HEAVY METAL POLLUTION STATUS OF A TROPICAL FRESHWATER LENTIC ECOSYSTEM, NIKE LAKE, SOUTH-EAST NIGERIA

Department of Applied Biology, Department of Fisheries and aquaculture,
Key words : Pollution, Heavy metals, Nike Lake, Nigeria


Abstract - The heavy metal (Iron (Fe), Zinc (Zn), Copper (Cu), Manganese (Mn), Lead (Pb) and chromium (Cr) pollution status of a tropical fresh water lentic ecosystem, Nike Lake, South-East Nigeria was assessed from five different stations between March 2006 and February 2007 using Hach Br-300 Model atomic absorption spectrophotometer. The results revealed that out of the 60 samples of water analysed, Fe, Zn, Cu, Mn, Pb and Cr were detected in 45, 39, 29, 26 and 15 in concentrations ranging from 0.079 to 0.509, 0.010 to 0.0307, 0.002 to 0.016, 0.020 to 0.056 and 0.001 to 0.008 mg/L respectively. Pb was detected in 16 samples in the range 0.001 to 0.003 g/L. Results obtained indicated that the concentrations of all the metals were within the WHO and FEPA prescribed limits for lentic water quality. These metal and the physicochemical parameters measured though did not significantly vary among the locations showed significant variation (P<0.05) between the seasons. The toxicological implications of this finding in relation to aquatic ecosystem and human health are discussed.

INTRODUCTION
Fish constitutes one of the major protein sources for humans around the world. Over the years however, increasing human influences have led to the decline in the vast and varied fish diversity. Advancement in technology as well as growth in population have led to high levels of industrialization and urbanization which in turn have led to environmental pollution resulting from the discharge of industrial effluents repute with most common heavy metals into the environment particularly natural waters. The characteristic feature of heavy metals is their strong attraction to biological tissues and in general their slow elimination from biological systems. They are environmentally stable, non-degradable and induce toxic effects (Pande and Sharma, 1999). Li (2008) noted that Man's activities have increased the quality and distribution of heavy metals in the atmosphere, on land , in rivers, lakes and sea. Some of our industries may endanger public health by discharging toxic substances (including heavy metals) into the water which may cause taste and odour problems, contaminating irrigated food crops and killing fishes and other natural life in water. Roberto et al. (2008) noted that some vehicle parts such as breaks and clutches have metals that can accumulate on road ways and later could be carried into water bodies such as lakes by storms and water run off. Due to urbanization of the Nike metropolis, the lake receives huge amount of domestic, agricultural and industrial wastes. Lake pollution is a serious water pollution problem which needs concern as it is known that lakes are less assimilating than the flowing water. The stagnant water of lakes can only undergo aerobic purification which is a slow process. Although several reports on heavy metals have been published on some lotic water boides in Nigeria (Ekpo and Ibok, 1999; Oguzie ,2003; Obasohan, 2006, 2007) very little information is available about the heavy metals pollution status of most lentic water bodies including Nike Lake. Nike Lake apart from its recreational values and source of domestic water and fish provides facilities for wildlife, vegetable and irrigation farms for the people. In view of the multipurpose use of the lake, it becomes relevant to assess the heavy metal pollution status of the lake to obtain baseline data important for conservation, management and sustainable utilization of the water resources.

MATERIAL AND METHODS
Water samples were collected monthly in precleaned polythene bottles from five different stations (each station being about 500 meters apart) of Nike Lake and preserved according to standard methods (APHA 2005). The water sample was aspirated to the flame Atomic Absorption Spectrophotometer (Perkin Elmer Model A Analyst and Vanan AA-575) for determination of the metals (Fe, Zn, Cu, Mn, Pb and Cr). Temperature was measured by Celsius thermometer, pH by digital pH meter and conductivity was determined on the spot using Hanna conductivity model H 198303. Dissolved oxygen was also determined on the spot using dissolved oxygen meter WTW GmbH multi 3401 Germany. The months of April to October constitute the rainy season while November to March constitute the dry season. RESULTS AND DISCUSSION The mean concentration of the various metals obtained from samples collected as well as the physicochemical parameters in five different stations form Nike Lake during the one year study period (March, 2006 - February 2007) is presented in Table1. Fe, Zn, Cu and Mn were detected in 75, 65, 48 and 43 per cent of the sample in concentrations ranging from 0.080-0.509, 0.010-0.019, 0.002-0.016 and 0.0110.040 mg/L respectively. Lead (Pb) was not detected in station I but was detected in 27 per cent of the samples in other stations in concentration ranging from 0.001-0.003 g/L. Similarly chromium (Cr) was not detected in station V but was present in 25 per cent of the samples in other stations in concentration that range from 0.001-0.008 mg/L. While the highest concentration of the metals was recorded in station III, the lowest was recorded in station I except for Cr where the lowest was recorded in station V. When compared with the standard permissible values by WHO (1994) and FEPA (2003), the values obtained for all the metals at Nike Lake fall within the prescribed limits in all the stations thus indicating that the Lake is safe for both aquatic organism and human. Our result even though agrees with Amoo et al. (2005) who reported that concentration of heavy metals in fishes, water and sediments of Lake Kainji Nigeria are within the acceptable standards of WHO (1994) however differed from Akabugwo et al. (2007) who showed significant (P<0.05) higher values for the heavy metals (cadmium, chromium, copper, lead and Zinc) in Uburu salt lake Nigeria. Our report also differed from Ochieng et al. (2007) who reported high concentration of heavy metals in water and surface sediments in five rift valley lakes in Kenya. The results of the physicochemical parameters (Table 1) showed that the mean temperature varied from a minimum of 26.50 0 C at station III to a maximum of 27.20oC at station II. The annual mean seasonal temperature variation (Table 2) shows that temperature was significantly higher (P<0.05) during the dry season (26.330C) and lower during the rainy season (25.46 0 C). The pH value ranged between a minimum of 6.53 in station II to a maximum of 6.81 in station I. The annual mean pH shows that it was higher during the dry season (6.63) but lower during the rainy season (6.42). The fall in pH during the rainy season may be attributed to the incursion of water recorded from rainfall in neighboring ponds and streams (Odo et al. 2007). The fall in pH during the rainy season may be responsible for the higher concentration of most of the metals during the rainy season as solubility of the metals increase with increasing acidity. Conductivity ranged from a minimum of 39mhos/cm in station V to maximum of 44 mhos/cm in station II and IV. Annual mean indicated that conductivity was higher during the dry (42 mhos/cm) than the rainy season (40 mhos/cm). Dissolved oxygen (DO) varied form a minimum of 5.55 mg/L in station V to 5.80 mg/L in station II. The annual seasonal mean indicate that DO was higher during the rainy season (5.61 mg/L) than dry season (5.48). The higher values of DO during the rainy season may be due to oxygenation resulting from high rainfall and mixing up of the run off coming from the surrounding areas. The concentration of the heavy metals in Nike Lake even though generally falls within the prescribed limits by WHO (1994) and FEPA (2003) are influenced significantly by the sampling season except Pb and Cr. The maximum concentration of Fe, Cu and Mn were recorded during the rainy season.

Table 1. Heavy metals concentrations and physiochemical parameters of fresh water lentic ecosystem, Nike Lake, South-East Nigeria. Mean concentrations of metals Station I II III IV V Mean Overall range Fe (mg/L) 0.080 (8) 0.089 (8) 0.509 (10) 0.097 (9) 0.089 (10) 0.173 (45) 0.0780.640 Zn (mg/L) 0.012 (6) 0.019 (7) 0.037 (9) 0.010 (8) 0.012 (9) 0.018 (39) 0.0090.050 Cu (mg/L) 0.002 (4) 0.003 (6) 0.016 (7) 0.005 (5) 0.006 (7) 0.006 (29) 0.0010.025 Mn (mg/L) 0.011 (3) 0.020 (7) 0.040 (6) 0.020 (4) 0.030 (6) 0.024 (26) 0.0100.055 Pb ( g/L) ND 0.001 (3) 0.003 (5) 0.001 (3) 0.001 (5) 0.001 (16) 0.0010.005 Cr (mg/L) 0.001 (2) 0.001 (3) 0.008 (4) 0.002 (3) ND 0.002 (15) 0.0010.009 Physicochemical parameters Temp. 26.67 27.20 26.50 27.01 26.60 26.80 26.0527.50 pH 6.81 6.53 6.64 6.67 6.70 6.67 6.486.87 Cond. Do2 42 44 42 44 39 42 41-46 5.56 5.80 5.66 5.58 5.55 5.63 5.456.20

Temperature, conductivity and dissolved oxygen are in 0C, mhos/cm and mg/L respectively, ND=Not detected. Figures in parenthesis indicate the number of samples out of 12 observations for each station and out of 60 samples for all. Table 2. Seasonal variations of heavy metals and some physicochemical parameters in Nike Lake, South ­ East Nigeria. Mean concentrations of metals Station I II III IV Mean Overall range Fe (mg/L) Dry: 0.874 Rainy: 0.079 Dry: 0.080 Rainy: 0.090 Dry: 0.450 Rainy: 0.490 Dry: 0.085 Rainy: 0.090 Dry: 0.077 Rainy: 0.083 Dry: 0.153 Rainy: 0.162 Zn (mg/L) 0.011 0.007 0.017 0.015 0.033 0.024 0.009 0.007 0.011 0.009 0.016 0.012 Cu (mg/L) 0.001 0.002 0.001 0.002 0.011 0.015 0.002 0.004 0.004 0.006 0.004 0.006 Mn (mg/L) 0.007 0.010 0.013 0.016 0.028 0.034 0.015 0.017 0.021 0.027 0.017 0.021 Pb ( g/L) ND ND 0.001 0.001 0.003 0.003 0.001 0.001 0.001 0.001 0.002 0.002 Cr (mg/L) 0.001 0.001 0.001 0.001 0.007 0.007 0.002 0.002 ND 0.003 0.003 Physicochemical parameters Temp. 25.69 24.63 26.83 25.94 26.05 25.75 26.90 25.20 26.20 25.80 26.33 25.46 pH 6.80 6.37 6.50 6.47 6.60 6.30 6.60 6.46 6.67 6.50 6.63 6.42 Cond. Do2 42 40 43 42 42 40 44 42 38 37 42 40 5.43 5.55 5.69 5.78 5.45 5.65 5.50 5.57 5.35 5.50 5.48 5.61

ND= Not detected. Dry Season = March, Nov- Dec 2006, Jan - Feb 2007, Rainy Season = April ­ Oct, 2006. Temp. Conductivity and dissolved oxygen are in 0C mhos/cm and mg/L respectively.

Roberto et al. (2008) had reported that rain water infiltrate the soil and underlying geologic formations, dissolve metals causing them to seep into aquifers and finally to the river system and lakes thus increasing their concentration. The fall in pH during the rainy season may have also resulted to higher concentrations of these metals during the rainy season as solubility of the metals increase with increasing acidity. This is in agreement with the report of Ochieng et al. (2007) that when pH falls, metals solubility increases and the metals particles become more mobile. The concentration of zinc was higher during the dry than rainy season as higher temperature favour the concentration of zinc. The results of the present study enabled us to determine the background concentrations of these metals in the water body. These data will constitute a reference to future studies on the evolution of contamination in this area. Periodic monitoring of these and other heavy metals not only in the Lake water body but in the fishes there in to ensure continuous safety of people in the area is highly recommended. Safe disposal of domestic wastes and industrial effluents should be practiced and where possible recycled to avoid these metals and other contaminants from going into the environment.

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