Boltovskoy et al., 1997; Bilos et al., 1998).

Corbicula flumineaMuller (1774), an introducedAsian clam, was first reported in Argentina in 1981(Ituarte, 1981) and has undergone a remarkable ter-ritorial expansion since then to become a commonbenthic invertebrate in many major freshwater drain-ages of the country (Darrigran, 1992; Darrigran &Coppola, 1994).Limnoperna fortuneiDunker (1857),an Asian freshwater mussel, was first reported in Ar-gentina in 1991 (Pastorino et al., 1993), and hasquickly expanded from the Río de la Plata Estuary intothe Paraná River (Villar et al., 1997). At the presenttime, both bivalves represent a large and common

42

Figure 1. Map of the study sites in the Parana River and Rıo de la Plata Estuary.

component of the macroinvertebrate assemblages inthe lower portion of the basin (Darrigran, 1994).

C. flumineahas been used successfully in labor-atory and field studies as a freshwater bioindicatororganism (Foe & Knight, 1987; Doherty & Cherry,1988; Farris et al., 1989; Belanger, 1991; Colomboet al., 1995), while there is no available information ofthe usefulness ofL. fortuneias a biomonitor. AlthoughL. fortunei is a potential invasive pest, information

about its biology and susceptibility to metal pollutionis scarce.

With an area of 3× 106 km2 and a mean dis-charge of 25 000 m3 s−1, the Río de la Plata basinis the second largest hydrographic system in SouthAmérica, after the Amazon. The Río de la Plata basinis composed of two main watersheds, the Paraná andthe Uruguay Rivers, which together form the Río dela Plata Estuary (Figure 1). The Paraná basin has ex-

43

perienced damming, deforestation followed by intenseagriculture and erosion; large urbanisations and hugeindustrial settlements are particularly intense. SaoPaulo and Buenos Aires, the two largest urban settle-ments in South América, lie at the opposite extremesof the basin.

The aim of the present study was to assess Cd, Cuand Zn content in water, sediments and bivalves at thelower 500 km of the Paraná River and of the rightmargin of the Río de la Plata Estuary, and to assessthe utility of L. fortuneiandC. flumineaas indicatorsof trace metal pollution in the region.

Materials and methods

Sampling sites

Samples were collected from a number of sites in theLower Paraná River and right margin of the Río dela Plata Estuary, near Buenos Aires (Figure 1). Thesampling locations were chosen to cover a wide rangeof potential anthropogenic inputs of trace metals.Samplings at the river were performed at Santa Fe,Arroyo Seco, Lima and Otamendi (Paraná de las Pal-mas River, right arm), Brazo Largo (Paraná GuazúRiver, a less disturbed left arm), c.a. 480, 270, 70,50 and 40 km upstream of the river mouth, respect-ively (Figure 1). At Lima, emphasis was focused onevaluating the impact caused by the thermal effluent ofthe Atucha Nuclear Power Plant. At this site, sampleswere taken in a coastal area of the river upstream theplant and at the thermal effluent. The samplings in theestuary were performed at Quilmes, Punta Lara andPunta Blanca, c.a. 22, 50 and 70 km downstream ofBuenos Aires.

Sampling and analytical procedures

Samples of water, bottom sediments and bivalves werecollected during October and November 1996. Duringthe study period, river water level was low, allowingthe manual collection of samples, while at the Río dela Plata Estuary samples were taken during low tide.All measurements and samples were done in triplicate.Dissolved oxygen concentrations were determinedinsitu with an YSI 51B recorder and pH with an OR-ION 250A pHmeter. Subsurface water samples fornutrient and macro ion determinations were imme-diately filtered through preweighed Whatman GF/Cfilters and brought to the laboratory in dark and cold

boxes. Suspended matter was calculated from pre-weighed and sample filters, and dissolved nutrientswere determined in the filtrate. Soluble reactive phos-phorus (SRP, molybdate-ascorbic) and nitrates plus ni-trites (cadmium reduction) were determined followingStrickland & Parsons (1972). Ammonium (NH4

+, in-dophenol blue) was measured according to Mackerethet al. (1978). Calcium and magnesium (EDTA ti-tration), sodium and potassium (flame photometry),bicarbonate (Gran titration), sulphate (turbidimetry)and chloride (silver nitrate titration) were determinedfollowing APHA (1985). Total phosphorus (TP) andtotal organic carbon (TOC) were determined in un-filtered samples after Strickland & Parsons (1972).Particulate organic carbon (POC) was determined onthe filters used for suspended matter determination,using the same procedure as for TOC.

Subsurface water samples for metal analysis weretaken with plastic bottles previously washed withnitric acid and rinsed in Milliq deionized water.Samples were acidifiedin situ with pro analysis nitricacid. Bottom sediments were taken with a 4.5 cmdiameter Plexiglas core. The upper 3 cm layer wasdried at 60◦C and ground. Subsamples were taken forthe determination of texture (wet sieving and pipettemethod; Folk, 1959) and organic matter content (losson ignition; Dean, 1974). Bivalves were collected witha plastic spatula. Each replicate consisted of a compos-ite sample of 50 individuals pooled together. Mollusksof similar size were collected at all the sampling sites(mean shell length± SD: L. fortunei 13.4 ± 1.8mm, C. fluminea23.4± 4.1 mm). In the laboratory,mollusks were cleaned of fouling growth, scrubbedand rinsed in deionized water, and shucked and driedat 60 ◦C. Bivalves were not depurated prior to ana-lysis, as no significant variations were observed inCd, Cu and Zn content comparing depurated againstnon-depurated organisms sampled at the Río de laPlata basin (Bilos et al., 1998). Traceability of heavymetals results was assessed by the use of bovine liver(SRM 1577b, NIST), and estuarine sediment (SRM1646a, NIST) certified reference materials. The resultsobtained were in close agreement with the certifiedvalues for both materials.

Samples were prepared for metal analysis as fol-lows: concentrated ultrapure HNO3 was added to wa-ter samples and refluxed in 250 ml flasks. Sediments(0.5 g DW) were digested with a hydrofluoric-nitric-perchloric mixture and bivalves soft tissue samples(0.5 g DW) were digested with concentrated ultrapureHNO3 in a CEM Microwave Sample Preparation Sys-

44

tem MDS-2000 with advanced composite vessels. Ex-treme care was taken during sample manipulation toavoid contamination. Sediment solutions were dilutedto 50 ml and bivalves solutions to 25 ml. The lab-ware was carefully cleaned and analytical grade (ACSspecifications) reagents were used. Standard curveswere constructed using Merck certified stock solu-tions. To overcome matrix interferences, all reagentsused to treat the samples were added to the work-ing standards in the same proportions. Every time aset of samples was digested a procedural blank wasmade to correct sample readings. Total Cd, Cu and Zndeterminations were performed by atomic absorptionspectrophotometry (Shimadzu AA6501) with graph-ite furnace atomiser (Shimadzu GFA6000), followingBettinelli et al. (1989). The limits of detection wereexpressed as 3 s for the blank signal. Obtained valueswere: Cd 1µg l−1, Cu and Zn 2µg l−1, variationcoefficient 2%. Precision expressed as %RDS,n=10was better than 2% in all measurements.

Bioaccumulation factors (BAF) of Cu and Zn werecalculated on a dry weight basis relative to mean sed-iment concentration of metals. Units were mg kg−1

for both bivalves and sediments. As metal content insediments may variate with time, our point sampleestimate of Cu and Zn in sediments was pooled withdata from Villar et al. (in press) from B. Largo (ParanáRiver) and P. Blanca (coastal estuary) to estimatea mean value of Cu and Zn content in sedimentsthrougout 1995–96.

Statistical analysis

Metal content in sediments and bivalves was comparedbetween sites with ANOVA and Tukey tests (Sokal &Rohlf, 1979). Prior to analysis, data were tested fornormality and homogeneity of variance using the Lil-liefors and Bartlet tests, respectively (Sokal & Rohlf,1979). Data which did not conform to the assumptionswere transformed to logarithms. Pearson correlationswere calculated between metal content in water, sed-iments and bivalves. Software used to execute theanalysis was Statistica (1993).

Results

Water composition

Table 1 shows the limnological variables measured atthe sampling sites. The river and estuary water showed

similar temperature and dissolved oxygen concentra-tions, but coastal estuary water showed higher valuesof conductivity, pH, suspended matter, organic matterand of all the other measured ions (p<0.05). At theParaná River, conductivity, sodium, potassium, chlor-ide and sulphate increased downstream of Santa Fe,being maximum at site A. Seco (p<0.01), without sig-nificant further changes downstream. Elevated valuesof NH4

+ and P were also observed at A. Seco. Allthe other physical and chemical variables measured inthe river did not differ among sampling sites. In theestuary, the highest values of most measured variateswere registered at Quilmes, while high concentrationsof NH4

+ and P were observed at P. Lara.Cu and Zn concentrations in water showed high

variability between replicates and, consequently, theyare not reported. Cd concentrations fell below thedetection limits (1µg l−1) in all the study area.

The main physical and chemical characteristics ofthe Nuclear Power Plant thermal effluent were not sig-nificantly different from those measured in river water,with the exception of temperature, which was 12◦Chigher.

Sediments

Cu and Zn content in bottom sediments of theParaná River increased downstream of Santa Fe, withthe highest values registered at A. Seco (p<0.05)(Table 2). B. Largo and Otamendi showed similarmetal content, being intermediate between those ob-served at Santa Fe and A. Seco. In the estuary,Cu and Zn content in sediments was significantlydifferent between sampling sites, being highest atQuilmes, intermediate at P. Blanca and lowest at P.Lara (p<0.05). Cu content was correlated with sed-iment texture (sand-Cu:r: −0.77, p<0.05; silt-Cu:r: 0.79,p<0.05) and organic matter content (r: 0.82,p<0.05), while Zn content was not significantly re-lated to sediment characteristics (Table 3). Cd con-centrations in sediments fell below the detection limits(0.1 mg kg−1) in all the study area.

Trace metals in bivalves

Metal content in soft tissue ofL. fortunei and C.fluminea is shown in Table 4. Metal content in thebivalves was not correlated with metal content in sed-iments. Cd content was similar in both species (exceptin P. Blanca), while Cu and Zn content was 3–5 and 2–3 times higher, respectively, inC. fluminea. L. fortuneidoes not bioaccumulate Cu (BAF: B. Largo, Paraná

45

Table

1.L

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otal

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ter)

Site

sT

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Con

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4+

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Ca2+

Mg2+

Na+

K+

Cl−

S–S

O 4−

◦ Cm

gl−1

µS

cm−1

mg

l−1

µg

l−1

µg

l−1

µg

l−1

µg

l−1

mg

l−1

mg

l−1

mg

l−1

mg

l−1

mg

l−1

mg

l−1

mg

l−1

mg

l−1

mg

l−1

Par

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Riv

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Fe

16.5

9.8

7.4

6563

2114

44

586

0.7

2.7

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15.

42.

34.

91.

7

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16.5

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519

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4

Lim

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312

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6.4

3.6

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664

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547

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P.L

ara

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7.7

236

120

304

771

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698.

112

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46

Table 2. Metal content in bottom sedi-ments (mg kg−1 DW) at each samplingsite. Mean± standard deviation

Cu Zn

mg kg−1 mg kg−1

Parana RiverSanta Fe 14± 1 36± 4

A. Seco 21± 1 74± 4

B. Largo 17± 1 54± 3

Otamendi 15± 1 47± 2

Rıo de la Plata EstuaryQuilmes 15± 2 94± 4

P. Lara 4± 0 21± 1

P. Blanca 11± 0 59± 1

Table 3. Sediment composition and organic matter content ateach sampling site

Sand Silt Clay Organic matter

————————-%————————-

Parana RiverSanta Fe 72 18 10 1.7

A. Seco 33 52 15 2.6

B. Largo 39 48 13 2.4

Otamendi 56 32 12 2.1

Rıo de la Plata EstuaryQuilmes 98 2 0 0.8

P. Lara 98 0 2 0.4

P. Blanca 96 1 3 1.6

River: 0.62; P. Blanca, coastal estuary: 0.22) and Zn(BAF: B. Largo, Paraná River: 0.96; P. Blanca, coastalestuary: 0.59), whileC. flumineaseems to bioaccumu-late both metals in the Paraná River (Cu: 2.38; Zn:3.26), but not in the coastal estuary (Cu: 0.68; Zn:1.37).

A significant trend in metal concentration in bi-valves differentiated the river from the coastal estuaryenvironment.L. fortuneishowed higher values of Cdand Cu in the river than in the estuary (p<0.05). C.flumineaalso showed higher concentrations of Cd,Cu and Zn in the river (p<0.05) (Table 4). Cd andCu concentrations in the clam tissues were negativelycorrelated with conductivity (r=−0.96 and−0.99, re-spectively,p<0.05) and, although not significant, asimilar trend was apparent for Zn (r=−0.83).

At the Paraná River,L. fortunei showed a signi-ficant increase in Cd content downstream of A. Seco(p<0.05), with Lima containing the highest value forthe study area (2.1 mg kg−1). In the estuary, Cdcontent increased downstream, being maximum at P.Blanca (p<0.05). Cu concentrations in both speciesdid not differ between sampling sites in each water-shed. Zn content inL. fortunei at the Paraná Riverwas lower at B. Largo and Otamendi than at the othersampling sites (p<0.05). In the estuary, Zn contentwas lower at Quilmes than at P. Blanca and P. Lara(p<0.05). Zn content inC. flumineain the estuarydid not show significant differences between sites.L.fortunei sampled at the thermal effluent showed thehighest Cu and Zn values for the study area (Table 4).

Discussion

Higher levels of Cd and Cu inL. fortuneiand of Cd,Cu and Zn inC. flumineawere present in the ParanáRiver rather than in the coastal estuary, in contrastwith previously reported higher metals load at the later(Villar et al., 1998a, Villar et al., in press), suggestthe influence of water chemistry on metal bioavail-ability. The observed differences in the limnologicalvariables between the river and coastal estuary wa-ters reported in this work are coincident with previousreported figures in the area (Villar et al., in press).While seasonal variations indeed occur, differencesbetween both environments are maintained throughoutthe year (Villar et al., in press). Increased conductivity(Sanders et al., 1991; Riedel et al., 1995), hardness(Wang & Evans, 1993; Muller & Mackay, 1980) andorganic matter content (Bodek et al., 1988) are knownto decrease metal uptake by organisms. Since thesevariates are simultaneously higher at the coastal es-tuary water, it is not possible to ascertain the relativeinfluence on metal bioaccumulation of each one alonebased on present data. Lower tissue metal content inthe coastal estuary water most probably reflects a di-minished metal bioavailability. Consistently, Gómez etal. (1998) reported higher Zn toxicity in the fishCnes-terodon decemmaculatusin acute bioassays performedwith Paraná River water than with coastal estuarywater, with the LC50 roughly double in the latter.

Local small affluents draining the Pampa plain, in-dustrial discharges, and the untreated sewage disposalof Buenos Aires, located close to Quilmes samplingsite, contribute the higher ion and organic matter con-centration in the coastal waters of the Río de la Plata

47

Table 4. Metal content (mg kg−1) in L. fortuneiandC. fluminea(on a dry weight basis). Mean± standarddeviation

Sites Cd Cu Zn

L. fortunei C. fluminea L. fortunei C. fluminea L. fortunei C. fluminea

Parana RiverSanta Fe 0.7± 0.1 – 12.5± 0.1 – 63± 3 –

A. Seco 1.1± 0.2 – 13.0± 0.0 – 69± 1 –

Lima 2.1± 0.2 – 13.5± 0.1 – 72± 5 –

T. effluent 1.3± 0.1 – 42.0± 2.8 – 133± 62 –

B. Largo 1.5± 0.1 1.6± 0.1 13.0± 0.0 50± 6 48± 1 163± 1

Otamendi 1.5± 0.1 – 14.0± 1.4 – 48± 1 –

Rıo de la Plata EstuaryQuilmes 0.4± 0.1 0.3± 0.0 10.5± 0.1 31± 1 48± 1 117± 3

P. Lara 0.6± 0.1 0.5± 0.0 11.0± 0.0 28± 6 66± 1 125± 6

P. Blanca 1.2± 0.3 0.3± 0.1 8.7± 0.0 27± 3 58± 2 134± 13

Estuary. Comparatively high metal figures in river sed-iments (Table 4) are related to their higher content ofsilt and clay. The highest metal content in sedimentsat Quilmes, in spite of their coarse texture, reflects animportant load at this site. P. Blanca, further than P.Lara to the main loads, showed higher metal contentbecause of its higher clay and organic matter content.

The significative human impact observed at A.Seco is originated at Rosario, a large city (1 millionhabitants) with important industries lacking sewagetreatment plants, located roughly 20 km upstreamthe sampling site. The increase of ions concentrationbetween Santa Fe and A. Seco has already been re-ported (Villar et al., 1998b), and likely reflects thecontribution of the Salado River, an affluent of com-paratively low discharge but high salinity (Maglianesi& Depetris, 1970).

The higher water temperature at the Power Planteffluent can be partially responsible for the highestmetal content inL. fortunei. Bivalves exposed to theeffluent were meagre and smaller than those of theother sites (Villar et al., 1997), possibly related tothermal stress. Higher temperatures generally implygreater solubility of metal salts, increased absorptionthrough cellular membranes, and higher absorptionsdue to higher metabolic rates (Macinnes & Calabrese,1978; Bodek et al., 1988). Belanger et al. (1990)reported thatC. flumineaexposed to Cu at highertemperatures suffered larger impacts, and explainedthis as increased siphoning and sensitivity to toxic-ants. Another cause of increased bioaccumulation at

the Power Plant effluent could be chronic expositiondue to metal corrosion of the refrigerating system.Roosemburg (1969) and Abbe & Sanders (1986) re-ported that metals eroded from the condenser tubes ofthe Chalk Point electric generating station, PatuxentRiver, U.S.A., led to higher Cu content in oysters inthe vicinity of the plant. However, Villar et al. (1998c)were unable to detect significant differences in metalconcentrations between the inflow and outflow waterof the Power Plant cooling system during a one yearsurvey.

L. fortuneiandC. flumineaseem to be successfulbiomonitor organisms for Cd pollution in the basin.Although Cd content in water and in sediment samplesfell below the detection limits, mollusks showed signi-ficative variations in metal content among sites, mostprobably reflecting different Cd loads in the environ-ment. The downstream increase in Cd concentrationin L. fortunei tissues in the Paraná River is indicativeof the significative anthropic impact in the region. Atthe estuary, higher Cd values distant from the maineffluents near Buenos Aires suggest the importance ofsmall rivers and channels draining industrial areas inthe southern suburbs. AGOSBA et al. (1992) reportedvalues of 2.3, 9.0 and 10.6µg Cd l−1 in water samplesof 3 small rivulets located near P. Lara. An increasein metal bioavailability because of decreased organicmatter content downstream Quilmes may be anotherpossible explanation for the observed differences inCd bioaccumulation. Bodek et al. (1988) reported di-minished absorption rates of metals in fishes of the

48

west coast of the United States affected by sewagedischarges, suggesting the effect of organic load onmetal speciation.

In contrast, variations in Cu and Zn concentra-tions in bivalve tissues were apparently not relatedto metal load. Considering the river and the estuaryseparately, Cu content in both species did not differsignificantly between sites, in spite of different metalloads. Extreme care must be taken in using bivalveaccumulation patterns to assess Cu and Zn contamin-ation in the region. This pattern was also observed byBelanger et al. (1990) inC. flumineaheld in effluent-impacted sites, which accumulated Cu to levels similarto downstream, unimpacted sites. They observed thateach reach had divergent growth rates, but not dis-tinctive bioaccumulation patterns. As Cu and Zn areessential metals, levels in live bivalves would be regu-lated by complex physiologic mechanisms (Couillardet al., 1993 quoted by Bilos et al., 1998).

Concentration of heavy metals in water reportedat the sampling sites (Villar et al., 1998a,c, in press;Boltovskoy et al., 1998) are close to or above levelsthat could affectC. flumineapopulations. ReportedZn and Cu concentrations ranged between 21–170and 3–28µg l−1, respectively. Farris et al. (1989,quoted by Eisler, 1993) reported that 34µg l−1 Znreduced the growth and the cellulase enzyme activityof C. flumineaafter 30 d of exposure; and Belangeret al. (1986, quoted by Eisler 1993) reported thatZn concentrations between 50–500µg l−1 inhibitedclams growth between day 20 and 30 of exposure.The highest Cu concentrations measured in the studyarea (28µg l−1) was two fold higher than the me-dian lethal concentration forC. flumineaafter 42 dof exposition in laboratory bioassays (Graney, 1980,quoted by Doherty & Cherry, 1988). Boltovskoy et al.(1997) suggested that the Cu concentration in water(up to 28µg l−1) at the San Antonio River, a site loc-ated near industrial and sewage effluents in the lowerDelta of the Paraná River, could be associated with thepresence of a dwarfed population, and with the lack ofdiscernible cohorts.

Cd concentrations in both species lie within thelowest range reported in the literature. Measured val-ues are similar to those reported for 3 clam speciesfrom the Illinois River, U.S.A., (0.2–1.4 mg Cd kg−1

DW; Eisler, 1985); are lower than those reported by 1.Broman et al. (1991) for the musselMytilus edulisincoastal areas of Sweden (4.7–10.8 mg Cd kg−1 DW);2. by Couillard et al. (1993 quoted by Bilos et al.,1998) forAnodonta grandisin Small Lakes, Canada,

(16–129 mg Cd kg−1 DW); and 3. by Abaychi &Mustafa (1988) forC. flumineain the Shatt al-ArabRiver, Iraq, (2.2–70 mg Cd kg−1 DW). Zn concen-trations in the bivalves tissues (48–133 mg kg−1 forL. fortunei and 117–163 mg kg−1 for C. fluminea)were within previously reported values for other bi-valves present in areas with contrasting environmentalZn levels: Eisler (1993) reported 81–115 mg kg−1 Znin soft parts of clams, and 34–110 mg kg−1 Zn inthe common musselMytilus edulis; and Abaychi &Mustafa (1988) reported 31–83 mg kg−1 Zn in C. flu-mineaof the Shatt al-Arab River (Iraq). The existenceof homeostatic regulations of this essential element issuggested by this uniformity in Zn concentrations inthe bivalves tissues. Cu concentrations inL. fortunei(8.7–13.5 mg kg−1) lie within the lowest range, whilein C. fluminea(27–50 mg kg−1) lie within the upperrange of freshwater bivalves from unpolluted to mod-erately polluted areas (6.3–52 mg kg−1) (Bilos et al.,1998). As previously reported by Bilos et al. (1998),the Cu concentration inC. flumineain the study areasuggests a significant Cu load.

Acknowledgements

This research was possible thanks to financial supportby the International Foundation for Science (IFS) andUBACyT.

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