Baseline Information on the Metallic Pollution of Sediments of the Lakes of the Ossa Complex, Dizangue, Cameroon

The aim of this study was to evaluate the state of metallic pollution of the sediments of the three main Lakes (Mevia, Ossa and Mwembe) of the Ossa lake complex by the analysis of some heavy metals from May to July 2011. Copper, Nickel, Lead, Zinc, Cadmium and Iron were measured using atomic absorption spectrophotometry method. According to the standard values of GESAMP (group of Experts on the Scientific Aspects of Marine Pollution), lead concentrations were low in Lake Mwembe (4.985±0.060 mg.kg -1 , dry weight), closer to standard values in Lake Mevia (12.935±0.815 mg.kg -1 , dry weight) and very high in Lake Ossa (39.935±2.480 mg.kg -1 , dry weight). BJAST, As for Zinc and Cadmium, the contents were superior to standard values in the three lakes. The level of sediment contamination was evaluated using the Pollution Index (PI) and Enrichment Factor (EF). Only cadmium had a PI value greater than 3 and the highest EF (491.456 in Lake Mevia, 480.310 in Lake Ossa and 1761.099 in Lake Mwembe). All heavy metals presented EF values superior to 50 exception being observed for lead in Lake Mwembe (EF value less than 30). Generally, gross values of the different heavy metals measured, associated with EF values, indicated an anthropogenic pollution of these waters. This pollution could be linked to the use of chemical fertilizers and biocides around the neighborhood of Ossa lake complex by the African Forestry and Agriculture Company of Cameroon (SAFACam).


INTRODUCTION
For the preservation of environmental equilibrium and its biodiversity, man has raised across the world so-called protected areas. The Ossa lake complex is one of such protected wildlife reserves in Cameroon and is classified as a "World Heritage site" by UNESCO. Like all wetlands, this complex is part of the most productive areas of the world, which serves as a source of goods, varied and multiple services and the cradle of biodiversity [1]. The classification of the Ossa lake complex as a protected zone takes its international significance in terms of ecology, botany, zoology, limnology and hydrology [1]. The vegetation surrounding these lakes is a young coastal forest of anthropogenic origin [2], which is made up of Pandanus, Mytragyne, Raphia, Uapaca, and also industrial palm oil crops and rubber plantation [3,4]. This forest unquestionably contributes to the development of the Cameroon economy, but it could equally constitute, nowadays, a factor of aquatic ecosystem disturbance in the Ossa lake complex. Agricultural activities (establishment of nurseries, use of fertilizers and biocides, production of palm oil, resin processing etc.) contribute to the discharge of large quantities of pollutants into the environment. The development of intensive agriculture, with intensive use of synthetic pesticides and fertilizers to fight against weeds and pests, contributes to soil pollution. These products contain non-degradable elements that can remain in the soil or be driven by the runoff into groundwater, surface water and even be transferred to plants, animals and humans. Agriculture is thus an important factor of chemical pollution in the environment [5]. Among the chemical pollutants found in the environment are heavy metals. Their sources are multiple and sometimes unexpected.
Industrial wastewater, sewage wastewater, fossil fuel combustion and atmospheric deposites related to industrial activities are important pathways of aquatic pollution by heavy metals deposition [6][7][8][9][10][11]. Rapid economic development, urbanization and industrialization together with agriculture, have added huge loads of metal contaminants into rivers [12,13]. Heavy metals are indeed contained in pesticides (fungicides, molluscicides, herbicides, insecticides and others) and fertilizers.
Heavy metals which constitute some elements of the food chain can be essential as trace in living organisms (copper, cobalt, iron, manganese, nickel) while others (such as cadmium, mercury and lead) can be harmful to the development of species in hydrosystems and even to humans [14,15]. Due to their toxicity, durability in soils, groundwater, lakes or streambed sediments [16][17][18][19], their non-degradability by biological and chemical processes and their bioaccumulation in the food chain [20,21], it is crucial to control the concentrations of heavy metals (density > 5 g.cm -3 ) in hydrosystems [22]. The main metals considered in hydrosystem studies are copper, nickel, chromium, lead, zinc, cadmium and arsenic [23,24]. Their presence determines the type of pollution. In addition, if the trace metals are transported in liquid form by suspended solids, or are present in the form of colloidal particles [25], heavy metals meanwhile settle more easily and are concentrated in sediments. Sediments are the main repository and source of heavy metals in aquatic systems and can play an important role in the transport and storage of potentially hazardous metals [26]. The bioavailability and environmental behavior of metals are related to their total concentrations and their chemical speciation [27]. According to [28] and [29], heavy metals concentrations in sediment can be used to reveal the history of the type and quantity, and the intensity of local and regional pollution. Its study has become one of the most important areas of environmental research [30]. Measuring the amount of heavy metals in sediments of a hydrosystem proves to be an important tool for assessment, continuous monitoring and control of pollution in aquatic environments [31]. It gives a clearer indication of the contributions and accumulation of heavy metals in aquatic environments [25][26][27][28][29][30][31][32]. The toxicological quality of sediments reflects indeed the level of pollution of anthropogenic activities [33,34].
Heavy metals in soils and sediments have a direct impact on aquatic systems, water quality and bioassimilation and bioaccumulation of metals in aquatic organisms. Indeed, elevated concentrations of heavy metals, exceeding certain thresholds, are known to endanger ecosystems and introduce a potential risk to living organisms [35][36][37][38]12,39], since these are biomagnified in the food chain [40,41]. Under certain conditions such as dredging, chemical elements accumulated in the bottom sediments of water bodies can migrate back into the water [42,43]. Resuspension may mean that all of the animals in the water, and not just the bottom dwelling organisms, will be directly exposed to toxic contaminants [44].
Despite the importance of the Ossa lake complex and its vulnerability to pollution, very few studies have been conducted on its waters to determine its trophic status, its evolution in time and the structure of living communities, as well as to understand its operation. The works so far known are those of [45,46]. Likewise, there is a paucity of information on pollution by heavy metals of lentic ecosystems in Cameroon. However, one can cite the works of [47,48] who evaluated the concentrations of heavy metals in the marine habitat in Cameroon, those of [49,50] on heavy metals in water and sediments of Yaounde Municipal Lake and its main tributary Mingoa and that of [51] on the level of pollution by heavy metals of two lakes in Adamaoua region. The results of these studies cannot be generalized to the whole country, given that ecosystems are different and pollution sources are varied, as well as the amounts of pollutants. It is in aim of enriching the documentation of pollution of Hydrosystems in Cameroon that this study was carried out to evaluate the concentration of heavy metals (copper, nickel, lead, zinc, cadmium and iron) in the sediments of lakes Mevia, Ossa and Mwembe. Results obtain could be vital to undertake measures for the protection of these habitats and their biodiversity.

Study Area
The basin of Lake Ossa (245 km 2 ) is located about 40 km East from the Atlantic coast of the Gulf of Guinea. It lies between latitude 3°45'42" and 3°53' North and between longitude 9°9' and 10°4 '12" East [52]. The relief is dominated by flat tops of 80 meters altitude, but the slopes and lake depressions are quite strong [4].
The Ossa lake complex ( Fig. 1) submerges the bottom of an ancient deep valley that was blocked by the alluvia of the Sanaga River, probably at the end of the last ice age. It is located at an altitude of 8 m above sea level and is part of a North-South rectangular orientation of about 12 km by 10 km [4]. The maximum water depth is 10 meters for an average of 3 meters [53]. The lakes are fed by rainfall (≈ 2.9 m/yeardata of the SAFACam station and those of the city of Dizangue for the period of 1930-1990) and draining of two watersheds that extend respectively over 55 km 2 and 110 km 2 .
The waters from the largest watershed transit Lake Mevia. Losses are carried out by evaporation (678 mm/year on average over a period of 14 years - [4]) and an outlet towards the Sanaga River. The water surface area of Ossa complex is estimated at 3779 hectares, for a total area of 4507 ha, including the islands. The complex is formed from two basins; the Ossa basin (3103 ha) and the Mevia basin (676 ha) communicating with a water arm [4]. A third arm of the complex Ossa, Lake Mwembe, which is less important, is not far from the agglomeration of Dizangue.
The Ossa lake complex habours the West African manatee (Trichecus senegalensis), the dwarf crocodile (Osteolaemus monticola), pythons, soft-shelled turtles, fishes, rodents such as the brush-tailed porcupine (Atherurus africana), the aulacode (Trynomys swinderianus) and the Gambian rat (Cricetomys gambianus). It also includes about 30 species of aquatic and mangrove birds . The vegetation around the complex is a swampy forest on the Southeast border [3]. The western border is occupied all along the riparian zone by industrial palm oil crops and rubber plantation of the (SAFACam) [4]. The forest which is made up of planted palm trees and rubber exist since 1897 and covered 95 hectares in 1914 [4], which extended to 6800 hectares in 1967, with only 3850 hectares being exploited as of the 1st of January 1989 [55-4].

Sampling
Samples for physicochemical analysis and heavy metal analysis were collected thrice during the months of May, June and July 2011, at the same station in lakes Mevia, Ossa and Mwembe. The choice of the sampling period of sediments (mud) of the lakes was justified as a result of earlier studies carried out by [46] who showed during that particular period, a decrease in dissolved oxygen (O 2 ) thus favoring the release of heavy metals into the aqueous phase and therefore, its reduction in the sediment. The sediments were collected using an Eckmann tipper. 2 mL of 65% nitric acid (a conservative substance) was added to the samples and then stored in polyethylene bottles of 250 cc and placed in a dark chamber maintained at 4°C for the analysis of the following heavy metals: lead, cadmium, iron, nickel, copper and zinc [56]. Samples were analysed at "OCEAC" (Coordinating Organization for the Fight against Endemic Diseases in Central Africa) laboratory in Yaounde.

Sediments analysis for heavy metals
Heavy metals in sediments were determined by the atomic absorption spectrophotometric method using the technique of [57]. The degree of pollution of hydrosystems by heavy metals [58] and their origin in sediments [59,60] were determined using the calculation of the PI and the EF. These two expressions take into account the reference values of the levels in the earth's crust. Comparison of the mean concentration of heavy metals in the sediments and sediment quality guidelines of Environment Agency [61] were done to identify the contaminants of the different component in the ecosystem.

Physicochemical analysis of water samples
Temperature, pH and dissolved O 2 content of waters were measured in situ between the surface and a 50 cm depth, respectively using a graduated (1/100 th of a degree) column mercury thermometer, a pH meter (SCHOTT, CG818) and an oxymeter (52YSI). These parameters were chosen because of their impact on the form, and toxicity of heavy metals.

Pollution Index (PI)
A solution in determining the degree of pollution by heavy metals in bottom sediments of water systems is the calculation of the PI [58]. Where: [M]s = Concentration of metal M in the sample; [M] RM = Concentration of metal M in the reference material.
-If PI < 3, the origin of metal is terrestrial, lithogenic or natural -3 < IP < 9, the origin is suspicious -If IP > 9, the pollution is certain, the zone is risky.

Enrichment Factor (EF)
In such studies, it is recommended to calculate the EF, which provides the number of times an element is enriched compared to its abundance in the reference material , and its determine whether an element has a natural or anthropogenic origin [59,60].

Sediment Quality Guidelines (SQGs)
The works of [67][68][69] estimate that sediment quality guidelines (SQGs) are important for the protection of benthic organisms in aquatic ecosystems and can be used to identify contaminants of concern in these ecosystems and to rank areas of concern on a regional or national basis. For this study, the set of sediment for freshwater sediment were used. The threshold effect level (TEL) and the probable effects level (PEL) values were considered (Table 1) and compared to the metal concentrations from lakes. The two assessment levels were used to define three ranges of concentrations: rarely (below the TEL), occasionally (above the TEL) and frequently (equal to and above the PEL) associated with adverse biological effects as clearly proposed by [70].

Physicochemical Quality of Water
Values of temperature, pH and O 2 of the three lakes are presented ( Table 2).
The temperature of the water varied between 28.90°C and 29.30°C in Lake Mevia, between 29.30°C and 30.20°C in Lake Ossa and between 29.90°C and 30.20°C in Lake Mwembe. As in most tropical lakes, water temperature was relatively stable. The small amplitude in variation is a characteristic of most tropical lakes [71]. In general, water temperature fell between May and July, the months corresponding to the rainy season. The cooling of water during this period was definitely related to the floody season as described by [72,73].
Mean pH values in the three lakes were respectively 6.55; 6.48 and 6.56 for Mevia, Ossa and Mwembe. Overall, the values of this parameter increased in the waters but these waters remained slightly acidic. This slight acidity of water could be attributed to the total hydrolisation of primary minerals resulting from sedimentary ferralitic yellow rocks found in Lake Ossa. It could equally result from the elimination of the major part of the exchangeable bases, especially calcium and magnesium [74,75]. Water acidification can be accelerated by the contributions of mineral salts in some fertilizers [76]. [77] had already identified a pH of 7.00 in Lake Ossa, which shows that the waters became more acidic since 1987, and that agricultural activity around the complex contributes to water acidification. When pH is low, heavy metals dissolve easily and are found in the aqueous phase of the hydrosystem while neutral or basic pH lead to their precipitation and concentration in the sediments [78]. The pH values of the waters of the three lakes therefore portend a reduction in the content of heavy metals in the sediment.
The O 2 saturation values varied between 59.92% and 75.48% in Lake Mevia, between 57.32% and 72.16% in Lake Ossa and between 51.23% and 72.43% in Lake Mwembe. The average of this parameter in the three media was respectively 69.19%, 62.60% and 60.86%. Except for Lake Mevia where an increase in O 2 content was observed, the quantity of this gas decreased in lakes Ossa and Mwembe. Generally, oxygen solubility decreased with increase temperature although in the last two lakes the rate of oxygen in water decreased with temperature. This could be attributed to the greater concentration of fermentable organic matter during the rainy season due to the mixing of the Sanaga River water with runoffs of the watershed, concentration which causes rapid consumption of O 2 by bacteria, consequently leading to its decrease.

Heavy Metal Contents in Sediments of Lakes Mevia, Ossa and Mwembe
The concentrations of heavy metals in sediments are reported ( Iron is by far the metal whose concentration was highest in the sediments. Yet, it is the only metal whose values are much lower than in nonpolluted sediments (41000 mg.kg -1 , dry weight) and the continental crust (30890 mg.kg -1 , dry weight). Copper concentrations in the sediments were inferior to standard concentrations in nonpolluted sediments (33 mg.kg-1, dry weight), but all higher than the rates in the continental crust (14 mg.kg-1, dry weight). Generally, the mean concentrations of Cu and Pb were higher in the Lake Ossa while those of Ni, Zn, Cd and Fe were higher in Lake Mwembe. , dry weight). The harmful properties of this metal are among others its high bioaccumulation in food chains and its high toxicity to living organisms [80,81]. Its impact on human health has not yet been established [82]. It is therefore considered a toxic trace element [81]. Its presence in large quantities in lake sediments could reflect risks to human health because of its potential accumulation in the tissues of fishes and other organisms [83][84][85].
With regards to zinc, it is a soluble contaminant in water [86,87]; the rainy season and low pH values could have favoured the release of a certain amount of this metal in the aqueous phase. Zinc concentrations however, remained very high in the sediments and exceeded the threshold values in the continental crust (79 mg.kg -1 , dry weight). This suggests that it was highly concentrated in these waters. The values of zinc in the Ossa complex were compared to those obtained in some lakes in Cameroon. They remained below the values recorded in the Yaounde Municipal Lake [51], but very high compared to those of lakes Bini and Dang [51].
Lead presented higher mean values with respect to the reference values in Lake Ossa while in lakes Mevia and Mwembe, the values were lower than those of the continental crust (17 mg.kg -1 , dry weight). In Lake Ossa, its value was twice the concentration in the continental crust, and even in sediments considered unpolluted (19 mg.kg -1 , dry weight). The values recorded for this pollutant whose accumulation in sediments was relatively high due to its low solubility in water [88] testify their high concentrations in this environment. The low concentration of lead in Lake Mwembe and its high concentrations in lakes Mevia and Ossa could be justified by the fact that the most important part of the rubber and palm oil plantation is found on the largest watershed which harbours lakes Mevia and Ossa, thus, by leaching, water passing through Lake Mevia to reach Lake Ossa bring more lead contained in fertilizers. This water input could be accompanied by particles, including organic material to which heavy metals bind [89,90]. The water which enters Lake Mevia, then Lake Ossa could bring and maintain in these hydrosystems enough lead clogged to particles. When the rain stops and at the beginning of the dry season, suspended particles might be deposited, causing sedimentation of the metal. Compared to other lakes in the country, lead concentrations in the Ossa lake complex were lower than those in lakes Bini and Dang of the Adamaoua region, two lakes polluted by heavy metals (especially cadmium and lead) [51], and in the Yaounde Municipal Lake in the Centre region, a eutrophic to hypertrophic lake [91].
Taking into account Cu, Pb, Zn, Cd and Fe, the PI and the EF were calculated. The results are shown ( Table 4). The comparison of the results was made with respect to the concentrations of these metals in the earth's crust, with reference to iron. The PI varied between 0.006 (Fe) and 3.200 (Cd) in Lake Mevia. The index ranged from 0.008 (Fe) and 4.200 (Cd) in Lake Ossa and from 0.010 (Fe) and 18.240 (Cd) in Lake Mwembe. With the exception of Cd whose values were higher than 3, all the other metals had PI values lower than 3.
For the EF, values fluctuated between 116.856 (Pb) and 491.456 (Cd) in Lake Mevia, between 149.076 (Cu) and 480.310 (Cd) in the Lake Ossa and between 28.312 (Pb) and 1761.099 (Cd) in Lake Mwembe. Cd also presented highest values. Apart from Pb whose EF value in Lake Mwembe was less than 30, all the other heavy metals had values superior to 50.
The gross values of various heavy metals measured showed an anthropogenic pollution of water. This pollution could be linked to the intense agricultural activity around the Ossa lake complex and the use of chemical fertilizers and fungicides by SAFACam, with consequent of leaching these phytosanitary products to the hydrosystems [92,93]. It can also be due to the presence of poorly managed solid waste that generate many uncontrolled landfills [93].
The results of PI did not seem to confirm this anthropogenic pollution of the waters of the Ossa complex by heavy metals. Only Cadmium had a PI higher than 3. A PI value ranging between 3 and 9 indicate a suspicious situation of pollution [58]: Cd could be the only metal whose enrichment in the lakes could be of exogenous origin. Nevertheless, [62,59,60] recommend the calculation of the EF to complete the PI and determine whether an item is of natural or anthropogenic origin. From this point of view, the EF values were greater than 50. According to [66], an EF value superior to 50 expresses an extreme enrichment of the environment by heavy metals. Considering these values and taking into account that gross values of Cu, Zn, Cd and Pb were higher than the concentrations in the earth's crust, one is able to say that the waters of these lakes were enriched with heavy metals. Indeed, biocides (pesticides, herbicides, preservatives) contain As, Hg, Pb, Cu, Sn, Zn, Mn and fertilisers contain Cd, Hg, Pb, Al, As, Cr, Cu, Mn, Ni and Zn [94]. These various products are used in intensive farming.
Moreover, in this study, Cd had the highest and Pb had the lowest EF and PI values in lakes Mevia and Mwembe while in Lake Ossa, the lowest EF or PI value is that of Cu. For [95], generally, refining, manufacture and application of phosphate fertilizers are the main anthropogenic sources of Cd in the environment. [96] and then [97] showed that the distribution of Cd is closely related to the intensive use of phosphate fertilizers, as the later contain significant amount of metals, particularly Cd, as impurities. This has been found by [98] in a water reservoir between Iran and Afghanistan. The high enrichment of Cd in lakes can then be explained by the agricultural activity of SAFACam. According to these results, Lake Mevia has the highest values of EF for Cd, Zn and Cu compared to the two other lakes, meaning that it is the most enriched lake especially due to Cd, Zn and Cu while Lake Mwembe is the less enriched.
Comparison of the mean concentration of heavy metals in the sediments and sediment quality guidelines indicate that all sites studied were below the PEL guideline for Pb, Cd, Cu and Zn, exception being for Ni whose value were greater than 35.9 (Table 5). This shows that Ni could frequently have adverse biological effects on benthic organisms. The higher value of Ni has already been seen in China [69]. The results also show that for Pb (in Lake Ossa), Cd, Zn (in Lake Mwembe) and Ni (in the three lakes), the concentrations are above the TEL guideline but below the PEL guideline. This suggests that those metals in indicated lakes can be occasionally responsible for adverse effects on organisms. For metals with values below the TEL guideline, they rarely constitute a risk for living organisms.

CONCLUSION
Water temperature values between May and July remained within the range of characteristic values of tropical lakes. The pH was slightly acidic, favouring the release of heavy metals in the water column. As for dissolved oxygen, its variation in time justifies a high concentration of fermentable organic matter in water: waters of the Ossa lake complex could be undergoing a process of eutrophication. Analysis of heavy metals in the sediments revealed that mean concentrations of Cu and Pb were higher in Lake Ossa while those of Ni, Zn, Cd and Fe were higher in Lake Mwembe. Moreover, with respect to the standards of [79], Pb, Zn and Cd in Lake Ossa presented high values. If the PI presents Cd as the single metal whose enrichment in lakes have an exogenous origin, the EF reveals an extreme enrichment of the environment by heavy metals. The waters of the Ossa lake complex, given the results of this research, are enriched with heavy metals. The main cause of this pollution could be the practice of intensive agriculture around the lakes by SAFACam which uses pesticides whose composition is generally rich in heavy metals. The high values of heavy metals measured in lake sediments portend a risk of bioaccumulation in lake complex fish products, which should be studied.