Distribution pattern of ambient cd in wetland ponds ditributed along an industrial complex

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Distribution pattern of ambient cadmium in wetland ponds distributed along an industrial complex

Aquaculture and Applied Limnology Laboratory, Department of Zoology, University of Kalyani,

Kalyani 741 235, West Bengal, India Received 27 February 2003; received in revised form 11 September 2003; accepted 7 October 2003

Abstract

Water and sediment samples collected from 18 wetland ponds within and outside industrial areas were examined for cadmium concentration and water quality parameters during the period of January to July 1996 The Cd contents in gill, liver, mantle and shell of freshwater mussel (Lamellidens marginalis) as well as leaves and roots of water hyacinth Eichhornia those occurred in these ponds were also estimated Cd concentration ranged from 0.006 to 0.7025 mg/l in water and from 7 to 77 lg/g dw in sediments of all the ponds investigated The amount of Cd occurring in water and sediment was much higher in concentrations in the ponds located in Captain Bheri and Mudiali farm close to industrial areas, compared to remaining ponds located outside the industrial belt Lamellidens marginalis procured from Mudiali and Captain Bheri ponds showed regardless of size, tissue and season of collection signiﬁcantly higher Cd concentration than did those from other ponds Likewise, tissue Cd in Eichhornia collected from Mudiali pond was as high as 125–152 lg/g dw in root and 21–63 lg/g dw in leaves compared to 40–108 lg/g dw in root and 9–43 lg/g dw in leaves in the remaining ponds Seasonal variability of Cd was clear-cut; the concentration was relatively higher in water and sedi-ment in all ponds during summer than during monsoon season or winter Size-wise, smaller groups showed the highest concentrations of Cd in all tissues of Lamellidens compared with medium and large size groups Concentration fac-tor for all tissues of Lamellidens regardless of size and season, was inversely proportional with the ambient Cd con-centrations Concentration factor estimated for all tissues in all ponds and all seasons was in the order: liver > gill > shell > mantle As all ponds located outside the industrial belt showed Cd concentrations ranging from 0.006 to 0.049 mg/l, it is suggested that these wetlands do not pose serious risk to the environment

Keywords: Wetland ponds; Cadmium; Lamellidens marginalis; Eichhornia crassipes

1 Introduction

Cd with an average concentration of 0.15 ppm

(Weast, 1969–1970), ranks as the 67th element in order

of abundance in the earth’s crust (Trotman-Dickenson,

1973) It normally occurs as an air-borne and aquatic contaminant associated with Pb-smelting and electro-plating processes Increased concentrations of Cd deposition in impounded water occurs through atmo-spheric fallout and runoﬀ (Borg et al., 1989) The global anthropogenic emission to the atmosphere is estimated

Cd contamination of impounded waters has posed an important threat to human health because of its estab-lished harmful eﬀect in the food chain of ﬁshes and human health

*

Corresponding author Tel.: 033-5826-323; fax:

+91-033-5828-282.

E-mail address: bbj@cal2.vsnl.net.in (B.B Jana).

doi:10.1016/j.chemosphere.2003.10.016

www.elsevier.com/locate/chemosphere

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A large number of growing industrial complexes,

such as a electroplating, cable, alloy, vehicle, plastic

pigments, dyes in many parts of India, and especially in

the vicinity of Calcutta city often have resulted in

indiscriminate discharge of industrial eﬄuents with high

Cd load These contaminants ﬁnally ﬁnd their ways into

the neighboring wetlands (Fig 1) and thereby damage

ecosystem health

According to the World Health Organisation (1971),

the maximum permissible limit for Cd in drinking water

is 0.01 mg/l However, detected concentrations in many

natural bodies of water are often much higher (Roth and

Hornung, 1977; Murphy et al., 1978; Mathew and

Menon, 1983) For example, in some ﬁsh inhabiting

natural lakes, a concentration of 13.6 lg/g Cd has been

detected (Murphy et al., 1978), while provisional

toler-ance for humans range from 0.4 to 0.5 lg Cd/person/

week

In general, ﬁshes growing even in sublethal

contam-inated environments show high levels of Cd in their

tissues due to bio-accumulation through the aquatic

food chain For example, Cd concentration in ﬁsh from Bombay ﬁsh markets ranges from 16 to 176 lg/g (Pillai, 1983), and is in the range of 2–417 lg/g from other parts

of the world (Anand, 1978) Such concentration is potentially hazardous to human health as they exceed the tolerable Cd intake (0.4–0.5 mg/person/week) Thus, ﬁsh or animals living in Cd contaminated aquatic habi-tats pose hazard to human health if they are part of human food chain

Information pertaining to Cd distribution in fresh-water habitats in relation to Cd accumulation in animals

is sparse in the Indian sub-continent With respect to other regions of the world, data available on alpine lakes suggest a total anthropogenic discharge of 3.3 t Cd/year into Ontario waterways (Environment Canada and Health Canada, 1994) Fifty seven lakes in Central Ontario, Canada have a geometric mean Cd concen-tration of 10 ng/l (Stephenson and Mackie, 1988) Wavy Lake within the regional municipality of Sudbury, On-tario, Canada, had a water Cd concentration of 4780 ng/l in 1992 (Taylor et al., 1995)

Fig 1 General location of study ponds.

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The ability of certain freshwater plants and animals

to accumulate metals above ambient water

concentra-tion is well documented Using these organisms as

indicators, bio-availability of metals from the

environ-ments can be monitored over extended period of time

However, environmental metal level is not the only

factor which inﬂuences the metal content of mussels, as

both the size of the animals and the seasons markedly

aﬀect these parameters (Penthreath, 1973; Boyden, 1977;

Majori et al., 1978) Among molluscs, bivalve and

gas-tropods are excellent bio-accumulators for a wide range

of pollutants (Simkiss, 1983; Everaarts, 1990;

Living-stone, 1991; Das and Jana, 1999) (Table 1) In general,

they are ﬁlter feeder, herbivorous and have the potential

to bio-accumulate contaminants that normally occur in

the water or sediment at concentrations too low for

detection by routine monitoring technique Thus, they

are considered ideal species for environmental

moni-toring Further more, the sedentary nature of these

animal is helpful in the interpretation of

bio-accumula-tion data (Short and Sharp, 1989; Livingstone, 1991)

Information about the level of metal pollution and the distribution of bivalves in freshwater habitats of India is scarce The purpose of this study was to examine the distribution of Cd concentrations of water and sedi-ment in relation to tissue Cd concentration in freshwater bivalve, Lamellidens marginalis in freshwater ponds along a Cd gradient from industrial complex to uncon-taminated areas

2 Materials and methods Eighteen wetland ponds were selected for the present investigation These wetlands are located within a radius

of 60 km of the University of Kalyani Some rain-fed wetlands receive industrial eﬄuents; others are situated

in an uncontaminated area (Fig 1) These wetlands are used for irrigation of agriculture crops, domestic use and ﬁsh culture The selected wetlands were distributed along a pollution gradient ranging from very high level

to low or uncontaminated areas The pond area ranged

Table 1

Cadmium accumulation in various tissues of bivalve molluscs under diﬀerent ambient Cd concentrations

Lamellidens marginalis

concentra-tion: 0.006–0.7025 ppm)

Present study

Lamellidens marginalis

exposure time: 40 days

Das and Jana (1999)

Unio elongatus

exposure time: 60 days

Badino et al (1991)

Crassostrea virginica

Elliptio complanata

Mytilus edulis

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from 0.1 to 6 ha with a depth range of 1.5–3 m (Table 2).

Natural populations of ﬂoating macrophytes Eichhornia

are common in about 39% the wetlands Freshwater

bivalve, Lamellidens marginalis is found in almost all

ponds

Samples of water and surface sediment were collected

from each pond during summer, monsoon and winter

seasons in 1996 The samples from each pond were

pooled into a composite sample As ponds located in

contaminated area had moderate water spread area and

received industrial eﬄuents from a point source, the

collected sample represented the true picture of that

body of water

Samples were collected in one liter polythene bottle,

the laboratory Each one liter sample was then

concen-trated to 10 ml volume by slow evaporation 20 ml

mixture was evaporated to near dryness The residue

was extracted with 50 ml double distilled water Cd

content of the sample was analysed by direct aspiration

of the aqueous digest extract into atomic absorption

spectrophotometer (Model UV 2201), following the

method described in APHA (1995) The instrument was

calibrated with metal standards,and Oyster Cd

stan-dards procured from of National Research Council of

Canada

The surface sediment (0–2.5 cm) of pond was

col-lected by suitable bottom sampler (Van Raaphorst and

Brinkman, 1984) from diﬀerent sites in the pond and

then mixed to make a homogenous sediment sample One hundred ml sample was transferred to acid-washed

added and the mixture was digested to a volume of approximately 3 ml Digests were allowed to cool to 55

hot plate and digested to approximately 3 ml The di-gests were diluted to 25 ml with double distilled deion-ised water and transferred to a glass bottle prior to analysis Cd concentration was analysed by direct aspi-ration of the aqueous digest into AAS as described by Walsh et al (1994)

Freshwater bivalves (Lamellidens marginalis) were collected from various sites in the pond using a 50 cm quadrate hand grab sampler (APHA, 1995) The speci-mens were washed thoroughly in tap water, blot-dried and their length and wet weights were recorded Animals were sorted into small (12 ± 1.3 g; 3.8 ± 1.3 cm), medium (31 ± 2.5 g; 6 ± 1.5 cm) and large (55 ± 4 g; 9.5 ± 2.5 cm) classes, each comprising 12 animals

Water and sediment samples of the pond were monitored for Cd concentration using standard AAS described by Walsh et al (1994) Water samples from each pond were also monitored for other water para-meters (temperature, pH, dissolved oxygen, total hard-ness, total alkalinity) to speciﬁcation given by APHA (1995)

Changes in fresh weight in Lamellidens were recorded

on each sampling day Lamellidens were carefully

dis-Table 2

Physico-chemical parameters of investigated ponds throughout the investigated period

(ha)

Depth (m)

Temp.

(C)

DO (mg/l)

alkalinity (mg/l)

Total hardness (mg/l)

Ambient cadmium concentration Water (mg/l)

Sediment (lg/g dw) Captain

Bheri

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sected for gill, liver, mantle and shell The shell valves

were opened with a shell-valve opener The mantle, liver

and gill were carefully removed and placed on separate

watch glasses on top of chipped ice in ice-buckets The

wet weight of tissues was recorded on an electrical

bal-ance after blotting surface moisture with ﬁlter paper In

addition to Lamellidens, ﬂoating macrophyte,

Eichhor-nia crassipes were collected manually and washed

thor-oughly in tap water The root and leaves were separated

and dried for tissue Cd extraction and estimation by the

method of Walsh et al (1994)

Bio-concentration factor (CF), reﬂecting the

accu-mulation ability for Cd was calculated for each tissue

using the formula given by Taylor (1983):

WCd or SCd

concen-tration of water (mg/l) or sediment (lg/g dw) during the

period of experiment

Mean Cd concentrations (SE±) for each pond was

obtained All pond, on the basis of their Cd

concentra-tion were grouped in to (i) with values within the

per-missible limit, 0.05 mg/l for drinking water imposed by

Environmental Protection Agency (1986) and (ii) with

value above permissible values or highly contaminated

One way analysis of variance and LSD test were used to

determine Cd distribution among ponds during various

seasons and tissues as well as the size-group of animals

The relationship between Cd concentration factor and

ambient Cd concentration was determined by the use of

the ﬁrst order equation The level of signiﬁcance was

accepted at P < 0:05

3 Results

3.1 Ambient water cadmium

The concentration of ambient Cd in water was highly

variable according to the season and the location of the

ponds Cd concentration ranged from 0.006 to 0.7025

mg/l (in or for) all ponds throughout the investigation

period Cd concentration was higher (0.63–0.7025 mg/l)

in ponds designated as numbers 1 and 2 and located in

contaminated areas Uncontaminated ponds designated

as numbers 3–18 had a Cd concentration of 0.006–0.049

mg/l Cd concentration in each pond was signiﬁcantly

higher during the summer than during the monsoon and

winter (Fig 2) There was about 86 fold variability in

ambient Cd (0.0081 and 0.7025 mg/l) among all ponds

during the summer (ANOVA, P < 0:05) Ponds (11%)

located in industrial areas showed as high as 0.617–

0.7025 Cd mg/l, implying direct impact of industrial

eﬄuents Cd concentrations in the remaining 16 ponds (89%) ranged from 0.0081 to 0.049 mg/l, but, remained within the permissible Cd limit (0.05 mg/l) for drinking water as per standards set by Environmental Protection Agency––US (1986) for the USA, and therefore posed

no potential threat to the environment under local conditions

(P < 0:05) during winter and monsoon Similar to the scenario found for the summer, two industrial ponds (11%) showed higher values than the remaining 16 ponds (89%) during winter and monsoon

In general, Cd concentration in water varied in three seasons of the year (ANOVA, P < 0:05) depending upon the leaching, runoﬀ water or dilution by rainfall Winter values (0.007–0.6175 mg/l) were lower than those of summer (0.0081–0.7025 mg/l), but higher than those of monsoon (0.0062–0.502 mg/l) In these ponds, summer values were 12% and 31% higher than those observed during winter and monsoon, respectively

3.2 Ambient sediment cadmium The Cd content in the sediment ranged from 7 to 77 lg/g dw Cd concentration was distinctly higher in

pond-1 (63–77 lg/g dw) and in pond-2 (55–65 lg/g dw) than in remaining ponds located at uncontaminated sites (7–26 lg/g dw) Summer Cd values varied between 8.5 and 77 lg/g dw, indicating a signiﬁcant diﬀerence (ANOVA,

ponds were characterized by extremely high Cd con-centration that ranged from 65 to 77 lg/g dw in summer Remaining 89% ponds showed concentration ranging from 8.9 to 35 lg/g dw (Fig 2)

The spatial distribution of sediment Cd varied (AN-OVA, P < 0:05) during winter and monsoon Two ponds (11%) had higher values than remaining 89% ponds during winter and monsoon The remaining ponds showed winter (7–30 lg/g dw) and monsoon (7–27 lg/g dw) values that were 64–90% lower than the former two ponds Seasonal variability (ANOVA, P < 0:05) in sediment Cd remained was identical to that of water The values were signiﬁcantly higher (ANOVA,

11.7–18% higher than monsoon) During the summer,

Cd concentrations in Captain Bheri pond (pond-2) was 13.8% higher than during monsoon and 6.2% higher than in winter Similar trend was found in the rest of the ponds

3.3 Cadmium in Lamellidens

Cd was detected in the shell and liver of Lamellidens collected from all 18 ponds However, the gills were analysed from only six and mantle from only seven ponds

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Lamellidens from Mudiali (pond-1) and Captain

Bheri (pond-2) regardless of their size, tissue and

col-lection season showed signiﬁcantly higher (P < 0:05) Cd

concentration than those from other ponds Cd

con-centration in smaller animal ranged between 254 and

502 lg/g dw in liver, 189–258 lg/g dw in gill, 160–196 lg/

g dw in shell and 41–67 lg/g dw in mantle In other

ponds it compared to ranged from 10 to 189 lg/g dw in

liver, 9–128 lg/g dw in gill, 4.7–100 lg/g dw in shell and

2–23 lg/g dw in the mantle (Figs 3–5)

Cd concentrations were highest in small animals (10–

502 lg/g dw in liver, 9–258 lg/g dw in gill, 4.7–196 lg/

g dw in shell and 2–67 lg/g dw in mantle) and lowest in large individuals (7.5–451 lg/g dw in liver, 3.7–189 lg/

g dw in gill, 3–196 lg/g dw in shell and 2–57 lg/g dw in mantle) With the exception of Captain Bheri and Mu-diali, variability in tissue Cd in smaller animals from 16 ponds was highest in liver (99.8%) followed by shell (90%), gill (80%) and mantle (78%) In general, summer values for Cd were about 11–28% higher than they were

(A) WATER

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

PONDS

(B) SEDIMENT

0 10 20 30 40 50 60 70 80 90

Fig 2 Mean (±SE) concentration of water (A) and sediment (B) cadmium in 18 investigated ponds during three diﬀerent seasons Note the clear-cut diﬀerences in water and sediment cadmium distribution.

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0

100

200

300

400

500

PONDS

LIVER

0

100

200

300

400

500

SHELL

0 100 200 300 400 500

MANTLE

0 100 200 300 400 500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Fig 4 Cadmium accumulation in four different tissues of medium group Lamellidens procured from 18 ponds in three different seasons Note the clear-cut variability of tissue cadmium in different seasons and investigated ponds.

GILL

0

100

200

300

400

500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

PONDS

Summer Winter Monsoon Summer Winter Monsoon

LIVER

0

100

200

300

400

500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Summer Winter Monsoon

SHELL

0 100 200 300 400 500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

MANTLE

0 100 200 300 400 500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Fig 3 Cadmium accumulation in four different tissues of small group Lamellidens procured from 18 ponds in three different seasons Note the clear-cut variability of tissue cadmium in different seasons and investigated ponds.

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in winter (liver––20–492 lg/g dw, gill––12–221 lg/g dw,

shell––9–177 lg/g dw, and mantle––2–55 lg/g dw) and

5–15% higher than monsoon (17–389 lg/g dw in liver,

9–198 lg/g dw in gill, 4–16 lg/g dw in shell and 1.5–44

lg/g dw in mantle)

4 Concentration factor (CF)

CF for all tissues of Lamellidens regardless of size

and season, were higher in those ponds that had

low Cd concentration and low in those with high

ambient Cd concentration CF estimated for all tissue

in all ponds during all seasons was in the order:

liver > gill > shell > mantle The Cd concentrations in all

tissues were less in the summer than in the winter and monsoon CF among smaller animals were 9–15% higher than those for medium-sized animals and 14–37% higher than the larger animals (Table 3)

5 Cadmium in Eichhornia The natural population of Eichhornia was observed

in seven out of 18 ponds Eichhornia collected from Mudiali (pond-1) and Captain Bheri (pond-2) had sig-niﬁcantly higher (P < 0:05) Cd concentrations than did the remaining ponds In general, tissue Cd in Eichhornia collected from Mudiali pond was as high as 125–152 lg/g dw in root and 21–63 lg/g dw in leaves compared to

Table 3

Range of the values of concentration factor for diﬀerent tissues of Lamellidens marginalis procured from 18 ponds during the period of investigation

Each value represents the data calculated from six animals.

LIVER

0

100

200

300

400

500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

SHELL

0 100 200 300 400 500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

MANTLE

0 100 200 300 400 500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

GILL

0

100

200

300

400

500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

PONDS

Fig 5 Cadmium accumulation in four different tissues of large group Lamellidens procured from 18 ponds in three different seasons Note the clear-cut variability of tissue cadmium in different seasons and investigated ponds.

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40–108 lg/g dw in root and 9–43 lg/g dw in leaves in the

remaining ponds (Fig 6) Pond variability of tissue Cd

was as high as 65–81% for leaves and 74–85% for roots

of Eichhornia from the seven ponds

Seasonal variability of tissue Cd showed higher

val-ues during summer winter and monsoon Summer Cd

concentration (root––55–214 lg/g dw, and leaves––

11.75–63.8 lg/g dw) were 5–27% and 11–46% higher

than monsoon (29–168 lg/g dw in root, and 5–34 lg/g

dw in leaves) and winter (39–187 lg/g dw in root, and

5.9–48 lg/g dw in leaves)

6 Water quality

Water pH (6.0–6.5) and dissolved oxygen (3.5–4.8

mg/l) regardless of the seasons were signiﬁcantly lower

in Mudiali (pond-1) and Captain Bheri (pond-2) ponds

than 16 ponds (pH––6.9–8.5; DO––7.5–11.5 mg/l)

There was no marked diﬀerence of total alkalinity and

total hardness among the 18 ponds investigated

In general, pH was higher during monsoon (7.7–8.5) followed by winter (6.5–7.6) and summer (6.0–6.8) Values of DO were higher in winter (11.5–4.9 mg/l) and lower in summer (7.6–3.5 mg/l) Total alkalinity (115–

175 mg/l) and total hardness (130–175 mg/l) were highest

in summer and lowest in monsoon (total alkalinity–– 109–125 mg/l, total hardness––115–130 mg/l) (Table 2)

7 Discussion Spatial Cd distribution in wetlands ponds depended upon their degree of contamination About 80–86 fold increase in Cd concentration in two ponds situated in the industrial belt of Calcutta (Captain Bheri and Mu-diali farm) over remaining ponds outside the city industrial complex may be due by the discharge of high anthropogenic Cd through wastewater eﬄuents

As all ponds located outside the industrial belt show

Cd concentration within range of 0.006–0.049 mg/l, (the permissible limit, Environmental Protection Agency––

US, 1986) it is suggested that these wetlands do not pose serious environmental threat

Wide range Cd variability among the ponds located outside the city industrial complex with low Cd con-centration (water––0.006–0.049 mg/l; sediment––7–35 lg/g dw) perhaps caused large variability of tissue Cd of the test animal This implied that the ambient Cd of uncontaminated ponds remained far below the accu-mulating potentials of test animal and, hence, are quite useful as bio-ﬁlter On the other side, less variability of

Cd at higher concentrations of Cd (0.458–0.7025 mg/l) indicated that Lamellidens population occurring in these habitats, were almost at the plateau, and might not be considered suitable for bio-removal of Cd from the environments

As observed by other investigators of various lakes (Stephenson and Mackie, 1988; Stephenson et al., 1996),

Cd concentration in sediment was signiﬁcantly higher than that in the water column in all water bodies It has been shown that Cd was rapidly lost from the water column to suspended particles (Stephenson et al., 1996) The loss may also be due to the presence of humic substances and the organic content of the sediment (Stephenson and Mackie, 1988; Pempkowiak and Kozuch, 1994)

Cd tissue distribution in Lamellidens was in the order: liver > gill > shell > mantle This order was found

to be true, regardless of the size group, ponds and sea-son As hepatopancreas acts as a sink for metal ions (Rajalekshmi and Mohandas, 1993), it is possible that it bears the Cd load of the main body and thus shows the highest Cd concentration among all tissues examined Similar results were reported by other investigators (Merigomez, 1989; Merigomez and Ireland, 1989) Al-though the ability to regulate the internal concentration

LEAVES

0

10

20

30

40

50

60

70

80

Mudiali C Bhery P4 P6 P14 P16 P17

PONDS

ROOT

0

50

100

150

200

250

300

Mudiali C Bhery P4 P6 P14 P16 P17

Fig 6 Cadmium accumulation in leaves and root of Eichhornia

in investigated ponds.

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of Cu and Zn over a wide range of dissolved

concen-trations have been demonstrated in intertidal shrimp

Palaemon elegans, the Cd concentration, on the other

hand, is not regulated resulting in body concentration of

metal directly proportional to external metal

concen-tration of the environment (White and Rainbow, 1986)

Cd uptake by Lamellidens gill was appreciably higher

than the uptake by the shell and mantle Gill was the

primary site for Cd accumulation because of its

rela-tively large surface area and ﬁltration activity (V-Balogh

and Salanki, 1984; Holwerda et al., 1989; Salanki and

V-Balogh, 1989; Everaarts, 1990) It was estimated that

about 90% of Cd uptake occurred through absorption

the gill or by complexation with a high molecular

weight-compound present on the gill surface (Carpene

and George, 1981) It was proposed that large surface

area and the gill mucous, which might act in ion

ex-change, contributed to high metal concentration found

in gill tissue (Brooks and Rumsby, 1967; Pringle et al.,

1968)

Relatively low Cd accumulation in mantle and shell

might be due to the shell acting as a safe storage matrix

for toxic contaminants resistant to soft tissue

detoxiﬁ-cation mechanism (Walsh et al., 1995)

Computation of correlation coeﬃcient between tissue

Cd and ambient Cd revealed no signiﬁcant relationship

between the Cd content of diﬀerent tissues of

water The Ca content of water was, however reported

to have an inverse relationship with tissue Cd

(Bjerreg-aard and Depledge, 1994; Jana and Das, 1997)

Though Eichhornia was known to be an important

bio-ﬁlter for the removal of metal in many experimental

studies (Nir et al., 1990; Xiang et al., 1994), their

occurrence in these wetlands did not exert any clear-cut

relationship either between tissue Cd of Lamellidens and

the ambient Cd concentration or between the tissue Cd

of Eichhornia and the latter This is perhaps due to the

fact that the Eichhornia population was quantitatively

less exerting any substantial bio-ﬁlter eﬀect on pond

wetlands

Acknowledgements

This research was supported by a grant Department

of Environment and Forests, Government of India (to

B.B Jana) Shamik Das is grateful to DoEn for

pro-viding him with a Junior Research Fellowship AAS

calibration and Oyster Cd standards (National Research

Council of Canada) were provided by Professor M.A

Alikhan of the Department of Biology of Laurentian

University at Sudbury, Ontario, Canada

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