STUDY OF HEAVY METALS IN THE GASTROPOD, …

Ife Journal of Science vol. 17, no. 3 (2015)

STUDY OF HEAVY METALS IN THE GASTROPOD, PACHYMELANIA AURITA (MULLER, 1774), SEDIMENT AND WATER FROM OLOGE LAGOON, SOUTH-

WESTERN NIGERIA

*Samuel, O. B., Osibona, A. O. and Chukwu, L. O.

Department of Marine Sciences, Faculty of Science, University of Lagos, Akoka – Yaba, Lagos, Nigeria*Corresponding Author: [email protected]; [email protected]

(Received: 5th June, 2015; Accepted: 22nd June, 2015)

The levels of Cu, Fe, Pb, Mn and Zn in the gastropod, Pachymelania aurita in relation to heavy metal contaminations in sediments and surface water of Ologe Lagoon, Nigeria were investigated over a twelve- months period. The mean concentration of metals in whole tissue of P. aurita and sediment collected from Ologe Lagoon was in the descending order; Fe > Mn > Zn > Cu > Pb while the mean concentration in the surface water of the site where P. aurita was collected was in the descending order; Fe > Zn > Cu > Mn > Pb. The computed metal pollution index (MPI) (46.76 - 99.03) indicated that there was high level of heavy metal load in the whole tissue of P. aurita, and Iron contributed the highest concentrations of metal load. Based on the bio-sediment accumulation factor (BSAF) and bio-water accumulation factor (BWAF), the amount of Cu, Fe, Pb, Mn and Zn accumulated by P. aurita was found to be about 1.49, 1.46, 1.87, 0.69 and 1.28 times higher than the level detected in the sediment respectively while it was about 4.34, 55.19, 1.67, 83.11 and 20.94 times higher than the level detected in the surface water respectively. Based on the concentrations of Cu and Fe exceeding World Health Organisation (WHO) permissible limits in fish food, P. aurita from Ologe Lagoon may be said to be unsafe for human consumption.

Keywords: Heavy metals, Mollusc, Pachymelania aurita, Bioaccumulation, Sediment, Surface water

ABSTRACT

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INTRODUCTIONThe accumulation of heavy metals in our environment has intensified in recent years due to population growth, industrialization and new technological developments. This phenomenon is of great concern because heavy metals constitute considerable hazards to human health due to their toxicities, accumulative tendencies and persistence in the environment (Ayenimo et al., 2005). The aquatic environment is the ultimate sink of toxic chemicals generated by man's industrial, agricultural and domestic activities. Pollutants discharged into streams and rivers are transported over long distances affecting ecosystems miles away from the point of discharge. In view of the above-mentioned, environmental managers are constantly faced with having to determine the extent of environmental contamination and identifying localities and habitats at risk (Galloway et al., 2002).

The fact that heavy metals cannot be destroyed through biological degradation and have the ability to persist in the environment make them deleterious to aquatic environment and consequently to humans that depend on aquatic products for sources of food (Farombi et al.,

2007). Moreover, there is a growing concern among citizens about chemicals and metals that are suspected of disrupting reproduction in aquatic organisms. These are tied to observations in humans and wildlife over the last 40 years of worrying trends of adverse effects (Almeida et al., 2001; Licata et al., 2003; Berkun, 2005).

Biological communities respond constantly to physical forces, chemical dynamics, and ecosystem processes that change minute-to-minute and year-to-year but the difficulty is establishing to what degree these changes affect an ecosystem, how much they are outside the natural range of variability or disturbance frequency (Nedeau et al., 2003).

Ologe Lagoon meets several socio-economic needs (aquaculture, fishing, sand dredging and drainage) of the various towns and villages bordering it. More importantly, it drains river Owo, into which partially treated/untreated effluents from the Agbara industrial estate is discharged (Clarke et al., 2004). These discharges coupled with domestic inputs from the Agbara residential estates and satellite communities cause significant levels of pollution (Clarke et al., 2008).

566

A previous study (Kusemiju, 2010) has revealed the predominance of heavy metals in industrial effluents discharged into this environment. With respect to Ologe Lagoon, data exist on algal composition (Clarke et al., 2008; Onuoha, 2009), the fish and fisheries (Anetekhai et al., 2003) and heavy metal concentrations in fish (Kumolu-Johnson et al., 2010). However, there is a dearth of information on the heavy metal level of benthic macro-invertebrates in Ologe Lagoon.

Benthic macro-invertebrates are an integral component of the aquatic ecosystems. In addition to some being a source of protein to humans, they play important roles in energy flows, nutrient cycling and maintaining community balances in these ecosystems. This study therefore investigates the level of heavy metals (Cu, Fe, Pb, Mn and Zn) in the gastropod, Pachymelania aurita of Ologe Lagoon in relation to heavy metal levels in sediment and surface water. Pachymelania aurita (Mollusca; Gastropoda; Caenogastropoda; Melaniidae) commonly known as periwinkle in Nigeria is one of the commonest and most

dominant gastropod molluscs in Ologe Lagoon and other south-western lagoon systems (Oyenekan, 1987; Ajao, 1990; Brown, 2002; Uwadiae, 2009) of Nigeria. It is endemic to West Africa (Egonmwan, 2007) and is sought after by natives of coastal towns and villages in Nigeria as a staple source of protein.

MATERIALS AND METHODSDescription of Study AreaThe Ologe Lagoon (Figure 1) transverses Lagos and Ogun States, in Nigeria and is one of the nine lagoons in south-western Nigeria (Nwankwo, 2004). It is a freshwater lagoon system that lies between latitudes 6° 27′ N and 6° 30′ N and longitudes 3° 02′ E and 3° 07′ E and has a surface area of 9.4 km2 (Anetekhai et al., 2007). Rivers Imede, Owo, Oponu and Ilo are major rivers emptying into Ologe Lagoon. Its main outlet empties into Elete creek through which it connects with other creeks and lagoons such as Iyagbe lagoon, Badagry lagoon and the Lagos Lagoon.

Figure 1: Description of the Study Area

Samuel et al.: Study of Heavy Metals in the Gastropod, Pachymelania aurita

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Collection of Pachymelania aurita for Heavy Metal AnalysisPachymelania aurita samples were collected for a period of 12 months (January – December, 2008). The animals were collected by handpicking at the edge (latitude 060 29.496'N, longitude 030 03.722'E) of the lagoon at depth less than 0.75 m. The collection was done in three replicates each month and three individual animals were combined to serve as a replicate. The collected samples were taken to the laboratory and stored in the freezer prior to digestion for analysis of the heavy metal content. Collection of P. aurita from this station was based on its availability throughout the year. All samplings were carried out between the hour of 10.00 and 15.00.

Collection of Water SamplesWater samples for physicochemical analysis were collected using acid washed and distilled water cleaned 2-Litre polyethylene containers by dipping them 10 cm below the water surface. Water samples for heavy metal analysis were collected into fresh 120 ml plastic bottles. To prevent adsorption of metals into the walls of the 120 ml containers, the samples were acidified with 2 drops of concentrated HNO (APHA, 1998) and 3

0stored in a refrigerator (4 C). Water samples were collected for the duration of 12 consecutive months.

Collection of Sediment Sediment samples were taken for a period of 12

2months, using a 0.1m Van Veen grab. The samples were placed in a labelled polythene bags for preservation in the laboratory deep freezer (0 0C) prior to sediment analysis.

Heavy Metal Concentrations of Water (mg/l)The metals analysis was determined by the Atomic Absorption Spectrophotometric (AAS) method. 100 ml of the sample was placed in an Erlenmeyer flask and 10ml concentrated nitric acid added. The

oresulting solution was heated at about 60 C for two hours. The remaining volume was then transferred into a 50 ml standard flask and made to mark with distilled water. This was then filtered into an acid pre-washed polyethylene bottle. The metal content of this digest was then determined by flame AAS against known standards using the Perkin Elmer Analyst 200.

Sediment Heavy Metal Analysis 0Sediment samples (10 g) were oven-dried at 50 C.

After drying, visible remains of organisms and debris were removed. The dried sample was

crushed in a mortar and sieved through a 200 mm sieve to normalize particle size. From the dried sieved sediment sample, 2 g was placed in a kheljahl flask and 10 ml aqua regia (HCl: HNO ) 3

0added. The mixture was digested at 60 C for about two hours. The digest was cooled and transferred to a 50 ml standard flask and made up to mark with distilled water. The resulting digest solution was subsequently filtered using Whatman© filter paper into acid pre-washed polyethylene bottles. The metal content of the digest was then determined by Flame Atomic Absorption Spectrophotometry on Perkin Elmer AAnalyst 200.

Heavy Metal Analysis of Pachymelania auritaWhole animal tissue of collected P. aurita was extracted from the shell and dried in an oven at 50 0C for several hours until constant weight was achieved. The dried tissue was crushed and homogenised using a domestic blender. Two grams (2 g) of each sample was then digested with concentrated nitric acid for 180 minutes in a

oKheljahl flask at 60 C on a hot plate. The digest was transferred into a 50 ml standard flask and made up to volume with distilled water. The resulting mixture was subsequently filtered through a Whatman© filter paper into an acid pre-washed polyethylene bottles. The metal content of this digest was determined using Flame Atomic Absorption Spectrophotometer (AAS) (Perkin Elmer AAnalyst 200).

Statistical AnalysisOne way analysis of variance (ANOVA) and comparison of means by Duncan Multiple Range Test (DMRT) was used to test for significant differences in the level of heavy metals in the whole tissue of periwinkle samples collected from Ologe Lagoon. Pearson correlation (r) analysis was used to test for significant relationship among heavy metal levels in P. aurita, water and sediment of Ologe Lagoon.

Metal pollution index (Usero et al., 1997; Adeniyi et al., 2008) was used to compare the total metal


568

level in P. aurita at the different months of sampling. The MPI equation is as given below:

1/n MPI = (Cf x Cf x Cf . . . . . . . Cf ) 1 2 3 n

where Cf = concentration of the metal n n

in the sample

Bioaccumulation factor was estimated as the ratio of the concentration of heavy metal in animal tissue to the concentration of heavy metal in the sediment (BSAF = bio-sediment accumulation factor) or as the ratio of the concentration of heavy metal in animal tissue to the concentration of heavy metal in water (BWAF = bio-water accumulation factor)

RESULTS

Copper level in P. auritaThe monthly mean concentrations of copper (Cu) in the whole tissue homogenates of P. aurita are presented in Figure 2. The highest monthly mean concentration of Cu (21.68 ± 1.52 mg/kg) in P. aurita was recorded in February while the lowest monthly mean (1.03 ± 0.94 mg/kg) was recorded in November. Analysis of Variance (ANOVA) showed that the mean concentrations of Cu in P. aurita varied significantly (p<0.05) among months of collection. Further analysis using Duncan Multiple Range Test (DMRT), revealed that the mean level of Cu measured in P. aurita collected in February was significantly (p<0.05) different from the mean level of Cu in P. aurita collected in every other month but not significantly (p>0.05) different from that of January (Figure 2). There was no significant (p>0.05) difference in the concentration of Cu measured in P. aurita tissue in June, July, August, September, October, November and December (Figure 2).

BSAF =Heavy metal concentration in animal tissue

Heavy metal concentration in sediment

BWAF = Heavy metal concentration in animal tissue

Heavy metal concentration in water

Means (± SE bars) with the same letter on the same bar plots are not significantly different (p>0.05; Duncan)

Figure 2: Temporal variations in the mean level of copper in whole tissue of P. aurita collected from Ologe Lagoon.


Tab

le 1

: Pea

rso

n c

orr

elat

ion

co

-eff

icie

nt

(r)

mat

rix

for

rela

tio

nsh

ip a

mo

ng

hea

vy m

etal

leve

ls in

Pac

hym

elan

ia a

urita

, wat

er a

nd

sedi

men

t o

f O

loge

Lag

oo

n

Par

amet

ers

P. a

urita

W

ater

Se

dim

ent

Cu

Fe

Pb

M

n

Zn

C

u F

e P

b

Mn

Z

n

Cu

Fe

Pb

M

n

Zn

P. a

urita

Cu

1

Fe

-0.4

42

1

Pb

0.

126

0.24

9 1

Mn

-0

.504

0.

459

0.09

7 1

Zn

0.

550

0.18

7 -0

.164

-0

.196

1

Wat

er

Cu

0.65

4*

-0.2

10

0.07

9 -0

.361

0.

244

1

Fe

0.44

7 -0

.007

0.

141

-0.3

38

0.40

3 0.

543

1

Pb

-0

.230

0.

439

0.26

7 0.

246

-0.0

28

-0.4

24

0.02

3 1

Mn

0.

446

-0.2

68

-0.0

15

-0.6

53*

0.41

0 0.

328

0.41

7 -0

.162

1

Zn

0.

572

-0.2

02

-0.0

36

-0.1

21

0.16

1 0.

557

0.49

5 -0

.132

0.

242

1

Sedi

men

t

Cu

-0.5

15

-0.0

08

-0.2

21

0.47

1 -0

.392

-0

.437

-0

.055

-0

.110

-0

.445

-0

.034

1

Fe

-0.5

34

0.22

3 -0

.632

* 0.

370

0.03

6 -0

.390

-0

.191

0.

186

-0.2

20

-0.2

94

0.18

2 1

Pb

-0

.022

-0

.038

-0

.148

-0

.250

0.

076

-0.3

83

-0.4

34

-0.3

13

-0.0

90

-0.1

74

0.13

4 -0

.254

1

Mn

0.

322

-0.3

66

-0.2

94

-0.6

04*

0.19

5 0.

657*

0.

295

-0.4

90

0.64

8*

0.04

7 -0

.403

-0

.061

-0

.252

1

Zn

-0

.472

0.

101

-0.4

80

0.48

0 -0

.208

-0

.688

* -0

.523

0.

361

-0.3

85

-0.1

15

0.36

6 0.

699*

0.

075

-0.5

11

1

*

Co

rrel

atio

n is

sig

nif

ican

t at

th

e 0.

05 le

vel

Samuel et al.: Study of Heavy Metals in the Gastropod, Pachymelania aurita 569

Analysis of relationship between Cu in P. aurita and that of sediments and water using Pearson correlation coefficient (r) revealed that Cu in P. aurita exhibited a positive significant correlation with Cu in water column while it was found to be negatively but not significantly correlated with the concentration of Cu in the sediment (Table 1). The concentration of Cu measured in the P. aurita whole tissue correlated positively with the concentration of Pb and Zn but negatively with the concentration of Fe and Mn.

Iron level in P. auritaThe monthly mean concentrations of iron (Fe) in the whole tissue homogenates of P. aurita are presented in Figure 3. The highest monthly mean concentration of Fe (1036.04 ± 196.48 mg/kg) in

P. aurita was recorded in October while the lowest monthly mean (408.87 ± 87.71 mg/kg) was recorded in March. Analysis of Variance (ANOVA) showed that the mean concentrations of Fe in P. aurita varied significantly (p<0.05) among months of collection. Duncan Multiple Range Test (DMRT), revealed that the mean level of Fe measured in P. aurita collected in October was significantly (p<0.05) different from the mean level of Fe in P. aurita collected in January, February, March, September and December (Figure 3). However, the mean concentration of Fe in whole tissue homogenates of P. aurita collected in October was not significantly (p>0.05) different from the concentration of Cu measured in P. aurita tissue in April, May, June, July, August and November (Figure 3).


Figure 3: Temporal variations in the mean level of iron in whole tissue of P. aurita collected from Ologe Lagoon.

Analysis of relationship between Fe in P. aurita and that of sediments and water using Pearson correlation revealed that Fe in P. aurita exhibited a positive non-significant correlation with Fe in the sediment while it was found to be negatively but not significantly correlated with the concentration of Fe in the water column (Table 1). The concentration of Fe measured in the P. aurita whole tissue correlated positively with the concentration of Pb, Mn and Zn but negatively with the concentration of Cu.

Lead level in P. auritaThe monthly mean concentrations of lead (Pb) in the whole tissue homogenates of P. aurita are presented in Figure 4. The highest monthly mean concentration of Pb (0.08 ± 0.049 mg/kg) in P. aurita was recorded in August while the lowest monthly mean (0.02 ± 0.023 mg/kg) was recorded in June. Lead was only detected in the whole tissue homogenates of P. aurita in the month of January, June and August. Analysis of variance (ANOVA) showed that the mean concentrations of Pb in P. aurita varied insignificantly (p>0.05) among months of collection.


Correlation of Pb level in P. aurita with that of sediment and water using Pearson correlation revealed that Pb in P. aurita exhibited a positive non-significant correlation with Pb in water while it was found to be negatively but not significantly correlated with the concentration of Pb in the sediment (Table 1). However, Pb in P. aurita correlated negatively and significantly with Fe in the sediment (Table 1). The concentration of Pb measured in the P. aurita whole tissue correlated positively with the concentration of Cu, Fe and Mn but negatively with the concentration of Zn.

Manganese Level in P. auritaThe monthly mean concentrations of manganese (Mn) in the whole tissue homogenates of P. aurita are presented in Figure 5. The highest monthly mean concentration of Mn (191.78 ± 59.50 mg/kg) in P. aurita was recorded in November while the lowest monthly mean (39.06 ± 3.00 mg/kg) was recorded in March. Analysis of variance (ANOVA) showed that the mean concentrations of Mn in P. aurita varied insignificantly (p>0.05) among months of collection.


Figure 4: Temporal variations in the mean level of lead in whole tissue of P. aurita collected from Ologe Lagoon


Figure 5: Temporal variations in the mean level of manganese in whole tissue of P. aurita collected from Ologe Lagoon


Correlation of Mn level in P. aurita with that of sediments and water using Pearson correlation revealed that Mn in P. aurita exhibited a negative significant correlation with Mn in water column and with the concentration of Mn in the sediment (Table 1). The concentration of Mn measured in the P. aurita whole tissue correlated positively with the concentration of Fe and Pb but negatively with the concentration of Cu and Zn.

Zinc level in The monthly mean concentrations of zinc (Zn) in the whole tissue homogenates of P. aurita are presented in Figure 6. The highest monthly mean concentration of Zn (88.44 ± 7.54 mg/kg) in P.

P. aurita

aurita was recorded in April while the lowest monthly mean (36.29 ± 0.71 mg/kg) was recorded in November. Analysis of variance (ANOVA) showed that the mean concentrations of Zn in P. aurita varied significantly (p<0.05) among months of collection. Further analysis using DMRT, revealed that the mean level of Zn measured in P. aurita collected in April was significantly (p<0.05) different from the mean levels of Zn in P. aurita collected in March, August, September and November but not significantly different from the means of Zn in the P. aurita tissue collected in other months (Figure 6).


Figure 6: Temporal variations in the mean level of zinc in whole tissue of P. aurita collected from Ologe Lagoon

Analysis of relationship between Zn in P. aurita and that of sediments and water using Pearson correlation coefficient (r) revealed that Zn in P. aurita exhibited a negative non-significant correlation with Zn in sediment and with the concentration of Zn in the water column (Table 1). The concentrations of Zn measured in the P. aurita whole tissue exhibited a positive non-significant correlation with Cu and Fe while it exhibited a negative non-significant (p>0.05) correlation with Pb and Mn.

Metal Pollution Index and Bioaccumulation Factors of The metal pollution index (MPI) and the bioaccumulation factors (bio-sediment and bio-

P. aurita

water accumulation factors) are presented in Table 2. The computed MPIs (lead level not included) indicated that there was high level of heavy metal load in the whole tissue of P. aurita. The highest MPI (99.03) was recorded in the month of July while the lowest (46.76) was recorded in the month of November. Iron contributed highest concentrations of metal load. With the inclusion of lead (Pb) concentration detected in whole tissue of P. aurita, the MPI reduced in the month of January from 79.36 to 17.38, in June (from 75.10 to 14.48) and in August from 83.39 to 20.77 (Table 2).

The mean concentration of metals in whole tissue of P. aurita and sediment collected from Ologe


Table 2: Metal pollution index and bioaccumulation factors of Pachymalania aurita collected from Ologe Lagoon

Sampling months Heavy metals (mg/kg)

MPI Cu Fe Pb* Mn Zn

Jan 16.76 463.58 0.04 66.59 76.67 79.36 (17.38)

Feb 21.68 511.59 0.00 81.73 76.61 91.29

Mar 11.76 408.87 0.00 39.06 45.20 53.98

Apr 12.27 709.33 0.00 58.59 88.44 81.95

May 9.11 677.63 0.00 59.12 65.00 69.79

Jun 8.21 867.07 0.02 85.12 52.51 75.10 (14.48)

Jul 7.82 934.81 0.00 179.17 73.42 99.03

Aug 8.57 953.97 0.08 136.79 43.24 83.39 (20.77)

Sep 4.81 577.19 0.00 56.04 31.20 46.94

Oct 6.01 1036.04 0.00 97.50 84.08 84.53

Nov 1.03 666.73 0.00 191.78 36.29 46.76

Dec 5.98 617.16 0.00 145.23 51.23 72.39

Mean level in P. aurita

9.50 702.00 0.0125 99.73 60.32

Mean level in sediment

6.36 482.26 0.0067 144.51 47.24

Mean level in water

2.19 12.72 0.0075 1.20 2.88

BSAF 1.49 1.46 1.87 0.69 1.28

BWAF 4.34 55.19 1.67 83.11 20.94

Lagoon was in the decreasing order; Fe> Mn> Zn> Cu> Pb while the mean concentration in the water column of the site of P. aurita collection was in the order; Fe> Zn> Cu> Mn> Pb (Table 2). On the basis of computed bio-sediment accumulation factor (BSAF), Pb has the highest BSAF of 1.87 and Mn has the least BSAF of 0.69. Highest bio-water accumulation factor (BWAF) of 83.11 was recorded for Mn and the lowest BWAF of 1.67 was recorded for Pb. Based on the BWAF, the amount of Fe, Mn and Zn accumulated by P. aurita was found to be about 55.19, 83.11 and 20.94 times higher than the level detected in the water sample respectively. Generally, the levels of Mn detected in P. aurita was less than the ones detected in the sediment (BSAF = 0.69).

DISCUSSIONThe levels of all the investigated heavy metals (except Pb and Mn) in the whole tissue of

Pachymelania aurita collected from Ologe Lagoon varied significantly (p<0.05) for all the sampled months with no particular seasonal trend. Kumolu-Johnson et al. (2010) had attributed low levels of heavy metals in fish, Cynothrissa mento during the wet season at Ologe Lagoon to the dilution of the metals as a result of increase in rainfall and runoff. In contrast, Nussey et al. (2000) attributed high concentrations of heavy metals in tissues of moggel, Labeo umbratus from Witbank dam, Mpumalanga, South Africa during summer to higher rainfall causing pronounced leaching of metals into water. Otitoloju (2001) also attributed slight increase in metal levels of the gastropod, Tympanotonus fuscatus collected from Lagos lagoon during the wet season (study based on selected months) to the increased influx of fresh inland waters from the adjoining water bodies such as River Majidun, Ogun, Ona, Sasa and Osun which drain the neighbouring states


574

and discharges its contaminants including heavy metals into the Lagos lagoon. The monthly heavy metal variations in P. aurita observed in this study were largely influenced by the industrial and domestic activities of the study area rather than rainfall. Manganese, zinc, lead, iron have been reported to be rainfall independent (Egborge, 1991).

The computed metal pollution index (MPI) (46.76 - 99.03) indicated that there was high level of heavy metal load in the whole tissue of P. aurita with Fe contributing highest concentrations of metal load. The recorded mean concentrations of Cu (9.50 mg/kg) and Fe (702 mg/kg) in P. aurita were higher than the maximum allowable limits (Cu = 3.0; Fe = 100.0 mg/kg) in food fish set by the World Health Organization while the mean Zn and Pb was lower than the permissible limits of 75.0 and 2.0 mg/kg respectively (WHO, 2008). The levels of Cu (1.03 – 21.68 mg/kg), Fe (408.87 – 1036.04 mg/kg) and Zn (36.29 – 88.44 mg/kg) observed in P. aurita in the present study were higher than these values reported by Kumolu-Johnson et al. (2010) for Cynothrissa mentho of the same water body while the observed Pb (0.00 – 0.08 mg/kg) and Mn (39.06 - 191.78 mg/kg) was lower and higher respectively than the reported (Biney et al., 1994) Nigeria's freshwater fish species levels. The recorded concentrations of Pb and Cu in P. aurita were lower than those of the periwinkle, Tympanotonus fuscatus (Pb = 8.1 – 17.2; Cu = 21.4 – 40.2 mg/kg) reported by Otitoloju (2001) while the recorded concentrations of Zn, Mn and Fe were higher. The implication of these findings based on the concentrations of Cu and Fe exceeding WHO (2008) limit is that the consumption of the periwinkle, P. aurita from Ologe Lagoon can be said to be unsafe.

The mean concentration of metals in whole tissue of P. aurita and sediment collected from Ologe Lagoon was in the descending order: Fe> Mn> Zn > Cu > Pb while the mean concentration in the water body of the site of P. aurita collection was in the descending order: Fe> Zn > Cu > Mn > Pb. Of all the metals examined, Fe was found to be the most abundant. This agrees with the findings of Aderinola et al. (2009) in Lagos lagoon. Higher heavy metals levels in sediment than those in the surface water and organisms have been reported

(Otitoloju, 2001; Aderinola et al., 2009) for T. fuscatus in Lagos lagoon. However, in the present study, the levels of heavy metals (except Mn) in the sediment and water of Ologe Lagoon were lower than that in the tissue of P. aurita. This is in agreement with the trends reported by Davies et al. (2006) for Tympanotonus fuscatus radula in Elechi creek, Niger Delta. The ability of molluscs to accumulate heavy metals to levels higher than the acceptable minimum metal concentration without any detectable adverse effect has been widely reported and led to their inclusion in monitoring programmes of environmental pollution (Rainbow et al, 2002). The sediment accumulated more heavy metals than water in this study as have been observed by Otitoloju (2001), Chindah and Braide (2003), Davies et al. (2006) and Aderinola et al. (2009). Sediment is the major depository of metals in some cases, holding more than 99 % of total amount of a metal present in the aquatic system (Odiete, 1999).

Based on the bio-sediment accumulation factor (BSAF), the amount of Cu, Fe, Pb, and Zn accumulated by P. aurita was found to be about 1.49, 1.46, 1.87 and 1.28 times higher than the level detected in the sediment respectively. The level of Mn detected in P. aurita was less than the level detected in the sediment (BSAF = 0.69). The exposure of the edible periwinkle, Tympanotonus fuscatus to sublethal concentrations of Zn, Pb, Cu and Cd compounds of heavy metals by Otitoloju and Don-Pedro (2002) resulted in the bioaccumulation of higher concentrations of Zn and Pb ions that were about 2 – 6 times higher than the levels accumulated in control animals. Comparisons between the concentration of heavy metals in whole body tissues of T. fuscatus and the sediment of the media reported by this author showed that the concentration of the metals accumulated in tissues of T. fuscatus were about 2 – 729 times higher than that in the sediment. Likewise, field experiment employed by Otitoloju and Don-Pedro (2004) in assessing the impact of metal-laden effluents on the tissue concentration of heavy metals in Lagos lagoon showed that T. fuscatus accumulated Zn and Pb about 10 times higher than the concentrations detected in control animals. The termination of metal-laden effluent discharges into the experimental creek, after 150 days of


575

contamination, resulted in a rapid and significant decline in the concentrations of the metals detected in the sediment. However, the concentrations of metals accumulated in tissues of the periwinkles remained high.

Based on the bio-water accumulation factor (BWAF), the amount of Cu, Fe, Pb, Mn and Zn accumulated by P. aurita was found to be about 4.34, 55.19, 1.67, 83.11 and 20.94 times higher than the level detected in the water body respectively. Despite the relatively low concentrations of trace metals in the surrounding medium, aquatic organisms take up and accumulate them in soft tissues to concentrations above ambient environmental levels that are capable of exerting biological effects (Ekweozor et al., 2003).

Copper (Cu) and Pb in P. aurita exhibited a positive correlation with Cu and Pb in water while it was found to be negatively correlated with the concentration of Cu and Pb in the sediment respectively. Iron (Fe) in P. aurita exhibited a positive correlation with Fe in the sediment while it was found to be negatively correlated with the concentration of Fe in the water sample. Manganese (Mn) and Zn in P. aurita exhibited a negative correlation with Mn and Zn in water column and sediment respectively.

The bioaccumulation of pollutants to toxic levels especially heavy metals is detrimental to organisms generally, including plants and animals as they may be biomagnified through the food chain and transmitted to man. The BSAF and BWAF values ob ta ined in th i s s tudy showed tha t bioaccumulation occurred. Therefore periodic heavy metal monitoring of Ologe Lagoon is recommended in view of the socio-economic importance of the lagoon.

ACKNOWLEDGEMENTSThis study was partly supported by the University of Lagos Graduate Fellowship. The authors are grateful to Dr. B.E. Emmanuel for his assistance during the field trips. The assistance of Mr. Ben Begusa and Dr. Abayomi Akeem of the Department of Chemistry, UNILAG is highly appreciated.

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