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Journal of Environmental Solutions Volume 3 (2014) 1-10
Journal of Environmental Solutions
Averroes Publisher
METALLIC CONTAMINATION ASSESSMENT OF THE LÉVRIER BAY (MAURITANIAN ATLANTIC COAST), USING PERNA PERNA AND VENUS ROSALINA
MHAMADA Mohammed1; OULD MOHAMED CHEIKH Mohammed1; KHANNOUS Soumaya1; DARTIGE Aly2; ER-RAIOUI Hassan1*
1 Geosciences and Environment team - Department of Earth Sciences - Faculty of Sciences and techniques, Tangier, Morocco 2 Office of Sanitary Inspection of Fishery Products and Aquaculture (ONISPA) Nouadibou, Mauritania
* Corresponding author. Er-raioui Hassan E-mail: [email protected] Tel: +212 5 39 39 39 54
A B S T R A C T
Keywords: By its proximity to Nouadhibou city (the economic capital of Mauritania), Lévrier bay (Atlantic coast of Mauri- Lévrier bay tania) is closely exposed and influenced by port and industrial activities effluents. Its coastline is highly coveted Atlantic coast of Nouadhibou for the establishment of several industrial firms accompanied by a growing urban development. In this context, trace metals the present study focuses on the marine coasts evaluation of the trace metals contamination (Cd, Hg, Pb, Fe, Co, Perna perna Cr, Cu, Mn and Zn) in two different species of bivalves (mussels and clams) using ICP- AES. The results showed Venus rosalina significant coastal contamination by iron and Cd. Their recorded results have to be considered in any integrated
coastal zone management in Mauritania. The trends of trace metal concentration values, recorded from 1991 to 2009, showed an undeniable increase in Cd and iron, mainly attributed to industrial and urban domestic wastes. Cd concentration values ranged from 3.9 mg/kg dry weight (dw) in 1991 to 23 mg/kg dw in 2009. On the other hand, iron concentrations increase has been found both in mussels and clams. Actually, mussels showed in- creasing concentration values ranged from 155.5 mg/kg dw in 1999 to 460 mg/kg dw in 2009 while in clams, concentration values ranged from 322 mg/kg dw in 1991 to 663.6 mg/kg dw in 2009.
I. INTRODUCTION
Mauritanian coasts extend over almost 720 km along the Atlantic Ocean. They are considered as an aquatic system that is more economically important than ecologically as they are hosting a wide variety of species. Thus chemical pollution of these coastal areas could have serious consequences on the balance of marine ecosystems. That is why this work focuses on the assessment of the contamination of the Lévrier bay by trace metals and the sanitary of fishery products in the maritime exclusive economic zone (MEEZ). The Lévrier bay has an exceptional potential due to both its strategic geographical position and its natural resources. It is recognized as a true "biogeographical crossroads" where the ecological characteristics of the southern and northern areas meet including those of Europe. Beside its proximity to Nouadhibou city, the economic capital of Mauritania, the Lévrier bay is intimately linked to industrial and harbour activities. Its coastline is highly coveted for setting several industrial enterprises as well as for growing urban development. In fact, it is designated to accommodate important development projects in the frame of the Mauritanian government orientations into a special economic zone (AECOM, 2011). Moreover, the bay is under increasing anthropogenic pressure due to the significant urbanization development of Nouadhibou city. This development generates important economic activities such as setting of fishing companies and flour and fish oil factories. There are also many shipwrecks as well as petroleum and ore ports that could be sources of potentially toxic substances. Table 1 sums up some examples of industrial and agricultural sources (Othmer, 1995) which may introduce chemical pollutants in the bay environment. Table 1. Industrial and agricultural sources of metals in the environment.
Uses Metals Batteries and other electrical appliances Cd, Hg, Pb, Zn, Mn, Ni, Pigments and paints Ti, Cd, Hg, Pb, Zn, Mn, Sn, Cr, Al, As, Cu, Fe Alloys and solders Cd, As, Pb, Zn, Mn, Sn, Ni, Cu Biocides (pesticides, herbicides, curators) As, Hg, Pb, Cu, Sn, Zn, Mn Catalyst agents Ni, Hg, Pb, Cu, Sn Glass As, Sn, Mn Fertilizers Cd, Hg, Pb, Al, As, Cr, Cu, Mn, Ni, Zn plastics Cd, Sn, Pb Dental and cosmetic products Sn, Hg Textiles Cr, Fe, Al Refineries Ni, V, Pb, Fe, Mn, Zn Fuels Ni, Hg, Cu, Fe, Mn, Pb, Cd 1 Mhamada et al.
Contamination of coastal ecosystems by these hazardous substances is one of the major problems in ecotoxicology. Unlike organic pollutants, some trace metals could not be submitted to biodegradation. Thus, they could be accumulated in food chains to reach toxic thresholds (Cumont, 1984) and could generate critical and even dangerous situations (Serghini et al., 2001). In natural aquatic ecosystems, metals are found at low contents, generally in limits of nano-gram or microgram per liter. However, in recent decades, the occurrence of high trace metals levels, above than their natural contents, has become a serious problem which could be attributed to fast population growth, increased urbanization and especially expanded industrial activities. When addressing the toxicity of metals, it is necessary to distinguish between essential elements and non-essential ones. A metal is considered essential when pathological symptoms appear with decreasing concentrations or even with its absence and disappear once it is added. It also requires that the symptoms should be associated to a biochemical defection (Förstner & Wittmann, 1979). However, an essential element could be toxic when it is present in excessive concentrations. According to these criteria, 17 metals are considered essential, including four (Na, K, Ca and Mg) are present in large amounts (> 1 μmol kg-1 fresh weight), while the other thirteen (As, Cr, co, Cu, Fe, Mn, Mo, Ni, Se, Si, Sn, V and Zn) are present in trace (0.001 to 1 mmol kg-1 fresh weight) or even in ultra- trace (< 1μmol kg-1 fresh weight) (Mason & Jenkins, 1995). Unlike those previous elements, non-essential metals have no currently known biological role. This is the case of Hg, Ag, Cd and Pb (Mason & Jenkins, 1995). They are considered as harmful once they are present in the environment and lead to noxious biological effects at very low concentrations. In this focus, analyzed organisms belong to bivalves; that is mussels and clams. Those bivalves are widely used as bio-indicators in the evaluation of marine environmental contamination studies, given their ability to concentrate contaminants at levels well above those found in their surroundings. Indeed, by their lifestyle and/or their metabolic and physiological characteristics, bio- accumulative species have the ability to accumulate contaminants directly from the environment (bio-concentration) or by other means (e.g. food) (bio-magnification) to levels well above the levels present in the physical environment (water and sediment). Therefore, the bio-accumulative species are frequently used in monitoring networks (bio-monitoring) of the marine environment quality (Lagadic et al., 1998; Phillips & Rainbow, 1993). Moreover, the organisms chosen for this study are sedentary and may reflect pollution levels by geographic location. This research complements previous works conducted in the northern Mauritanian zone (Sidoumou, 1991; Sidoumou et al., 1997, et 1999; Roméo et al., 2000; Dartige, 2006; Wagne et al., 2013) and shows the actual state of metal contamination of the bay of Lévrier.
II. MATERIALS AND METHODS
II.1. The study area
The study area is located in the North Atlantic coast of Mauritania. It contains important agglomerations having different human activities as fisheries, trade and industry. Along its coast, there are four ports (oil port, fishing port, industrial port and Ore port). However, the fishing remains the main activity carried out by several industrial units located in this area. Two organisms were concerned by this study: clams (Venus rosalina) and mussels (Perna perna). The first ones were collected from five sampling points situated in the off-shore of the Cansado bay (table 2) while mussels were sampled from four sampling points from which three are located inside the Cansado bay (figure 1) and the fourth one is located outside the bay and is considered as a reference sampling point. The samples were the same age with the exception of mussels collected from the station Guerra which were smaller. The choice of sampling points was based on their abundance in mussels but also on their proximity to discharges. Cansado bay, being part of the Lévrier bay, is well known to host the majority of pollution sources of Nouadhibou city. It also accommodates many shipwrecks. • GUERA’s sampling point (used as control reference) is free from any significant pollution. It is exposed to strong waves that break on the coast area. This sampling point is located outside the bay in the north-west (20° 51' 26.09" N; 17° 01' 52.37" W); • Oil port sampling point is close to both Ore port, belonging to the National Industrial and Mining Company of Mauritania (SNIM) and Nouadhibou refinery (20° 49' 55.11" N; 17° 02' 08.34" W); • COMECA’s sampling point is located near the metallurgical company “COMECA”. This sampling point is near the oil port Cansado (20° 50’ 28.98" N, 17° 02' 03.93" W); • IMROP’s sampling point is located in the southeast bay Cansado (20° 51' 26.09" N; 17° 01' 52.37" W). This bay receives all urban wastes including Cansado effluent of domestic wastewater.
Figure 1. Location map of Perna perna and Venus rosalina samples (Lévrier bay) Journal of Environmental Solutions Volume 3 (2014): 1-10 2 Mhamada et al.
Regarding Venus rosalina, sampling stations are located off of the bay (figure 1), table 2 summarizes the details of the different sampling points.
Table 2. Coordinates of the different sampling sites Venus rosalina
Site Coordinates M5 20°19’ N 17°08’ W M6 20°13’ N 17°12’ W M7 20°12’ N 17°08’ W M8 20°15’ N 17°08’ W M9 20°18’ N 17°10’ W
II.2. Sampling and chemical analysis:
Bivalves sampling was made once a season (cold season [SF], transitional season 1 [ST1], hot season [SC] and transitional season 2 [ST2]). Using a stainless knife, mussels were manually collected at low tide in all the fourth coastal sampling points while clams were collected in the remaining fifth sampling point, using a dredger mounted onboard of IMROP’s boat (figure 1). The samples were stored in cool boxes and kept out of direct sunlight. Transport to laboratory was carried out in the best conditions. Before analysis, samples were thawed out in ambient temperature conditions and then shells removed using a stainless steel knife. The obtained tissues were rinsed with distilled water and then drained, crushed, refrozen and lyophilized. Once dried, the samples were reduced into a fine homogeneous powder using a pestle in a ceramic mortar. Then samples were mineralized.
Trace elements analysis (Cu, Pb, Cd, Zn, Co, Cr, Mn, Fe) Mineralization consisted in digesting 2 g of sample, first in cold conditions for 1 hour and then at 140 °C for at least three hours in 20 ml of concentrated nitric acid (HNO3) and 3 ml of concentrated sulfuric acid (H2SO4). After cooling, 5 ml of bi-distilled water and 5 ml of hydrogen peroxide were added to each sample. The mixtures were heated at 100 °C overnight. Once the samples had been cooled, treatment with hydrogen peroxide (5 ml) was performed at a temperature of 140 °C for one hour. This treatment was extended until the end of the mineralization (disappearance of black color). The volumes of the resulting digests were adjusted to 100 ml with bi- distilled water. The trace metals analysis was performed by ICP- AES instrument (Inductively Coupled Plasma - Atomic Emission Spectroscopy) type Jobin Yvon Ultima 2, having two detectors "High dynamic" to optimize sensitivity in the UV and visible range. The device has a holographic grating of high brightness (2400 r/min), of a spectral range of 120-800 nm, a linear dispersion (0.21 nm/mm for 120 nm to 320 nm and 0.42 nm/mm for 320 nm to 800 nm) and of a practical resolution (5 pm for 120 nm to 320 nm and 10 pm for 320 nm to 800 nm). The device has a PKI software analyst V5 under Windows 98/NT. The used standards were Precis Mono elements etalon of 1000 ppm. This technique has the advantage of being fast and multi-elementary.
Analysis of Mercury
An amount of 0.07 ± 0.001 g of sample was introduced into the mercury analyzer, equipped with UV photometry (DMA 80), through a cleaned nacelle. This instrument was calibrated using liquid standards (10 ppm and 1 ppm of Hg). Mercury results were directly displayed by the meter.
II.3. Statistical treatment
The analysis of variance (ANOVA and Kruskal-Wallis) and principal component analysis were used to show the state of metallic elements distribution in different locations.
III. RESULTS
The overall examination of the study results, related to the assessment of the contamination of the Mauritanian coast by trace metals, showed in both used organisms (mussels and clams) very heterogeneous and relatively high concentration levels of metallic elements.
III.1. Toxic elements
The results of spatio-temporal variations of the accumulation of elements Cd, Pb and Hg in mussels samples are shown in table 3. According to these results, these three metallic elements vary all in the same way and fluctuate successively from 0.26 to 1.88, from 0.02 to 1.13 and from 0.01 to 0.29 mg/kg dry weight. The highest values were recorded during the cold season in Guerra’s sampling point. Statistical tests showed significant spatio- temporal variations (respectively designed by s and t) for the mercury (Fs=5.51; ps=0.00; Ft=4.42; pt=0.00). Cd showed significant variations in time (Ft=7.258 and pt=0.001) but not in space (Fs=1.36; ps=0.27). Unlike Cd, Pb showed no significant changes in time (Ft=2.50; pt=0.07), but spatial variations were significant (Fs=12.17; ps=0.00).
Journal of Environmental Solutions Volume 3 (2014): 1-10 3 Mhamada et al.
Table 3. Toxic elements (Hg, Pb, and Cd) recorded in mussels in mg/kg dry weight at different study sites (Lévrier bay)
Site Season Cd Pb Hg SF 1.82 ± 0.15 1.13 ± 0.44 0.06 ± 0.05 Guera ST1 0.25 ± 0.06 0.52 ± 0.18 0.08 ± 0.01 SC 1.14 ± 0.31 0.40 ± 0.10 0.28 ± 0.10 SF 1.07 ± 0.13 0.02 ± 0.02 0.24 ± 0.04 ST1 0.85 ± 0.24 0.21 ± 0.03 0.14 ± 0.02 Cansado SC 0.55 ± 0.34 0.13 ± 0.01 0.29 ± 0.07 ST2 0.53 ± 0.37 0.02 ± 0.01 0.17 ± 0.04 SF 0.62 ± 0.18 0.03 ± 0.02 0.10 ± 0.03 ST1 1.03 ± 0.29 0.40 ± 0.01 0.13 ± 0.02 Comeca SC 0.92 ± 0.63 0.02 ± 0.01 0.12 ± 0.01 ST2 0.76 ± 0.38 0.13 ± 0.03 0.23 ± 0.05 SF 1.49 ± 0.49 0.20 ± 0.03 0.11 ± 0.02 ST1 1.07 ± 0.24 0.17 ± 0.02 0.13 ± 0.02 Oil port SC 0.46 ± 0.14 0.14 ± 0.03 0.14 ± 0.02 ST2 0.70 ± 0.11 0.89 ± 0.10 0.01 ± 0.01
However, the spatiotemporal variations of toxic elements in clams did not show the same profiles than in mussels. Cd concentrations ranged from 7.54 to 23 mg/kg of dry weight (dw) and were recorded respectively in M5 and M6 sampling points (table 4). Seasonally speaking, the results showed highest concentrations for both ST1 and SF seasons, while the lowest ones were recorded for SC season. Variances, expressed through the Fisher F parameter and the degree of significance, showed a significant temporal and spatial variation (Fs=66.5; ps=0.00 and Ft=151.12; pt=0.00).
Table 4. Toxic elements (Hg, Pb, and Cd) recorded in Perna perna as mg/kg dry weight at different study sites (Lévrier bay)
Site Season Cd Pb Hg SF 20.52 ± 1.78 0.64 ± 0.07 0.03 ± 0.03 ST1 7.63 ± 0.90 0.70 ± 0.09 0.02 ± 0.01 M5 SC 23.84 ± 1.15 0.24 ± 0.03 0.11 ± 0.01 ST2 25.26 ± 1.27 0.87 ± 0.07 0.02 ± 0.01 SF 47.01 ± 3.14 0.14 ± 0.03 0.03 ± 0.02 ST1 11.21 ± 1.37 0.62 ± 0.08 0.02 ± 0.01 M6 SC 24.90 ± 0.57 0.04 ± 0.01 0.01 ± 0.01 ST2 21.83 ± 1.05 0.27 ± 0.04 0.20 ± 0.02 SF 15.67 ± 1.79 0.35 ± 0.05 0.02 ± 0.00 ST1 16.26 ± 1.56 0.83 ± 0.18 0.03 ± 0.02 M7 SC 40.61 ± 1.50 0.09 ± 0.03 0.13 ± 0.03 ST2 24.43 ± 3.41 0.13 ± 0.01 0.01 ± 0.00 SF 17.59 ± 1.85 0.73 ± 0.19 0.03 ± 0.03 ST1 24.14 ± 1.34 0.13 ± 0.01 0.02 ± 0.00 M8 SC 20.37 ± 1.38 0.13 ± 0.04 0.02 ± 0.02 ST2 27.28 ± 2.04 0.10 ± 0.08 0.02 ± 0.01 SF 45.27 ± 3.54 1.38 ± 0.30 0.10 ± 0.04 ST1 21.08 ± 2.51 1.16 ± 0.22 0.03 ± 0.02 M9 SC 45.85 ± 1.85 1.67 ± 0.45 0.03 ± 0.02 ST2 17.76 ± 1.72 0.01 ± 0.00 0.04 ± 0.03
The seasonal results of the five studied sampling points showed Pb concentration averages ranging between 0.09 and 1.69 mg/kg dw, recorded both in the same sampling point (M9). SF and ST1 seasons showed highest concentrations of Pb. The results of the Kruskal-Wallis hypothesis test showed significant spatio-temporal variations (e.g. Fs=14.41, ps=0.00 and Ft=13.56, pt=0.00). Mercury concentrations in clam tissues were generally low and varied between 0.01 and 0.2 mg/kg dw, recorded in M6 sampling point. Seasonal variations showed highest concentrations for SF and ST1 seasons. However, these variations were not significant (Fs=0.61; ps=2.68, Ft=4.72, and pt=0.19).
III.2. Essentials elements
For essential elements, and given the heterogeneity in the data, the results obtained for mussels and clams analysis will be presented separately.
III.2.1. Results analysis in mussels (Perna perna)
The mean concentrations of metallic elements recorded in mussels during the four seasons in the different studied sites are shown in table 5.
Journal of Environmental Solutions Volume 3 (2014): 1-10 4 Mhamada et al.
Table 5. Metallic elements (Cd, Hg, Pb, Fe, Co, Cr, Cu, Mn and Zn) recorded in Perna perna as mg/kg dry weight at different study sites (Lévrier bay)
Site Season Co Cr Cu Fe Mn Zn SF 0.25 ± 0.04 3.79 ± 0.37 8.78 ± 0.81 710.83 ± 12.42 9.08 ± 0.84 120.02 ± 4.22 Guera ST1 1.80 ± 0.17 3.15 ± 0.56 4.85 ± 0.48 322.51 ± 35.80 2.25 ± 0.49 22.03 ± 3.71 SC 0.23 ± 0.04 7.05 ± 1.11 6.47 ± 1.03 588.55 ± 33.75 6.36 ± 1.07 79.42 ± 17.46 SF 0.22 ± 0.03 4.37 ± 0.56 6.55 ± 0.91 813.11 ± 29.42 10.90 ± 1.36 131.97 ± 12.71 ST1 0.27 ± 0.02 1.36 ± 0.24 4.55 ± 0.78 275.91 ± 36.16 3.47 ± 1.10 70.27 ± 7.53 Cansado SC 0.14 ± 0.02 1.52 ± 0.58 9.68 ± 2.34 367.80 ± 44.07 7.88 ± 1.35 75.71 ± 11.59 ST2 0.21 ± 0.03 13.66 ± 4.49 22.35 ± 3.42 522.76 ± 33.38 7.68 ± 1.28 88.70 ± 6.27 SF 0.22 ± 0.03 10.02 ± 1.42 4.99 ± 1.17 677.75 ± 24.43 6.28 ± 0.75 107.64 ± 8.28 ST1 0.34 ± 0.12 2.61 ± 0.41 4.64 ± 1.29 506.00 ± 37.50 5.67 ± 1.69 63.24 ± 4.92 Comeca SC 0.27 ± 0.14 1.82 ± 0.61 13.57 ± 2.01 405.72 ± 62.38 7.46 ± 1.22 91.66 ± 13.62 ST2 0.19 ± 0.03 8.15 ± 0.83 18.21 ± 2.38 180.09 ± 36.30 8.72 ± 2.77 83.80 ± 11.12 SF 0.44 ± 0.05 3.55 ± 0.38 6.16 ± 0.72 839.35 ± 65.81 10.31 ± 2.14 84.28 ± 9.17 ST1 2.70 ± 0.39 6.64 ± 0.70 13.97 ± 2.73 1084.49 ± 98. 38.43 ± 4.68 79.10 ± 10.49 Oil port SC 0.16 ± 0.03 1.91 ± 0.47 15.83 ± 2.47 384.05 ± 43.45 6.10 ± 2.09 50.09 ± 6.70 ST2 0.22 ± 0.03 2.22 ± 0.64 26.98 ± 3.25 323.28 ± 44.65 5.22 ± 0.89 73.85 ± 4.91
For cobalt, mean seasonal values ranged from 0.14 to 0.84 mg/kg dw. The maximum value has been recorded in the oil port in the transitional season 1, while the minimum value was recorded in Cansado during the hot season. This element showed a significant variation in time (Ft=1.684; pt=0.001) and no significant variation in space (Fs=3.64; ps=0.30). chromium element showed, for all the 4 sampling points, concentration values ranging from 1.36 to 10.02 mg/kg dw. The maximum value was recorded in Comeca sampling point during the cold season. The statistical test showed significant spatial and temporal variation (Ft=86.22; pt=0.03 and Fs=9.03; ps=0.02). For copper, mean seasonal concentration values ranged from 4.54 to 26.98 mg/kg dw. The maximum value was recorded in the oil port during the transition season 2, while the minimum was observed in Cansado sampling point in the transition season 1. Overall, the highest values were recorded in ST2 season. The inter-subject test has confirmed the existence of spatio-temporal variation of parameters F and p and shows the effect of space and time. The calculated values are respectively Fs=21.32; ps=0.00 and Ft=122.85; pt=0.000). For iron, the seasonal obtained concentration values varied from 180.09 to 1084.49 mg/kg dw. The maximum value was recorded in the oil port during the ST1 season while the minimum was recorded in Comeca sampling point during the ST2 season. For manganese, seasonal concentration values ranged from 2.25 to 10.9 mg/kg dw. The minimum value was recorded in Guerra sampling point during ST1 season while the maximum was recorded in Cansado sampling point during the SF season. The inter- subject test has confirmed a significant temporal variation and no significant variation in space (Ft=18.12; pt=0.00 and Fs=2.03; ps=0.13). For zinc, mean seasonal concentration values varied between 22.03 and 131.97 mg/kg dw with a maximum recorded in Cansado sampling point inside the bay. The minimum value was recorded outside of the bay in Guerra sampling point. However, the seasonal effect showed highest content of zinc in the SF.
The principal component analysis: The results presented in figure 2 are derived from a PCA with 9 variables and 12 modes (4 seasons x 4 sites). The first two components (C1 and C2) were extracted and, together, they could explain 99.9 % of the variability of the original data. So, the first two components are sufficient to explain the total variance. On the C1, where the iron element is the largest contributor to the explanation of this axis with a correlation of 0.99, the oil port sampling point as well as ST1 and SF seasons show that they are the most correlated with this axis. For the second axis, where Zn is the most correlated with this one, an overall exam of the map shows that the sampling points most correlated with this axis are successively Cansado, Guera and Comeca. Regarding the seasons, it is the cold one which has the greatest correlation.
Figure 2. PCA diagram showing the correlation between different trace metals in Perna perna (Cd, Hg, Pb, Fe, Co, Cr, Cu, Mn and Zn) and sampling sites (Lévrier bay). Journal of Environmental Solutions Volume 3 (2014): 1-10 5 Mhamada et al.
III.2.2. Analysis results in the clams (Venus rosalina)
For cobalt, the mean values ranged from traces to 1.91 mg/kg dw. The maximum value was recorded in M9 sampling point during SC season, while the minimum value was recorded in M6 sampling point during ST2 season. Variances, expressed through Fisher parameter (F) and the significance level (p) showed quite significant spatio-temporal variations (Fs=9.46; ps=0.00, Ft=18.64 and pt=0.00). The highest levels of Co were recorded during the SC season. For chrome, recorded values during the four seasons in the five studied sampling points (M5, M6, M7, M8 and M9) fluctuate between 1.97 and 13.23 mg/kg dw. The minimum and maximum values were recorded in M8 sampling point during ST2 and SF seasons respectively. The Kruskal-Wallis hypothesis test showed a significant time effect (Ft=10.69, pt=0.01), while the spatial variations were not significant (Fs=6.09; ps =0.19). For iron, the recorded results are shown in table 6. These results ranged from 91.91 to 1449.17 mg/kg dw. The maximum concentration value was recorded in ST2 season, while the minimum one in SC season. Moreover, the highest values were recorded in SF and ST2 seasons. The Kruskal-Wallis hypothesis showed no significant effect in terms of spatial and temporal variation (Fs=9.19; ps=0.56 and Ft=7.50; p =0.57). For manganese, mean concentration values ranged from 1.21 (in M8) to 7.84 mg/kg dw (in M6) during respectively SC and ST2 seasons. Variances, expressed through Fisher parameter (F) and the significance level (p), showed a significant spatial and temporal variation (Fs=66.560; ps=0.00 and Ft=151.12; pt=0.00). For zinc, the mean concentration values ranged from 8.64 to 282.25 mg/kg dw with a maximum recorded during SF season in M8, and a minimum observed in M9 during ST1 season. The inter-subject Kruskal-Wallis test showed significant spatio-temporal effects (Fs=1.64; ps=0.02 and Ft=9.21; pt=0.02).
Table 6. Metallic elements (Cd, Hg, Pb, Fe, Co, Cr, Cu, Mn and Zn) recorded in Venus rosalina (Averege ± standard deviation as mg/kg dry weight) at different study sites (Lévrier bay)
Site Season Co Cr Cu Fe Mn Ni Zn SF 0.86 ± 0.65 6.04 ± 1.39 9.82 ± 2.39 724.19 ± 66.28 3.45 ± 0.89 19.22 ± 1.32 92.10 ± 10.72 ST1 NI(*) 7.86 ± 0.17 22.03 ± 2.18 391.12 ± 36.39 17.16 ± 0.46 54.45 ± 6.28 493.30 ± 53.38 M5 SC 0.80 ± 0.27 9.31 ± 0.63 9.93 ± 1.68 1142.08 ± 32.44 2.95 ± 0.64 35.08 ± 2.88 189.81 ± 8.73 ST2 0.64 ± 0.16 28.92 ± 0.91 12.46 ± 0.73 1177.45 ± 15.91 7.84 ± 0.90 33.05 ± 1.29 93.17 ± 7.33 SF 1.185 ± 0.40 6.34 ± 0.81 11.64 ± 1.11 550.23 ± 54.17 2.66 ± 0.70 13.93 ± 1.32 61.40 ± 10.02 ST1 NI* 9.79 ± 1.31 11.86 ± 0.79 889.48 ± 61.36 3.40 ± 0.59 53.37 ± 6.96 207.07 ± 19.06 M6 SC 0.99 ± 0.23 14.33 ± 1.39 10.39 ± 0.95 91.91 ± 4.11 5.10 ± 0.97 44.09 ± 1.46 406.98 ± 4.45 ST2 0.38 ± 0.19 3.71 ± 1.09 8.88 ± 0.94 1449.17 ± 11.92 4.84 ± 0.50 52.98 ± 2.68 73.41 ± 3.49 SF 0.65 ± 0.32 32.94 ± 6.59 11.39 ± 1.07 768.76 ± 36.05 6.34 ± 1.19 50.32 ± 6.31 119.24 ± 16.56 ST1 0.58 ± 0.44 5.30 ± 0.50 7.75 ± 0.93 512.39 ± 60.45 1.79 ± 0.58 15.14 ± 1.31 66.85 ± 12.03 M7 SC 1.87 ± 0.12 4.24 ± 0.53 6.42 ± 0.94 820.38 ± 4.03 2.23 ± 0.32 13.47 ± 0.60 50.92 ± 2.58 ST2 0.78 ± 0.23 7.68 ± 0.99 8.40 ± 1.31 381.31 ± 38.51 3.39 ± 0.66 69.37 ± 10.67 56.82 ± 7.45 SF 0.83 ± 0.46 13.24 ± 1.39 11.48 ± 2.19 879.23 ± 21.51 4.65 ± 0.62 76.84 ± 7.98 282.26 ± 15.35 ST1 0.76 ± 0.15 4.14 ± 0.79 10.40 ± 0.74 340.34 ± 7.61 2.92 ± 0.28 18.59 ± 0.73 59.41 ± 3.32 M8 SC 0.95 ± 0.41 5.12 ± 0.39 7.91 ± 0.91 295.19 ± 12.47 1.21 ± 0.26 4.54 ± 1.13 40.07 ± 3.18 ST2 0.73 ± 0.25 1.98 ± 0.60 7.01 ± 0.27 293.59 ± 22.76 2.20 ± 0.36 10.47 ± 0.61 56.24 ± 4.96 SF 1.78 ± 0.36 11.89 ± 0.58 14.10 ± 1.59 1017.27 ± 58.10 4.59 ± 0.50 82.67 ± 8.46 206.21 ± 5359 ST1 0.99 ± 0.37 2.52 ± 0.66 9.69 ± 0.80 326.87 ± 9.57 2.46 ± 0.51 14.51 ± 1.51 38.64 ± 4.56 M9 SC 1.98 ± 0.28 7.25 ± 0.44 8.45 ± 0.64 1064.67 ± 22.58 3.01 ± 0.24 24.91 ± 1.17 133.90 ± 7.06 ST2 0.40 ± 0.29 2.08 ± 0.24 8.43 ± 1.36 156.33 ± 16.62 3.42 ± 1.08 8.05 ± 0.71 48.92 ± 6.42 * No Identifiable
Principal Component Analysis: In this case, the first two components explain 99.917 % of the variability of the original data; nevertheless, and for better results understanding, an analysis of the three components together that explains 99.992 % of data is required. For the first axis (C1) and according to the factorial map shown in figure 3, the iron element contributes most to the explanation of the axis C1 with a correlation of 0.99. The other elements have no contribution to C1. For the second axis (C2), the factorial map shown in figure 3 demonstrates that Zn explains this axis with a correlation of 0.99. The other elements have no contribution to C2. For the third axis, according to the same factorial map, Cd element is the metallic element which explains this axis with correlation of 0.991 while the other elements do not show contribution. For samples’ axis, the first axis C1, corresponding to iron axis (correlation 0.99), shows that M6, M5 and M9 sampling points as well as ST2 season present most correlation. On the second axis C2, M8 and M5 sampling points as well as SF and ST1 seasons are the most correlated to this axis.
Journal of Environmental Solutions Volume 3 (2014): 1-10 6 Mhamada et al.
Figure 3. PCA diagram showing the correlation between different trace metals in Venus rosalina (Cd, Hg, Pb, Fe, Co, Cr, Cu, Mn and Zn) and sampling sites (Lévrier bay).
IV. DISCUSSION
The contamination assessment of Lévrier bay’s marine coasts by trace metals accumulated in two bivalve species (mussels and clams) points out the iron coastal contamination. So, this significant contamination has to be considered in any integrated coastal zone management in Mauritania. In addition, the study underlines Cd contamination, revealed in high concentrations in clams.
Toxic elements: in this regard, the Cd contents recorded in clams were quite high, reaching 23 mg/kg dw in open sea. They are heavily influenced by bottom waters naturally loaded with Cd, exceeding thus European Community standards (5 mg/kg dw) (EC No 1881/2006) and could lead to a surely pollution risk. It should be noted that Cd is a non-essential element to life and causes severe toxic effects in aquatic organisms at very low concentrations. In the coasts level, Cd maximum content in mussels was 1.88 mg/kg dw and recorded in Guera sampling point during the SF season. Outside of this season, levels rarely exceeded 1.20 mg/kg dw. These seasonal variations could be explained by the presence of upwelling currents that occur during the cold season and strengthened by the life cycle of mussels of which maximum spawning period occurs in autumn (Zaouali, 1973). Variations related to space were general, but more important in Guera sampling point where collected mussels had a size 2 times smaller than those collected elsewhere in the bay and thus because of the exposure to strong waves in this area (Wagne et al., 2013). They could therefore concentrate more contaminants as they filtered more water. Indeed, a close relationship exists between metal concentrations and mussels size, individual weight and nutrition depending on seasons (Amiard et al., 1994). Regarding the other toxic elements, Pb and Hg concentrations rarely exceeded 1.69 and 0.2 mg/kg dw respectively, mainly recorded in clams in open sea where waters are enriched in elements brought from the bottom. Concentrations were quite below European sanitary thresholds for Pb (7.5 mg/kg dw) and for Hg (1.5 mg/kg dw) (EC No 1881/2006). However, the contribution of the pollutant load emanating from urban untreated sewage could not be excluded. Moreover, in near shore and under trade winds effect, superficial waters are pushing back allowing coastal upwelling happening. The cold waters of the coastal upwelling, main feature of coastal areas oceanography, are mixed with the cold sea surface waters and thus constitute the main source of the West African coastal ecosystems enrichment. This tends to puck buck the sea water away from the shore and then it generate the lifting up of the deep waters all along Mauritanian coast, creating in this way the coastal upwelling. This phenomenon contributes to spread towards open sea the contaminants drained by coastal effluents. In mussels, and like Cd, the maximum levels of Pb and Hg are recorded in Guera sampling point with respective values of 1.126 and 0.288 mg/kg dw. These values are of same magnitude as the ones recorded in clams.
Essential elements (Fe, Zn, Cu, Mn, Co and Cr): their analysis has revealed high levels of iron and zinc in bivalves (clams and mussels). Actually, iron concentrations reached 1,449.17 and 1,084.50 mg/kg dw in clams and mussels respectively and the maximum concentrations of zinc were respectively 282.25 and 131.98 mg/kg dw. Cu and Mn, essential elements of which concentration is strongly regulated by organisms, have respectively shown concentration values of 14.1 and 7.84 mg/kg dw in clams. In mussels, concentration values were 26.98 mg/kg dw for Cu, in the oil port and 0.89 mg/kg dw for Mn, in Guera sampling point. Hence the most likely direct effect of discharges of oil port and wastewaters of Nouadhibou and Cansado cities. Moreover, the area in question is located near to the ore harbor, where Mauritanian iron ore is exported. High contents encountered in Guera sampling point, relatively far from the sources of pollution, could be explained by the intensity of the upwelling currents in “Cap Blanc” which bring in sea surface mineral salts containing high amounts of nutrients and lead at the same time to contaminants dispersal. Apart from what may be induced as difference in concentrations related to different methods of analysis and sampling techniques and study periods, the trend curves developed from trace metals concentrations since 1991 until to 2009 showed an undeniable increase mainly of Cd and iron (figure 4). Cd concentrations showed values ranging from 3.9 mg/kg dw in 1991 to 23 mg/kg dw in 2009. Assuming the Atlantic bottom waters have relatively constant Cd contents, increasing Cd concentrations could be attributed in part to urban discharges drained toward coastal areas. As for iron, increasing concentrations occured in mussels and clams. In mussels, concentrations increased from 155.5 mg/kg dw in 1999 to 460 mg/kg dw in 2009. Concentrations in clams varied from 322 mg/kg dw in 1991 to 663.6 mg/kg dw in 2009. This fact reflects one more time the urban waste impact (industrial and Journal of Environmental Solutions Volume 3 (2014): 1-10 7 Mhamada et al. domestic), apart from the change of the iron ore loading ore which remained relatively constant (between 10,000 and 12,000 tons) during the period of 2001-2010. In general, in natural aquatic ecosystems, metals are found at low concentrations, commonly in the order of nannogram or microgram per liter. However, the presence of trace metals in concentration higher than natural contents has become an issue of increasing concern. It might be attributed to the rapid population increase, heightened urbanization, industrial activities expansion, natural resources exploration and exploitation, irrigation expansion and other modern agricultural practices spreading as well as environmental regulations absence.
Figure 4. Trending concentrations of some trace metals in coastal Nouadhibou
At Nouadhibou coast, and although the industrial activities level is relatively undeveloped, there is a need to manage rationally water resources and to control waste discharges in the environment. As it is expected that industrial and urban activities will intensify along the coastal area, the matter becomes now even more crucial especially when it concerns a highly sensitive environment, such as Lévrier bay. This requires taking necessary measures to reconcile the uses with biodiversity and coastal environment preservations in the context of sustainable development. On the other hand, this area generates more interest since it holds natural sites of high ecological importance: the satellite Reserve of “Cape Blanc”, World Heritage, and the Star’s bay expected to become a marine protected area for its role in safeguarding biodiversity and renewal of fishery resources in the Exclusive Economic Zone of Mauritania "ZEEM." Despite the natural dominating origin of some contaminants, urban industrial discharges remain incriminated as sources of potentially toxic elements detected in marine coast of Nouadhibou since their very diverse nature. Indeed, the coast, by its proximity to Nouadhibou city which knows an increasing urban development and which discharges large amounts of untreated waste, is closely influenced by port and industrial activities. It is highly coveted for multitude industrial firms’ establishment. Potential sources of pollution are Nouadhibou and Cansado cities, harbours (ore dock and fishing) and industrial companies, particularly industrial fishery products plants and economic offshore projects such as marine aquaculture farms. From these observations, the likely anthropogenic sources of trace metals, highlighted in this study, could be attributed to mining, industrial and domestic effluents and to urban streaming which include discharges from roofs, atmospheric deposition such as fossil fuels combustion, waste incineration and industrial emissions or petrochemical activities. For comparison, at the coast of Senegal, Sidoumou et al. (2006) reported Cd concentrations equivalent to 2.4 mg/kg dry weight in Perna perna. In addition, Chafik et al. (2001) found concentrations up to 11.9 mg/g dw in mussels collected in southern Morocco. Also, Banaoui et al. (2004) noted that, despite the absence of pollution sources in the area south of Cape Juby in Morocco (Sahara), analyzes show a high level of cadmium (6 μg/g dry weight), which seems to be explained by a normal presence of dissolved cadmium in the area and is probably due to the intense upwelling between Cape Ghir and Cape Juby. For Pb, In southern Morocco (between
Journal of Environmental Solutions Volume 3 (2014): 1-10 8 Mhamada et al.
Agadir and Dakhla), Benbrahim et al. (2006) found similar concentrations ranging from 0.66 to 1.11 mg/kg dw in Perna perna. At the western Morocco Mediterranean coast, analyzes reveal in Callista chione, Pb contents vary between 0.85 - 3.42 mg/kg dw (Khannous et al., 2013). At Havre (France), according to data from the National Surveillance Network (RNO), lead concentrations vary between 2 and 4 mg/kg in molluscs (Chiffoleau et al., 2001). For mercury, and in comparison with other regions, the generated results are similar to those obtained by Chafik et al. (1998) and Benbrahime et al. (2006) in south Morocco (0.06 mg/kg dw and 0.09 and 0.36 mg/kg dw). Other studies show mercury concentrations ranging from 00.3 to 0.26 mg/kg dw in coastal Ghana (Biney et al., 1991; Joiris et al., 2000 and Otchere et al., 2003).
V. CONCLUSIONS
The main objective of this study is the contamination state assessment of Nouadhibou’s coasts by trace metals as well as the ZEEM fishery products safety. In this regard, two bivalve shellfishes, mussels (Perna perna) and clams (Venus rosalina), were used for trace metals analysis. These two species are widely used in marine environment studies for the contamination assessment. A number of observations could be drawn from the present study. • Contamination by Cd has been found in high concentrations in clams collected in offshore of Nouadhibou (23 mg/kg dw). These organisms are largely influenced by bottom waters naturally loaded with Cd, exceeding the European standards (5 mg/kg dw) (EC No 1881/2006) and could consequently lead to a sure risk of pollution; • Contamination by iron is not negligible and has to be considered in any integrated coastal zone management in Mauritania. Recorded concentration values in mussels and clams are respectively 1,084.50 and 1,449.17 mg/kg dw; • Considering recorded concentration values that remain below the allowed levels, the other toxic elements (Pb and Hg) and essential elements (Zn, Cu, Mn, Co and Cr) do not constitute a danger to Nouadhibou environment. However, it should be noted that Zn contamination levels are quite high; • Trends developed from trace metal concentration values, recorded from 1991 to 2009, showed an undeniable increase especially in Cd and iron. Cd concentration values ranged from 3.9 mg/kg dw (in 1991) to 23 mg/kg dw (in 2009). Regarding iron, its increase in concentration occurred both in clams and mussels. The latter showed concentrations increasing from 155.5 mg/kg dw (in 1999) to 460 mg/kg dw (in 2009) while clams concentrations varied from 322 mg/kg dw (in 1991) to 663.6 mg/kg dw (in 2009). This reflects one more time the impact of urban waste, industrial and domestic, apart from the change in iron ore loading, which remained relatively constant during the period of 2001-2010. Despite the natural dominating origin of some contaminants, urban industrial discharges are incriminated as sources of potentially toxic elements detected in Nouadhibou marine coast since their very diverse nature. The coastline, by its proximity to Nouadhibou city knowing increasing urban development, is subjected to large amounts of untreated wastes, and is highly coveted for multitude industrial firms. Toxic elements may be present in large quantities in urban domestic and industrial waste. • In Nouadhibou coasts, although industrial activity levels are relatively undeveloped, there is a need for rational management of water resources and particularly the control of the waste discharges in the environment. As it is expected, industrial and urban activities will intensify along the coast.The matter becomes now even more crucial especially when it concerns a highly sensitive environment, such as Lévrier bay.
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