Physico-Chemical and Biological Characteristics of the Nklobisson Artificial Lake in Yaounde, Cameroon

The deterioration of water quality in lakes constitutes an environmental problem faced by humanity. The Nkolbisson artificial lake which is located downstream of the Abiergue and Mintotomo streams in Yaounde is under the influence of various forms of pollution, mostly resulting from human activities. There is proliferation of algae on the lake surface that consumes the available oxygen in the lake. The main objective of the current study is to evaluate the different forms of pollutants in the lake, in order to propose efficient solutions for a suitable management strategy, basing on the physico-chemical and biological analysis. The lake retains an average of 2535.49 tons/year of suspended solids in the rainy season and 1438.05 tons/year in the dry season. The range of dissolved oxygen (5.2 3.8 mg/l); pH (7.8 6.8); temperature (23.9 ̊C 23.1 ̊C); electrical conductivity (266.1 87.3 μS/cm); turbidity (22.3 10 UNT); suspended solid (240 40 mg/l); BOD (55 8 mg/l); COD (76 87.3 mg/l). Biologically, faecal coliforms fluctuates between (42 × 10 425 × 10 FCU/100 ml), faecal streptococci (8 × 10 158 × 10 FCU/100 ml) and total Coliform (15 × 10 78 × 10 FCU/100 ml) evidence that, the studied lake is highly polluted. Consequently the lake retains an average of 2888.7 tons/year of suspended matter.


Introduction
Urbanization and increasing industrialization of cities result in an increase in the load of wastewater that is enriched in organic matter and dissolved substances. The outcome is a deterioration of water quality, reduction of diversity, and on a long-term, the disappearance of water bodies [1]. Artificial lakes in Cameroon are increasingly faced with sedimentation problems. While many studies have been carried out on the filling of lakes in Europe and America in general, little is known in Africa [2]. In Cameroon, some works done by Zébazé Togouet et al. [3], Foto Menbohan et Njine [4] show respectively that, the Yaoundé municipal and the Effoulan lakes represent major assets to support the country's development but they are threatened by hydrological changes, pollution and siltation problems.
Siltation and eutrophication of lakes cause flooding, significant changes in the physicochemical and biological parameters and consequently rapid growth of aquatic plants [5]. The Nkolbisson artificial lake was created in the 1970s by the Higher National School of Agriculture in order to facilitate the training of engineering students in various disciplines (viticulture, irrigation experimental fields, fishing competitions and water skiing). After the closure of the school, all activities were definitely stopped in 1980. The continuous drainage into the lake of untreated wastewater from the Abiergue and Mintotomo streams of the Mefou watershed flowing across Cité Verte from the rivers of the Mefou watershed and the Abiergue, Mintotomo, Cité verte located upstream, as well as the wastewater treatment plant of Camps Sic Cité verte, has gradually degraded its quality and environment [6]. Despite the dangers inherent in the filling of lakes over time, no study has yet been undertaken on the Nkolbisson artificial lake; one of the oldest artificial lakes in Yaoundé Bessenassé [7]. Okasha et al. [8] indicate that siltation of lakes results in the reduction of their lifespan, one of the most harmful effects of water erosion. The management of lake ecosystems, therefore, requires a thorough knowledge of the hydrodynamical and sedimentary operation to propose scenarios of changes that are adapted to different environmental constraints. These include maintenance of water quality, improvement of fish habitat and aquatic organisms habitat, protection of structures, development of recreational and ecotourism activities [9] [10]. This study is one of the first studies that focus on the filling of the Nkolbisson artificial lake. This work aims at diagnosing the parameters that contribute to the degradation and filling rate of the Nkolbisson artificial lake and to propose approaches to ensure its restoration and sustainable management. Therefore, it is necessary to avoid refilling the lake that disequilibrates the Nkolbisson artificial lake which represents a touristic site to the urban community. Journal of Water Resource and Protection area is dominated by lateritic soils with minor hydromorphic subsoils [12]. The hydrographic network is dense, made up of many low and marshy funds including the Mefou River which is flowing through the southern part of the Nkolbisson neighborhood. The climate is characterized by two rainy seasons (September to mid-November and mid-March to June) with an annual average temperature of 23.5˚C.

Analytical Methods
The choice of the sampling points was not easy due to many factors including the rough morphology of the studied area, the multiplicity and the diversity of wastes. However, basing on the source of materials flowing into the lake, three water sampling points representing the streams feeding the studied lake were chosen: 1/Abiergue which is, situated upstream of the Lake, 2/Mintotomo, main tributary of Abiergue which crosses the undeveloped rural area and 3/the outlet of the studied lake ( Figure 1). Samples were collected both during raining season (two sets of sampling) dry season (two sets) within the period 2013 to 2014.
Given that the biodegradation of suspended matters in waste water takes place at a very fast rate, physicochemical parameters including temperature settings (T), pH, dissolved oxygen (DO) and electrical conductivity (EC) were measured using a mercury thermometer, pH meter (Schott CG 818 Geräte GmbH), an Oximeter (OXI model 315i WTW) and conductivity meter mark HACH, respectively. Turbidity was measured using turbidity meter (HANNA instrument model LP 2000) at LAGE Laboratory, Yaoundé. Suspended solids (SS) were Journal of Water Resource and Protection measured using a brand of Millipore filtration ramp WHATMAN GF/F 47 mm circular diameter with porosity 0.45 µm. The liquid debit was estimated by the volumetric method and the solid debit was calculated from the average concentration of suspended solid (SS) using the equation. The annual flux of suspended solids transported by the Abiergue and Mintotomo Rivers was finally obtained by adding the solid contributions of the two stations that feed the lake. BOD was determined by the breathing method using a brand-meter (WTW BOD Model 1020T) using the technique described by Din [14]. COD was determined by oxidation in a potassium medium at a tempera-   ganisms, induces unpleasant odour and increases the sedimentation rate of suspended matter. The results of this study are similar to those of Akono Ntonga et al. [6]. Water temperature plays an important role in the solubility of gases, the separation parting of dissolved salts and pH determination. Water temperature is also important to know the origin of the water and to identify the mixing of different water types is influenced by the source from where they originate [16].
Based on the pH of the lake water that ranged from 6.90 to 7.20 ( Figure 3), the lake is alkaline in nature. The highest pH values were obtained in the dry season. These results are similar with those obtained in other lakes such as the Yaoundé municipal lake [17], the Ebolowa municipal lake [18] and the Mefou lake [19]. These values could be due to the degradation of organic matter (plants, animals, faeces and domestic waste water).

Dissolved Oxygen
Dissolved oxygen (DO) values showed no significant seasonal variation and were generally low (<6 mg/l). They were higher (4 to 5.2 mg/l) during the rainy season with the highest value recorded at the lake's outlet while the lowest (3.8 mg/l) was along Abiergue River during the dry season ( Figure 4). These results show a strong deficit in DO, with no significant seasonal variation, although we noticed a slight increase at the flood season outfall. Oxygen is known as a good indicator of pollution of waterways and the monitoring of its self-purification.
At the studied sites, the DO was low, showing a low photosynthetic activity. This might be due to the extent of pollution of the lake. The average DO value obtained (<5.2 mg/l) was below the minimum (>6 mg/l) of DO characteristic of normal biological conditions [20]. These low DO values could be due to the influx of waste water enriched in organic matter and inorganic substances. The values further decreased in the lake due to the degradation of organic matter triggering the multiplication of aerobic microorganisms. If oxygen levels maintain at 3 mg/l, it could lead to the dead of fish [21]. Conversely, opposite results were observed in the Yaoundé municipal lake, (93.3 to 127.6 mg/l) [22].

Electrical Conductivity
The measure of Electrical conductivity (EC) indicates the degree of water and Journal of Water Resource and Protection   Figure 6). There was no significant seasonal variation observed at the Mintotomo station. The high turbidity values (22.4 NTU) observed at Lake outlet as described in Taabo Lake in Ivory Coast [25], may be linked both to highly charge water intakes from Abiergue and Mitotomo and also to particles transported during rainfall.

Total Coliform
The temporal evolution of load in total coliforms (TC) in the three sampling
Its spatial variation is closely related to that of total coliform, confirming the water contamination in the studied site by faecal matter. The seasonal variation showed that concentrations varied between 10 × 10 2 and 425 × 10 6 FCU/100 ml during the dry season, with the highest and lowest contents respectively recorded at Abiergue and Mintotomo (Figure 8). Whereas during the rainy season, concentrations ranged from 420 to 32 × 10 6 FCU/100 ml, with the highest value Journal of Water Resource and Protection found at Abiergue and the lowest is noted at the Mintotomo. These high concentrations might be due to discharges of either urban and domestic wastewater or faecal matter from shallow pit toilets directly into the runway as done by some of the surrounding population [26].

Faecal Streptococci (FS)
The FS are associated with faecal coliform, and are considered as good indicators of pollution, and also of treatment because they are significantly more resistant than coliforms and other pathogenic enterobacteria [27]. The amount of FS was generally higher than that of FC in the studied area (Figure 9). At Mintotomo, values of FS were higher during the dry season (158 × 10 3 FCU/100 ml) compared to the rainy season (12 × 10 3 FCU/100 ml). At the Abiergue station, FS varied from 33 × 10 3 FCU/100 ml during the dry season to 19 × 10 3 FCU/100 ml during the rainy season. The same trend was observed at the lake with 8 × 10 3 FCU/100 ml during the dry season and 3 × 10 3 FCU/100 ml during the rainy season. The highest concentrations were found at the Abiergue and at the lake outlet respectively during the dry and the rainy seasons. This is explained by the accumulation of organic load during the dry season followed by their transport and storage within the lake during the rainy season. During the study period, FC concentration was lower than that of FS and TC compared to faecal coliforms; faecal streptococci are reputed to be resistant to toxic conditions. It has been presented that prolonged exposure to toxic elements and to critical conditions of the medium is more letal for faecal coliforms than faecal streptococci. Ajibade et al. [28] found opposite results in the National Park lake in Nigeria, may be due lack of a constant treatment of the lake water. In general, the results of the bacterial counts indicate a very poor microbiological water quality.

Determining the Origin of Faecal Contamination (R = FC/FS)
The quantification of faecal flora contamination (Table 1)    values of these ratio characterise an origin of mix faecal contamination to human predominance since R falls between 2 < R < 4. This origin is linked to direct discharge of human excreta into streams by the population living along streams using them as privileged places for defecation [29].

Biochemical Oxygen Demand (BOD5)
Biochemical oxygen demand (BOD) is the amount of dissolved oxygen consumed by microorganisms in the dark at 20˚C for 5 days. It enables the evaluation of biodegradable organic matter present in water; Figure 10 shows  providing suitable conditions for the degradation of organic matter by microorganisms. This activity consumes oxygen and results to the self-purification of water [31]. Though, during the rainy season, the rain water significantly contributes to the dilution of organic load from waste water. The Mintotomo station samples are weakly loaded because of its location far from the highly populated neighbourhoods. According to Emere et al. [31], the waters of Abiergue and at the lake outlet are heavily polluted (BOD > 10 mg/l) while those of Mintotomo are moderately polluted (2 ≤ DBO5 ≤ 2.9 mg/l). The same results were found in the water of the Chikila Lake in Indian [32].

Chemical Oxygen Demand (COD5)
The measurement of Chemical oxygen demand (COD) is suitable to monitor municipal and industrial wastewater. It is representative both of the bulk organic compounds and of oxidizable mineral salts (sulphides, and chlorides. The COD contents recorded for the studied waters ranged from 16.5 mg/l (Mintotomo) to 68.8 mg/l (Abiergue) and between 19.5 mg/l (Mintotomo) and 76 mg/l (Abiergue), respectively during rainy and dry seasons. No significant variation between seasons was observed at the lake outlet, (45 -40 mg/l) (Figure 11). High COD values are related to the quality of wastewater and urban domestic water discharged (used as discharge outlet of different waste). The level of COD recorded throughout the study area was generally lower during the rainy season compared to the dry season. Furthermore, during the dry season the contents recorded in all stations are far beyond the WHO and UNICEF standards [33] of 30 mg/l. The increase of COD in the Abiergue station can be explained by the leaching of soils containing inorganic elements. It was noticed that during both the rainy and dry seasons the COD contents of Mintotomo waters remained low. According to Igbinosa and Okoh [34], lower concentrations of COD during the dry season could be attributed to a reduction in organic and inorganic substances in the receiving environment.

Evaluation of Organic pollution
The ratios of COD/BOD, BOD/COD (Table 2)

Relationship COD/BOD
The ratio COD/BOD helps to determine the nature of the pollution of the receiving environment. According to Singh et al. [35] COD/BOD > 2 means that there is chemical pollution and requires biological treatment. In the present study, the COD/BOD of wastewater of the three studied sites (Abiergue, Mintotomo and Lake Outlet) indicates two categories of outcomes depending on the season. During the dry season, the COD/BOD ratio varied from 1.38 to 3.0 mg/l and during the rainy season it ranged from 2.06 to 3.08 mg/l ( Table 2)

Relationship BOD/COD
When characterizing industrial pollution, the ratio BOD/COD is often used to provide insights on the origin of sewage pollution and its treatment options. In the present study, the ratio varies from 0.34 to 0.72 during dry season and from 0.35 to 0.48 in rainy season period ( Table 2). The values greater than 0.3 during dry season are consistent with those obtained by Nayak et al. [37], and are indicative of a strong inorganic pollution.

Solid Load
The liquid flow at the entrance of the artificial lake is 560 l/s) during the rainy season and 180 l/s during the dry season. These results are high compared to those observed in most lakes located near large municipalities. The suspended solids (SS) loads carried by the waters of the three sites experience significant seasonal fluctuations ( Table 3). The solid load entering the lake varies from 128.4 g/l during the rainy season to 13.6 g/l during the dry season. While at the lake outlet, the solid load leaving the lake increases to 48 g/l during the rainy season and to 2.  Figure 12). Furthermore, the decomposition of macrophytes induces an increase in the amount of organic matter in the environment which the degradation therefore favors suffocation of fish species [38]. This may cause an imbalance in the trophic chain (production/consumption) of a body of water and lead to significant ecological consequences [39].  The discharge of domestic waste contributes strongly to organic enrichment of the Nkolbisson artificial lake because the particulate contributions of output are lower than those of input throughout the study (Table 3).

Correlation between Physicochemical Parameters
The correlation matrix (Table 4)  During flood periods, the existing bond between all the variables in pairs and correlation coefficients between these variables were given by the correlation matrix. Table 5 shows that the temperature is strongly (T ˚C)

Conclusion
An investigation into the chemical and biological characteristics of the Nkolbisson artificial lake has been carried out. The study reveals that, the water of the lake is turbid; with conductivity values are higher than 100 µS/cm. The results of COD reflect the importance of the content of organic matter at the lake which ranges from 76 mg/l during the dry season to 68 mg/l during the rainy season. tons/year representing 64.5% of the total matter at the inlet. The lake is in an accelerated eutrophication process that led to the current stage of hypereutrophy. Journal of Water Resource and Protection The imbalance of the ecosystem comes from its continuous supply of organic matters. This study is a scientific showcase and, therefore, a tool to the decision of the authorities of the Yaoundé VII District, from which strong decisions must be made to facilitate the restoration of this urban river system which is rich in aquatic biodiversity.