ARTICLE IN PRESS Environmental Research 109 (2009) 690–696
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Heavy metal burdens in patches of asphyxiated swamp areas within the Qua Iboe estuary mangrove ecosystem J.P. Essien a,1, V. Essien a, A.A. Olajire b,,1 a b
Department of Microbiology, University of Uyo, Uyo-Nigeria Industrial and Environmental Chemistry Unit, Department of Pure and Applied Chemistry, Ladoke Akintola University of Technology, Ogbomoso, Nigeria
a r t i c l e in f o
a b s t r a c t
Article history: Received 6 September 2008 Received in revised form 26 March 2009 Accepted 14 April 2009 Available online 22 May 2009
This study examined the burden of Zn, Cu, Ni, Pb, Cr and V in patches of asphyxiated mangrove swamp areas within Qua Iboe Estuary mangrove ecosystem by sediments and surface water analysis; in order to establish natural background levels of these metals and to assess anthropogenic inﬂuences on them. The analysis shows that the mean concentrations (mg/kg, dw) of Zn, Cu, Ni, Pb, Cr and V in sediments from asphyxiated and healthy mangrove ecosystems of Qua Iboe vary from 36.3–179.4, 29.2–43.2, 3.6–37.4, 39.6–93.8, 0.15–0.53 and 2.9–9.3, with the former exhibiting higher metal accumulating potential. Although heavy metal concentrations in surface water of the asphyxiated swamp were low, their accumulative effect is signiﬁcant. The concentrations of Cu and Pb in surface water of this ecosystem exceeded the water quality criteria while Ni and Cr were under the maximum concentration for drinking water quality and protection of aquatic life. The values of pollution load index (PLI), which are generally greater than unity, show that the sediments and the surface water from the asphyxiated mangrove ecosystem were polluted with heavy metals, thus suggesting anthropogenic activities as a possible source of these metals. The mean concentrations of Zn, Ni and Pb exceeded the effects rangelow (ERL), indicating that there may be some ecotoxicological risk to organisms living in asphyxiated mangrove sediments. & 2009 Elsevier Inc. All rights reserved.
Keywords: Sediment Heavy metals Asphyxiated swamp Ecotoxicological risk Anthropogenic input
1. Introduction Habitat destruction through human encroachment has been the primary cause of mangrove loss (Pons and Fiselier, 1991; Farnsworth and Ellison, 1997). Diversion of freshwater for irrigation and land reclamation have destroyed extensive mangrove forests. In the past several decades, numerous tracts of mangrove have been converted for aquaculture, fundamentally altering the nature of the habitat. Sum estimates put global mangrove loss rates at one million hectare per year, with mangroves in some regions in danger of complete collapse (Kathiresan and Bingham, 2001). The Qua Iboe Estuary is a dominant hydrographic feature of the Niger Delta region of Nigeria. The geomorphology of the southernmost part of the river (lower reaches) consists of ﬁne sandy coastal beach ridges covering about 560 km2. The estuary with associated tidal creeks constitutes a rich assemblage of ecohydrological biotopes, dominated mainly by vast intertidal mangroves and mixohaline lagoons (Ukpong, 1992; Essien and Ubom,
E-mail address: [email protected]
(A.A. Olajire). Present Address: Health Safety and Environment Department, Shell Petroleum Development Company of Nigeria, Port-Harcourt, Rivers State-Nigeria. 1
0013-9351/$ - see front matter & 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.envres.2009.04.005
2004; Essien et al., 2009). The macrophytes of the coastal mangrove swamp include Rhizophora harrizoni, R. mangle, R. racemosa, Avicennia africana, Laguncularia racemosa and invading stands of brackish water palm Nypa fruiticans (Ekwere et al., 1992; Ukpong, 1995). Population growth and speciﬁcally residential developments along estuarine shores are usually associated with environmental impacts that include nutrient pollution, widespread eutrophication and inputs of metals or organic pollutants. Petroleum hydrocarbons and associated metals represent signiﬁcant sources of pollution for the aquatic bodies within the Niger Delta of Nigeria (Olajire et al., 2005; Essien and Antai, 2005; Essien et al., 2008, 2009). Asphyxiation is the process by which asphyxia (lack of oxygen) interferes with oxygenation of the ecosystem. It is a condition of severe deﬁcient supply of oxygen to the plant body that arises from being unable to respire normally. Asphyxia causes generalized hypoxia, a depressed concentration of dissolved oxygen in water, which primarily affects the tissue. Characterized in most cases by air hunger, asphyxia occurs when dissolved oxygen levels falls below 23 mg/l in aquatic ecosystems (Diaz and Rosenberg, 1995). Asphyxiated swamp chokes, the vegetation collapses (decays) into water supplying bacteria and fungi with more organic matter to mineralize but less or no oxygen for effective decomposition and thus creates the ‘‘dead zone’’ (Dodds, 2006).
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The urge to breathe is triggered by rising carbon dioxide concentration from decaying organic matter. Decreased concentration of dissolved oxygen may also result from eutrophication when nutrients (e.g nitrogen and phosphorus) are discharged in excess into aquatic ecosystem (Gunnarsson et al., 1995). In freshwater ecosystems, this often leads to algal bloom that can degrade aquatic habitats by reducing light levels and increasing the amount of organic matter in the system. As the organisms die and sink to the bottom, oxygen-dependent microbes, depleting the water of oxygen until none is left. Because of this depletion, almost nothing can grow in the swamp, which has now become dead or asphyxiated (Rabalais et al., 2002). Asphyxiation of swamps may also result from climate change. Sunlight is of major signiﬁcance to swamp dynamics, being the primary source of energy (Meade, 1995). As sun shines on a swamp it warms the surface water, the surface water becomes lighter than the cooler, denser waters trapped at the bottom. As a result, the water becomes stratiﬁed into layers (the top and bottom layers), which do not mix with each other (Hanifen et al., 1995). Thermal stratiﬁcation impairs the water quality because it affects dissolved oxygen levels. As water temperature increases, the water capacity to hold oxygen decreases (Aulenbach et al., 1997).
Previous studies on the impact of heavy metals in the southeastern part of the Niger Delta mangrove ecosystems (Eja et al., 2003; Benson et al., 2007a; Udosen et al., 2007; Benson et al., 2007b; Essien et al., 2008, 2009) had focused on its accumulation in sediment, ﬁshes and shellﬁshes. The present study has been initiated with the following objectives; (1) to evaluate metal concentrations and eutrophic conditions in two locations; an impacted site (asphyxiated swamp) and a reference location (healthy swamp); (2) to assess its ecotoxicological signiﬁcance, thus contributing to the knowledge and management of this region in future. Our results would provide a baseline against which future effects can be evaluated.
2. Materials and methods 2.1. Study site The Qua Iboe Estuary (Fig. 1), a mesotidal estuary is located in the coastal zone of Nigeria. The estuary lies within latitude 41 300 to 41 450 N and longitude 71 310 to 81 000 E. Although sandy beaches are known to develop in some portions of the estuary, most of them are fringed with tidal mud ﬂats and oligotrophic mangrove swamp (Essien and Ubom, 2004). The estuary constitutes a major inlet into the land and is often utilized by the inhabitants of the oil producing communities of
NIGERI LEGEND Sampling stations Mangrove swamp Water
Study area N W
Im o Ri ve
Qua Iboe Rive
AS -A Bight of
Fig. 1. Qua Iboe Estuary mangrove ecosystem showing the sampling location. Insert: Map of Nigeria showing the location of the study area.
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the Niger Delta as the main transport route. It is a multi use resource with ﬁshery as the most dominant. The estuary also serves as the receiving water body for domestic and industrial wastes especially petrochemical wastes. Three patches of asphyxiated mangrove areas (AS-A, AS-B and AS- C) were detected within the larger Qua Iboe mangrove ecosystem.
2.2. Sampling In the asphyxiated swamp, three sampling patches or locations–AS-A, AS-B and AS-C (Fig. 1) were selected. Similarly, locations HS-A, HS-B and HS-C were also mapped out in the healthy swamp to serve as control. Samples of the intertidal (epipelic) and subtidal (benthic) sediments, and the overlying surface water were collected in May 2007 from these sampling stations. A short core sampler was used to collect epipelic sediment with undisturbed sediment-water interfaces. Subtidal or benthic sediment samples were obtained with the aid of a Shipek grab sampler. At each sampling station, One (1) surface water sample and three (3) each of the epipelic and benthic sediment samples were obtained from different locations, homogenized and the sub-samples were carefully transferred into clean glass containers, and preserved in ice-cooled boxes. All water samples were collected from the surface (10–25 cm) in sterile glass bottles. The containers were opened to ﬁll and closed below the water. All containers were rinsed at least three times with the water being sampled before collection. Therefore, a total of 21 samples, comprising 3 samples of surface water, 9 each of epipelic and benthic sediments were collected per ecosystem (asphyxiated or healthy swamp). The samples were transported to the laboratory and analyzed within 12 h of collection.
2.3. Sediment and water characterization Sediment physicochemical parameters were determined using standard procedures (Radojevic and Bashkin, 1999). Fast changing parameters, such as pH, salinity, and temperature were measured on the ﬁeld using portable multi-probe quality meter (Model U7, Horiba Ltd). The soluble exchangeable cations (Ca2+, Na+, K+, Mg2+) were determined using ﬂame atomic absorption spectrophotometer after extraction using 1.0 M ammonium acetate (1 M ¼ 1 mol dm3), at pH 7. The nutritive salts (NH+4, SO2 4 , Cl , NO3 ) were determined by Nesslerization (Radojevic and Bashkin, 1999), turbidimetric (APHA, 1998), Argentometric titration (Mohr’s method) (APHA, 1985) and colorimetric method (APHA, 1985) respectively. Phosphorus as reactive orthophosphate was determined using the stannous chloride method (APHA, 1985) specially suited for determining low amounts of phosphate concentrations. Organic carbon content was determined by the rapid wet oxidation method based on Walkey and Black procedure (Jacobsen, 1992; Page et al., 1982; AOAC, 1975). Total nitrogen was determined by classical Kjeldahl digestion followed by distillation. Total nitrogen in the distillates was determined by spectrophotometry. Particle size distribution (grain size analysis) was determined by the hydrometer method (AOAC, 1975; Juo, 1979).
2.4. Analytical procedures The sediment samples were dried at 70–80 1C for 48 h. and thereafter gently ground with rolling pin to disaggregate the samples but not break down the grains themselves, and sieved to collect less than 63 m grain sizes. The sediment samples were digested as described by Ho, et al. (2003) and Miroslav and Vladimir (1999). Precisely, 2.0 g of sediment was digested with a solution of concentrated HNO3 (0.3 ml) and HCl (6.0 ml) to near dryness and allowed to cool before 20 ml of 5.0 M HNO3 (1 M ¼ 1 mol dm3) solutions were added. The solutions were allowed to stay overnight and ﬁltered. The ﬁltrates were transferred into 100 ml volumetric ﬂask and made up to the mark with 0.5 M HNO3 (Binning and Baird, 2001) and kept for metal analysis. A reagent blank (without sample) was prepared using a mixture of HNO3 and HCl, and the entire sequence of steps was followed as described for the sample preparation. The water samples were preconcentrated as described by Ramesh et al. (2001). One hundred mL samples were passed through 0.2 mm ﬁlter to separate suspended matter, and stored in polyethylene bottles (previously treated with dilute HNO3 and wash with warm organic detergent and distilled water). After ﬁltration, concentrated HNO3 was added to the samples to adjust the pH value to 4.0070.05; and buffered with 2 mL of 0.1 M (1 M ¼ 1 mol dm3) potassium hydrogen phthalate solution (pH 4). Two mL of 1% (W/V) methanolic solution of sodium dibenzyl dithiocarbamate (NaDBDTC) was added. The solution was stirred intermittently for 15 min. The solution was then ﬁltered under vacuum through a 25 mm (0.45 mm pore) membrane ﬁlter. The sorbed elements on the ﬁlter are subsequently eluted with 4 mol dm3 HNO3 and the acid eluates were kept for metal analysis. The digested sediment sample solution, acid eluates desorbed from the ﬁlter, and the blank were analyzed for the concentrations of heavy metals (Cr, Pb, Ni, V, Zn, and Cu) using an inductively coupled plasma spectrophotometer (Optima 3000-Optima 3000–Perkin Elmer). The analysis was duplicated to verify the precision of the method of digestion. The instrumental detection limits (IDL) were: 0.02, 0.002, 0.01, 0.002, 0.01, and 0.01 mg kg1 for Cr, Pb, Ni, V, Zn and Cu,
respectively. Duplicates and method blanks were employed to test for precision, accuracy and reagent purity used in the analytical procedures. In order to reduce the detrimental effects of overlapping spectral interferences on element quantitation during metal analyses, an inter-element correction standard was prepared by using standardized solution of metal ions prepared from their salts. A mixture of commercially available 100 mg kg1 stock solutions (Analar Grade) of Cr3+, Pb2+, Ni2+, V2+, Zn2+ and Cu2+ were prepared as interelement working standard solution to verify that the overlapping lines do not cause the detection of elements at concentration above methods detection limits (MDLs) (Popek, 2003).
3. Results and discussion 3.1. Physico-chemical characteristics The mean values of pH, organic carbon (%), total nitrogen (%), exchangeable cations, and percentages of sand, silt and clay and nutritive salts of sediments from asphyxiated and healthy mangrove ecosystems of Qua Iboe are given in Table 1. Mean values of organic carbon in sediments from asphyxiated and healthy mangrove ecosystems ranged from 5.0 to 12.4%. The low organic carbon contents (8.8% and 5.0%) obtained for the benthic sediment might be as a result of marine sedimentation and mixing processes at the sediment/water interface, where the rate of delivery as well as rate of degradation by microbial-mediated processes can be high (Canuel and Martens, 1993). The relatively high percentage of organic carbon (12.8%) in epipelic sediment of asphyxiated swamp can be related to the decomposition of mangrove litter and hydrolysis of tannin in mangrove plants releasing various kinds of organic matter and acids. (Liao, 1990). The total nitrogen ranged from 0.2% to 0.8%. The benthic sediment from healthy mangrove ecosystem has least nitrogen content of 0.20%, while the epipelic sediment from asphyxiated swamp has the highest value of 0.8%. The palaeoenvironmental signiﬁcance of C/N ratio and its usefulness as an organic matter identiﬁer has been emphasized by Meyers (1994). The C/N ratios have been used to distinguish between organic matter inputs in estuaries, since autochthonous marine organisms rich in protein material have C/ N values (4–10) much lower than terrestrial plants (420) (Kawamura and Ishiwatari, 1981; Meyers, 1994, 1997). In this work, the C/N ratios of all the sediments except epipelic sediment from asphyxiated swamp were greater than 20, indicating terrigenous organic matter, mainly from vascular plant detritus or grassy material. The epipelic sediment from asphyxiated swamp has C/N ratio of 16.0, indicating an input of different mixtures of land and aquatic organic matter. The pH of the sediments ranged from slightly acidic to weakly basic (6.08–7.6), where acidic nature is mainly pronounced in epipelic sediment from both asphyxiated and healthy mangrove ecosystems and this was partly ascribed to oxidation of FeS2 and FeS to H2SO4 and partly resulted from the decomposition of mangrove litter and hydrolysis of tannin to organic acids (Liao, 1990). The sediment samples show a variable admixture of sand, silt and clay. Results show that sand (463 mm) was the main component of all sediment samples, with a range from 52.2% to 65.3%. Mean clay contents were in the range of 18.3% to 31.4%. The dominance of sand fraction might be as a result of high energy level in the estuary, giving the depositional area a sandy beach environment. The high levels of nutritive salts such as CO2 3 (111 mg/kg); SO2 (201.5 mg/kg); Cl (142.5 mg/kg); NH+4 4 (178.8 mg/kg) especially in epipelic sediments from asphyxiated swamp were indicative of the inﬂuences of human mediated activities. Crude oil pollution has also been associated with increase in nutritive salt and salinity levels of aquatic ecosystems (Rhykered et al., 1995; Ward et al., 1980) and may have + contributed to the high concentrations of SO2 4 , Cl , and NH4 salts in the epipelic sediments of asphyxiated swamp. The
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Table 1 Ranges (means,7SD, n ¼ 9) of physicochemical characteristics of sediment samples from Asphyxiated and healthy Mangrove ecosystem of Qua Iboe. Parameter
Epipellic sediment Asphy.
Temperature (1C) pH Organic carbon (%) Total nitrogen (%) Available P Salinity (%) Dissolved oxygen Ca Mg Na K Nutritive salts SO2 4 NO–3 Cl NH+4 CO2 3 PSD (%) Sand (450 mm) Silt (42–63 mm) Clay (o 2 mm) THC
28.0–31.1 5.8–6.5 10.4–16.2 0.58–0.99 5.3–6.7 3.7–4.8 1.9–2.7 5.9–7.2 2.7–4.1 9.7–11.8 0.2–0.4 162.7–223.8 19.1–23.8 123.4–161.3 157.7–196.5 106.2–116.0 53.8–63.1 9.2–12.2 24.8–35.4 17.1–23.4
Benthic sediment Healthy
(29.270.8) (6.170.3) (12.872.3) (0.870.1) (6.170.4) (4.270.4) (2.170.3) (6.670.5) (3.270.4) (10.670.7) (0.2370.05) (201.5724.7) (21.371.5) (142.5710.2) (178.8714.0) (11173.6) (57.473.8) (10.971.1) (31.474.2) (20.271.9)
28.0–30.0 6.3–6.8 8.4–12.8 0.3–0.6 4.3–5.9 3.0–3.8 3.4–5.3 5.7–6.9 2.2–3.1 9.2–11.4 0.1–0.2 61.8–72.9 19.2–26.2 108.5–144.7 101.6–190.5 66.2–98.1 62.4–68.6 9.7–11.2 21.7–26.8 11.0–21.3
Asphy. (28.770.7) (6.570.2) (10.271.5) (0.570.1) (5.270.5) (3.470.3) (4.170.5) (6.270.4) (2.670.3) (10.070.7) (0.1570.02) (68.974.2) (23.772.4) (125.9712.6) (161.5727.4) (33.6711.1) (65.372.1) (10.370.6) (24.372.1) (16.774.2)
27.0–29.0 6.6–7.3 7.9–10.4 0.4–0.6 5.3–6.7 2.8–4.1 1.4–2.0 6.9–10.8 3.3–4.7 9.3–11.7 0.13–0.18 161.8–198.2 24.4–26.9 13.1–51.7 18.3–54.5 94.3–116.3 48.1–64.9 20.9–33.8 14.2–23.8 22.2–29.4
(28.170.6) (6.870.2) (8.870.8) (0.570.1) (5.970.4) (3.870.4) (1.770.3) (8.471.4) (4.170.8) (10.670.8) (0.1670.03)
27.0–28.0 7.4–7.8 4.3–5.6 0.2–0.3 1.2–2.4 2.1–2.7 2.0–3.4 6.0–7.3 3.0–3.9 10.2–11.9 0.16–0.19
(27.470.5) (7.670.2) (5.070.4) (0.270.02) (1.770.3) (2.470.2) (2.570.5) (6.870.4) (3.470.3) (11.070.5) (0.1870.02)
(176.2713.3) (25.670.8) (28.2714.3) (29.1712.9) (106.779.7)
17.8–23.4 26.7–31.4 2.9–4.8 10.4–16.6 47.7–56.2
(20.271.9) (29.371.7) (3.670.7) (12.172.1) (51.072.8)
(52.275.1) (29.573.6) (18.372.8) (27.176.8)
60.1–81.2 5.5–10.9 13.0–29.5 13.1–17.4
(64.576.6) (9.671.6) (25.274.9) (14.871.4)
All measurements are in mg/kg, except otherwise indicated; PSD, particle size distribution.
concentrations of the micronutrients obtained in the present study were relatively stable (RSDo20%) and did not exceed the natural occurrence levels reported for cations (e.g. 1350, 410, 10,500 and 390 mg/L for Mg, Ca, Na and K respectively) in tropical seawater (Hem, 1985). The physico-chemical characteristics of surface water from asphyxiated and healthy mangrove ecosystems are given in Table 2. The lower values of pH and dissolved oxygen were found in surface water from asphyxiated mangrove ecosystem where anthropogenic activities are obvious, introducing acidic industrial efﬂuent with higher content of organic matter, which is degraded by bacteria, utilizing dissolved oxygen in the process. The mean dissolved oxygen level (23.3 mg/l) of asphyxiated swamp was a little above 23 mg/l reported for dead zone (Diaz and Rosenberg, 1995), an indication that the swamp is dying. High level of nitrate (1.81 mg/l) was also found in surface water from asphyxiated mangrove swamp. The nutrient concentrations detected in surface waters of asphyxiated and healthy mangrove ecosystems were low, and ranged from non-detectable level for CO2 3 in surface water of the healthy mangrove ecosystem to 25.6 mg/l dissolved oxygen in surface water of the healthy mangrove ecosystem. The total suspended solid in surface water from asphyxiated swamp ranged from 3.44 to 6.66 mg/l with a mean value of 5.65 mg/l. The high total suspended solid also contributed directly to the high turbidity recorded (1.32 to 1.62 NTU) for surface water from this ecosystem. The available phosphorus of the surface water ranged from 1.32 to 2.64 mg/l with a mean value of 1.89 mg/ l for asphyxiated swamp; and from 1.28 to 2.01 mg/l, with a mean value of 1.62 mg/l for healthy mangrove ecosystem. These values greatly exceed 0.1 mg/l, the recommended maximum concentration value of available phosphorus in ﬂowing water to discourage excessive growth of aquatic plants (Sawyer et al., 1994).
3.2. Heavy metal contamination The range, mean and standard deviation of heavy metals in sediments and surface water from asphyxiated and healthy
Table 2 Ranges (means,7SD, n ¼ 9) of physicochemical characteristics of surface water samples from Asphyxiated and healthy Mangrove ecosystem of Qua Iboe. Parameter
Temperature pH Organic carbon (%) Total nitrogen (%) Available P Salinity (%) Dissolved oxygen BOD COD Turbidity (NTU) TSS SO2 4 NO 3 Cl NH+4 CO2 3 THC
26.0–28.1 6.2–6.7 3.2–7.6 0.2–0.5 1.3–2.6 1.9–3.6 20.3–24.3 2.8–5.4 21.3–38.0 1.3–1.6 3.4–6.7 2.7–3.9 1.3–2.0 1.4–4.0 1.2–2.4 0.00–1.1 1.2–4.1
Healthy (26.770.7) (6.470.2) (5.871.3) (0.470.1) (1.970.4) (2.770.6) (23.370.7) (4.170.9) (29.976.4) (1.470.1) (5.671.0) (3.370.4) (1.870.3) (2.970.8) (1.870.3) (0.470.3) (2.571.0)
25.0–27.1 6.8–7.7 2.8–4.0 0.1–0.2 1.3–2.0 1.2–2.1 24.7–26.4 0.6–0.9 18.8–24.1 1.0–1.4 5.4–6.4 1.4–2.1 0.8–1.4 1.1–1.9 1.0–1.6 ND 1.0–1.4
(26.070.7) (7.370.3) (3.370.4) (0.270.02) (1.670.3) (1.870.3) (25.670.5) (0.870.1) (18.576.8) (1.270.1) (5.970.4) (1.870.3) (1.270.2) (1.670.2) (1.670.2) (1.270.2)
All measurements are in mg/l, except otherwise indicated; BOD, biological oxygen demand; COD, chemical oxygen demand; ND, not detected.
mangrove ecosystems are given in Table 3. From all metals studied, Zn showed the highest mean concentration values of 179.4 and 146.8 mg/kg respectively in epipelic and benthic sediments of asphyxiated mangrove ecosystem; followed by Pb with concentration values of 93.8 and 78.1 mg/kg respectively in epipelic and benthic sediments of asphyxiated mangrove ecosystem. The mean concentration values of Cu (33.5 and 29.2 mg/kg in epipelic and benthic sediments respectively) from asphyxiated mangrove ecosystem were lower than mean concentration values of Cu (43.2 and 39.9 mg/kg in epipelic and benthic sediments respectively) from healthy mangrove ecosystem. The sediments from asphyxiated mangrove ecosystem are not as enriched with Cu as expected, and this might be due to dilution of contaminated sediment by natural sediments transported through the channel of healthy mangrove
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Table 3 Ranges (mean7SD, n ¼ 3) of heavy metal concentrations in sediment (mg/kg) and surface water (mg/l) samples from Asphyxiated and Healthy Mangrove Ecosystem of Qua Iboe. Metal Asphyxiated Mangrove swamp Epipelic Sediment Zn Cu Ni Pb Cr V V/Ni
157.2–191.4 33.1–34.0 36.9–38.4 90.2–96.6 0.12–0.31 8.7–10.1 0.25
Healthy Mangrove Swamp Surface water
(179.4719.2) 137.2–155.9 (146.879.4) 0.02–0.04 (0.0370.001) 99.0–103.8 (101.572.1) 24.0–46.5 (36.3711.4) (33.570.4) 28.5–30.1 (29.270.8) 0.02–0.05 (0.03670.015) 41.4–44.8 (43.271.7) 38.1–41.4 (39.971.7) (37.470.8) 26.8–28.7 (27.871.0) 0.01–0.02 (0.01370.005) 18.5–23.9 (21.272.7) 3.1–3.9 (3.670.4) (93.873.2) 75.4–82.6 (78.173.9) 0.005–0.017 (0.0170.006) 39.3–45.7 (42.273.2) 38.3–41.4 (39.671.6) (0.2170.09) 0.11–0.19 (0.1570.04) 0.019–0.021 (0.0270.001) 0.15–0.18 (0.1670.02) 0.47–0.62 (0.5370.07) (9.370.8) 3.4–5.1 (4.470.9) 0.05–0.08 (0.0770.02) 8.3–9.0 (8.670.4) 2.7–3.0 (2.970.2) 0.16 5.4 0.41 0.80
ecosystem of Qua Iboe. The V/Ni ratio of the sediments from asphyxiated and healthy mangrove ecosystems is given in Table 3. The values (0.16–0.80) comply with emissions from oil-related industries, and comparable with values reported for Nigerian crude oils (Olajire and Oderinde, 1993). Of all the metals studied, vanadium showed the highest mean concentration (0.07 mg/l) in surface water from asphyxiated mangrove ecosystem (Table 3). The mean concentration values of Zn (0.03 mg/l), Cu (0.036 mg/l), Ni (0.013 mg/l), Pb (0.01 mg/l) and Cr (0.02 mg/l) were obtained in surface water from asphyxiated mangrove ecosystem. Intensive ﬁshing and industrial activities, weathering of mineral and rocks are possible sources for the enrichment of these metals in this ecosystem. The V/Ni ratios of the surface water from asphyxiated and healthy mangrove ecosystems are 5.4 and 17.3 respectively. These values also comply with pollution from petroleum–related industries.
The evaluation of sediment quality is an important part of assessing the health of a mangrove ecosystem. The sediment quality of the present studied area was estimated by comparing the effects range low (ERL), effects range medium (ERM) and US EPA sediment quality guidelines. Results are given in Table 4 and clearly show high metal accumulation rate with respect to Zn, Ni and Pb. Over 58.5% of benthic sediment from asphyxiated mangrove ecosystem exceeded the ERL for Zn and 100% for Ni, Pb and Zn (for epipelic sediment only); while none (0%) exceeded the ERL for Cu and Cr. These high rates are clearly associated with anthropogenic impacts and may have detrimental effects on the benthic environments. Sediments were also classiﬁed as nonpolluted, moderately polluted and heavily polluted, based on the sediment quality guidelines (SQG) of US Environmental Protection Agency (Perin et al., 1997). Sediments contaminated with Pb were considered ‘‘heavily polluted’’ per the sediment quality guidelines (Fig. 2). Also, Zn, Ni, and Cu were ‘‘moderately polluted’’ using the sediment quality guidelines; while Cr was classiﬁed as nonpolluted. The concentrations of Cu and Pb in surface water of this ecosystem exceeded the water quality criteria for protection of aquatic life (Frits, 1990), and Pb concentration in the surface water from this ecosystem also exceeded the maximum concentrations for drinking water quality (WHO, 1993; USEPA, 1990). The concentrations of Ni and Cr in the surface water from this ecosystem were below the maximum concentration for drinking water quality and protection of aquatic life. 3.4. Pollution load index In order to understand the contamination state of heavy metals, Tomllinson’s pollution load index (PLI) (Tomlinson et al.,
0.01–0.03 0.004–0.024 0.001–0.006 0.00–0.012 0.003–0.005 0.01–0.08 17.3
(0.01670.007) (0.01170.001) (0.00370.002) (0.00670.006) (0.00470.001) (0.0570.04)
1980) was calculated using the heavy metal data and natural background concentrations. In the present study, we do not have a sufﬁcient number of sampling sites, which we can say are not likely to receive anthropogenic inputs. But because samples were collected from both asphyxiated and healthy mangrove ecosystems, we agree that the healthy mangrove ecosystem has the higher probability of being uncontaminated. Thus, for the purpose of calculating the PLI, we use data of samples taken from healthy ecosystem as natural background concentrations. The PLI is obtained as a contamination factor (CF) of each metal with respect to the natural background value in the sediment (Angulo, 1996) by applying the following equations: CF ¼
PLI ¼ 3.3. Sediment and water quality assessment
C¯ sample C¯ background ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ p n CF 1 CF 2 CF 3 CF n
where, CF is the contamination factor, n is the number of metals; C¯metal is the mean metal concentration in sediments from asphyxiated mangrove swamp; C¯background value is the mean metal concentration in sediments from healthy mangrove swamp. The PLI represents the number of times by which the metal content in the sediment exceeds the average natural background concentration, and gives a summative indication of the overall level of heavy metal toxicity in a particular sample. The contamination factor and pollution load index of sediments and surface water from asphyxiated mangrove ecosystem are given in Tables 4 and 5 respectively. Our results show that the values of contamination factor for Zn, Ni, Pb, V and Cr (for epipelic sediment) in the study area are high (41), suggesting inﬂuence of external discrete sources like industrial and urban runoff and anthropogenic inputs. However, the values of contamination factor for Cu and Cr (for benthic sediment) are low (o1). The values of pollution load index (Table 4) were greater than unity (41) in all the sediment types from this ecosystem. An overall assessment indicates that this asphyxiated mangrove ecosystem is considered to be of pollution concern in view of crude oil spillages that have been occurring in the region. The contamination factor and pollution load index of the surface water from this ecosystem are high (41), also suggesting anthropogenic inputs.
4. Conclusion The asphyxiated swamp has substantial accumulation of Pb in the sediments, Cr occurs at non-polluted level, while others occur at moderately polluted level. The surface water from asphyxiated swamp has high contents of Cu and Pb that exceed the water quality criteria, while Ni and Cr were below the maximum concentration for drinking water quality and protection of aquatic
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Table 4 Elemental concentrations, sediment quality guidelines from USEPA and National Oceanic and Atmosphere Administration (Long et al, 1995). Metal
ESAsp Zn Cu Ni Pb Cr V PLI
179.4 33.1 37.4 93.8 0.21 9.30 1.40
BSAsp (1.77) (0.77) (1.76) (2.22) (1.31) (1.08)
146.8 29.2 27.8 78.1 0.15 4.40 1.66
(4.04) (0.73) (7.72) (1.97) (0.29) (1.63)
o90 o25 o20 o40 o25 n.i
90–200 25–50 20–50 40–60 25–75 n.i
4200 450 450 460 475 n.i
150.0 34.0 21.0 47.0 81.0 n.i
% sample exceed ERL
410.0 270.0 52.0 220.0 370.0 n.i
100 0 100 100 0 –
58 0 100 100 0 –
ESAsp. and BSAsp. are the epipelic and benthic sediments of Asphyxiated mangrove ecosystem; values in parenthesis are the contamination factors; ERL, effect range low (NOAA); ERM, effect range medium (NOAA); n.i, not included.
200 180 160 140 120 100 80 60 40 20 0
50 45 40 35 30 25 20 15 10 5 0
Sediment 40 35 30 25 20 15 10 5 0
Sediment 100 90 80 70 60 50 40 30 20 10 0
Sediment 5 4.5 Cr (mg/kg) 4 3.5 3 2.5 2 1.5 1 0.5 0 EA
Sediment Fig. 2. Metal concentrations in sediments of asphyxiated (A) and healthy (H) mangrove swamps. Straight horizontal line represents a moderately polluted level as per sediment quality guidelines by US Environmental Protection Agency.
life. Based on the present study, the main sources of heavy metal burden in patches of asphyxiated swamp are probably from industrial and urban activities. Therefore, efforts should be made to improve the water quality of Qua Iboe River. In future study, the
concentration of persistent pollutants in various organisms in relation to those in sediments and to trophic level should be conducted to assess the ecological risk on the water birds and other aquatic animals.
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Table 5 Elemental concentrations (mg/l) of surface water sample from Asphyxiated mangrove ecosystem of Qua Iboe and water quality criteria. Metal
Zn Cu Ni Pb Cr V PLI
0.030 (1.88) 0.036 (3.27) 0.013 (4.33) 0.01 (1.67) 0.02 (5.00) 0.07 (1.27) 2.57
Water quality criteria (mg/l) WHOa Drinking water
USAc Protection of aquatic life
3000 2000 20 10 50 ni
ni 1300 100 0 100 ni
180 20 50 10 50 ni
Values in parenthesis are the contamination factors. a World Health Organization (1993). b USEPA (1990). c Frits (1990).
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