Influence of Anthropogenic Pollution on the Abundance Dynamics of Some Freshwater Invertebrates in the Coastal Area of Cameroon

The abundance dynamic of some freshwater invertebrates in relation with the abiotic factors of the medium was studied from March 2013 to March 2014 on the main streams of the Wouri and Sanaga basins. Individuals of Copepoda, Atyidae, Chironomidae and Physidae were collected, identified and counted. Samplings were carried out monthly; meanwhile measurements of the environmental variables were taken. Results of the physico-chemical analyses revealed that water of the Sanaga basin is globally well oxygenated (77% 92%) and faintly mineralized (14 107 μS/cm), excepted in the stations receiving household pollutants. Inversely, in the Wouri basin, waters are hypoxic with high content of organic matters (27.79 ± 9.43 mg/L). During rainy season (June-September), there is a dilution-dispersion of the organic matter due to stormwaters. Concerning biological analyses, the families of Physidae and Chironomidae were more abundant in urban stations than in stations situated in suburban area, especially during low water level period, showing that they are well adapted to anthropogenic and organic pollutions. The hypoxic conditions of urban waterways do not favor the proliferation of sensitive taxa such as Atyidae and Copepoda. These taxa preferred well oxygenated waters with rapid flow, and are more abundant during rainy season in stations located in suburban area or which are less subjected to anthropogenic disturbances.

Bio-Assessment

Introduction
The preservation and the rehabilitation of the ecological quality of surface waters in sub-Saharan Africa is one of the major stakes for the environmental, touristic and socio-economic sustainability.It is thus a necessity to carry out regular environmental monitoring of surface waters, using reliable and relevant indicators.The use of biological indicators associated with the physicochemical analyses constitutes one of the most informative and complete method for the water quality assessment.At this purpose, freshwater macroinvertebrates are broadly regarded as particularly sensitive indicators of the health of urban waterway ecosystems subject to anthropogenic activities [1] [2] [3].Indeed, as an important functional group in stream ecosystems, macroinvertebrates show predictable responses since they are sensitive indicators of long-term environmental changes in water and habitat quality [4] [5] [6].They are thus used in surface water assessment and monitoring [7] [8].
The physid individuals and chironomid larvae are well adapted to live in extremely polluted aquatic media [9] [10].Indeed, their ecological diversity is favor by their physiological adaptation to the high variability of the environmental factors of the medium such as salinity, acidity, the high thermal amplitudes and the dissolved oxygen fall [11].Inversely freshwater shrimps of the family Atyidae preferred well oxygenated and low mineralized water, and are mostly abundant in the upstream and middle stream [12] [13].Copepods are also sensitive to water pollution and to the variations of the environmental conditions of the medium [14] [15].Consequently, it becomes obvious that the abundance dynamics of aquatic invertebrates is controlled by various parameters of the medium, which also show spatial and temporal variations.This study aims to determine the influence of anthropogenic activities through the abiotic factors of the medium on the abundance dynamic of some freshwater invertebrates in the coastal region of Cameroon.

Materials and Methods
Activities such as deforestation, installation and functioning conditions of the hydroelectric dam have a strong anthropogenic pressure on the river from the town of Edea [18].
The samples were collected monthly, from March 2013 to March 2014 between 8 am and 12 am.A total of 16 sampling stations were selected in the basins of Wouri and Sanaga, respectively in the cities of Douala and Edea (Table 1).Eight stations were chosen in three streams of the lower Sanaga basin (Mabandi, Boué and Sanaga), and eight other stations located in seven streams in the Wouri basin (Kondi, Longmayagui, Mboppi, Mgoua, Tongo-Bassa, Mbanya and Wouri).

Measurement of Environmental Variables
The measurements of physicochemical parameters of water at each sampling station were done following standard methods [19] [20].Water temperature (˚C) (WT), pH, and dissolved oxygen (%)(DO) were measured in situ using an alcohol thermometer, a HACH HQ11d pH-meter, and a HACH HQ14d oxymeter, respectively.Likewise, electrical conductivity (µS/cm) (Cond) and salinity (‰) (Sal) were measured in situ using a HACH HQ 14d conductimeter.Current

Sampling and Identification of Aquatic Invertebrates
For the sampling of copepod individuals at each studied station, 60 L of water were filtered through a plankton sieve (10 cm diameter and 64 µm mesh size).For this purpose, subsamples of water were collected mainly under aquatic macrophytes using a 10 L bucket.Materials collected in the sieve were then preserved into a glass sampling bottles with 4% formalin.In the laboratory, all individuals of each sample were identified under stereomicroscope and optic microscope with the use of appropriate taxonomic keys [21] [22] and counted.The abundances were expressed as number of individuals per liter (Ind./L).Concerning macroinvertebrates, samples were collected at each station using a long-handled kick net (30 cm × 30 cm side, 400 µm mesh-size, 50 cm depth).For each station, samplings were done in a 100 m stretch following protocol described by Stark et al. [23].At each station, 20 drags of the kick net were done in different micro-habitat, each corresponding to a surface of 0.15 m 2 (30 cm × 50 cm).The materials that were collected in the sampling net were rinsed through a 400 µm sieve bucket and all macroinvertebrate individuals were sorted and preserved in plastics sampling bottles with 70% ethanol.In the laboratory, all aquatic insects belonging to the families Atyidae (Decapoda), Chironomidae (Diptera) and Physidae (Gasteropoda) were identified under a stereomicroscope using appropriate taxonomic keys [24] [25], and counted.The density was expressed as number of individuals per square meter (ind./m 2 ).

Data Analyses
The ANOVA tests followed by post-hoc tests were used to Banferroni pairwise comparisons for quantitative data.T-student test was used to compare the samples in pairs.A logarithmic transformation was performed prior to the matrix of quantitative variables to ensure compliance with normality hypothesis.The Principal Component Analysis was used to search the relationship between abiotic and biological variables.The method of classification by the K-means clusters was used to divide the study period according to the weather variables.XLSTAT.2014software was used for all analyses.

Environmental Variables
The mean values and standard deviation (SD) of environmental variables meas-ured at each sampling station are shown in Table 2.In the watershed of Sanaga, the maximum value of water temperature (31.5˚C) was observed in station S2 during the month of January.In the Wouri basin, the values of water temperature varied between 25˚C (KO) and 34˚C (MG), respectively during the months of July and May.The t-student test revealed that, mean value of water temperature registered in the Wouri basin (29.06˚C ± 1.99˚C) was significantly (P < 0.05) higher than the mean value obtained in the Sanaga basin (27.21˚C ± 2.11˚C).In Edea, the highest values of the suspended solids were obtained at station S2 (125 mg/L) in September.Whereas the values of suspended solids registered in Douala, fluctuated between 3 mg/L (July) and 358 mg/L (June) respectively in stations KB and WO.In the Sanaga basin, the values of turbidity oscillated around an average of 28.66 ± 26.43 NTU, with a maximum value of 144 NTU recorded at S2 station in September.
While in the watershed of Wouri, values of turbidity varied between 11 NTU (TW, June) and 316 (WO, February), with a mean value of 56.85 ± 55.14 NTU.This mean value was significantly (t-student test, P < 0.05) higher than the mean    while values of the air temperature and the relative wetness fluctuated between 24.8˚C (August) and 28.51˚C (February), and between 73.9% (January) and 86.9% (September), respectively.The monthly maximum values of rainfall were registered in September (746 mm), while the minimum were recorded in February (1.4 mm).Positive and significant correlation (r = 0.78; P < 0.05) was found between rainfalls and relative wetness.

Partitioning of the Study Period Using the K-Means Cluster's Method
The results of the K-means cluster's classification based on meteorological pa-

Composition and Distribution of Taxa
Table 4 presents the species identified in the 4 classes (Malacostraca, Maxillopoda, Insecta and Gasteropoda) of invertebrates studied in the watershed of Sanaga and Wouri during the study period.In addition to the Nauplii and Copepodits larvae that are not easy to identify to species level, 9 species of mi-  4).For freshwater shrimps, Caridina africana was the most represented species with 86.5% of total abundance, while Caridina nilotica registered just 13.5% of the total individuals.Physa acuta was largely dominant with 91.4% of relative abundance against 8.6% for Aplexa sp.Con-  4).

Spatial and Temporal Dynamic of Taxa Densities
As for Chironomidae, densities varied from 0 to 199 ind./m 2 , the highest value being obtained in station M2 in October (Figure 2  Thus, for the Physidae densities, no significant difference were detected between sampling sites located in Edea and Douala (t-student, P = 0.06).

Relationships between Environmental Factors and Abundance Dynamic of Invertebrates
The results of the first principal component analysis (PCA) based on the spatial gathering of parameters, revealed that the relationship between invertebrate Group A is constituted of parameters such as color, turbidity, suspended solids, dissolved carbon dioxide, oxydability, salinity, water temperature, electrical conductivity, air temperature and insolation which are positively correlated with the Physidae and Chironomidae densities.This association is more pronounced during the months of December, January, February and March, which are characterized by low water level and hydrous stress.Group B gathered the months of April, May and November which presented relatively the same abiotic factors.This low water flow period seem not to significantly influence the distribution of aquatic invertebrates.Group C is constituted of parameters such as current velocity, dissolved oxygen, pH, relative wetness and rainfalls which are positively correlated with the Copepda and Atyidae densities.This association is more pronounced during the months of June, July, August and September, which are characterized by over flood.

Characteristic of Environmental Variables
The analysis of meteorological parameters of the littoral region of Cameroon permitted to determine three periods: a low flood period (April, May, October and November), a low water level period (December, January, February and March) and an over flood period (June, July and September).This variability might be due to anthropogenic activities, influence of Atlantic Ocean and global climate change.Similar results were documented in Equatorial Central Africa on the variability of the hydrographic regime [26] [27].The strong correlation observed between rainfalls and relative wetness shows that the atmosphere of these zones, does not exchange only heat with the ocean, but also of water vapor and rains.Indeed, littoral water system such as marshy and lake maintain a high relative wetness all over the year.This observation is online with the work of Drijfhout et al. [28] on climate variability.Furthermore, negatively significant correlation recorded between rainfalls and current velocity revealed the impact of seasonal variability on the hydrological and even the ecological regime of wa-ter.Our results are closer to those of Diomandé [29] on some Ivorian rivers and Tchakonté et al. [10] on some urban rivers in Douala Cameroon.
Concerning physicochemical parameters measured the slightly high values of water temperature registered in Douala watershed should be related to urbanization, anarchical land used and the anthropogenic activities in this township.Indeed, in this situation water systems are subjected to light rays and industrials wastes.Our results are consistent with the work carry out Tchakonté et al. [30].
Although the impact of anthropogenic activities is perceptible on the heat flux, these values of water temperature remain in the optimal range for the development of aquatic organisms [8] [31] [32].Furthermore, the increase of temperature in stations M2 and M3 would have favored the degradation of organic matter, which thus increase the electrical conductivity and reduce the oxygen content these stations.Similar results were documented for other Cameroonian rivers surface [10] [33].High values of oxydability in stations M2 and M3 should be due to urban and industrials wastes.Inversely, low values of these parameters observed in stations B1, B2, B3, M1, M2, S1 and S2 could be due to the absence of houses and industries.These observations are consistent with the results documented by Tchakonté et al. [10] in urban and suburban streams of Douala.On the other hand, the significantly high values of oxydability in Douala watershed reflect the important input of domestic and industrial wastes.This conclusion is concomitant with that outlet by Onana et al. [34] on the stream Kondi in Douala.
High values of suspended solids observed during the beginning of the rainy season, could be due to the input of various matters by storm water drainage.
Moreover, high tide induces the increase of suspended solids in the coastal rivers [35].Similar observations were done by Diomandé [29] on the Agnéby River in Ivory Coast.The relatively high values of salinity in Douala watershed may be related to the high concentration of minerals due to anthropogenic disturbances.
Nevertheless, the significantly high values of salinity registered in Wouri river are surely linked to the contamination by brackish waters.

Influence of Environmental Factors on the Abundance Dynamic of Invertebrates
The families of Physidae and Chironomidae were more abundant in urban stations than in stations situated in suburban area, especially during low water level period, showing that they are well adapted to anthropogenic and organic pollutions.They are known to be saprophagous [36].The prolificness of the nonbiting midges Chironomidae is not surprising, because they are opportunists and tolerant of polluted waters where they may occur in large numbers [36] [37].
Indeed, Chironominae are known as 'blood worms' because they are bright red, due to the presence of hemoglobin in their body fluids that enables them to respire at low oxygen concentrations and live in hypoxic or occasional anoxic bottom mud [10] [37] [38].Moreover, they can build inter-connected galleries in mud, which are then ventilated by undulatory movements; or they can prolong their tubes above the surface of the sediment to capture oxygen molecules [39] [40].As such, the well adaptation of Physidae to harsher conditions may be due to the fact that their shell is a physical barrier to pollutants, and they are also able to assimilate atmospheric air via a vascularized mantle cavity [3] [41].
The hypoxic conditions of urban waterways do not favor the proliferation of sensitive taxa such as Atyidae and Copepda.These taxa preferred well oxygenated waters with rapid flow, and are more abundant during rainy season in stations located in suburban area or which are less subjected to anthropogenic disturbances.Similar results are recorded in other Cameroonian streams, revealing that freshwater shrimps Atyidae proliferate in well oxygenated rivers [12] [30].
In the same way, Mergeay et al. [42] noted a significant increase in the density of Copepoda in surface water during flood.Atyidae and Copepda organisms seem to be very sensitive to the variations of the organic matter loads and dissolved oxygen content.

Conclusion
This study highlights that the abundance dynamics of freshwater invertebrates in the watersheds of Douala and Edea are highly influenced by water pollution due to anthropogenic disturbances.On the one hand, input of organic matter induce the installation and the prolificness of resistant taxa such as Physidae and Chironomidae, which are well adapted to organic pollution and hypoxic condition of the medium.On the other hand, water pollution causes the fled and the extinction sensitive taxa such as Decapoda-Atyidae and Crustacea-Copepoda, which are very sensitive to oxygen content depletion and rainfalls variability.
These ecological characteristics might be used to increase the preciseness of water quality assessment methodologies of biomonitoring programs.There is therefore, the need for an elaborate characterization of wastewaters and water bodies, and the evaluation of treatment facilities of municipal waste so as to assess the status and control water pollution.
value obtained in the Sanaga basin.The values of water color fluctuated between de 27 Pt-Co (M1) and 776 Pt-Co (S2), with a mean value of 157.07 ± 137.81 Pt-Co in Edea, while in Douala, values ranged from 70 Pt-Co (TB) to 3072 Pt-Co (WO) with an average value of 363.39 ± 411.39 Pt-Co.The values of pH showed a range of 5.50 (M1, February) to 8.20 (S2, March) with a mean value of 7.27 ± 0.50 in Edea.At the level of Douala, pH values oscillated between 6.30 and 7.70 Table 2. Mean values and standard deviation of abiotic parameters measured at each sampling station during the study period (Oxy = oxydability, WT = water temperature, DCO 2 = dissolved carbon dioxide, Sal = salinity, Cond = conductivity, OD = oxygen dissolved, SS = suspended solids, Turb = turbidity, CV = water current velocity).
cro-crustaceans belonging to the family Cyclopidae were identified in the study sites.In the same way, 2 freshwater shrimp species of the family Atyidae, 1 species of the family Chironomidae (Insect-Diptera) and 2 species belonging to the family Physidae (Gasteropoda-Basommatophora) were identified during the study.A total of 17,555 individuals of Copepoda, 2588 individuals of freshwater shrimps, 5212 individuals of Diptera-Chironomidae and 18,055 individuals of Gasteropoda-Physidae were collected in the two river basins (Sanaga and Wouri) during the study period (Table Spatial and temporal variation of invertebrate densities revealed that in the Sanaga river basin, high densities of freshwater shrimps of the family Atyidae were obtained at stations S1 in June (145 ind./m 2 ) and S2 in July (147 ind./m 2 ) (Figure 1(a)); while in the watershed of Wouri these densities were globally high in station WO with maximum value observed in June (186 ind./m 2 ) (Figure 1(b)).One way ANOVA test showed that densities of Atyidae significantly varied (P = 0.0001) from one station to another, values recorded in stations S1, S2 and WO being significantly higher (Banferroni, P < 0.05) compared to the other (a)).In the Wouri river basin, Chironomidae densities relatively varied from one month to another, higher values being recorded at station LM in February (195 ind./m 2 ).Stations KO and TW also present higher densities during the month of February (Figure 2(b));

Table 1 .
Description of the physical environment of the study sites.

Table 3 .
Results of the K-means cluster's classification based on meteorological parameters.
rameters permitted to determine three clusters (Table3).Cluster 1 regroups the month of May, April, August, October and November.These months are characterized by relatively moderated rainfalls, with an average of 335.5 mm, and a mean value of insolation of 4.23 kWh/m 2 /day.These months correspond to low flood.Cluster 2 regroups the month of March, December, January and February which are characterized by very low rainfall (104.04 mm) and very high values of insolation and air temperature.These months correspond to the period of low water level.Cluster 3 is made up of June, July and September, characterized by high rainfalls (with a monthly mean value of 701.77 mm), high value of relative wetness (85.52%) and low insolation (3.74 kWh/m 2 /day).These months correspond to the over flood period, with high dilution of organic matters.

Table 4 .
Summary of abundances (relative abundances) of invertebrate taxa identified in the two watersheds during the study period.

Table 5 .
Spatial dynamic of abundances (relative abundances) of different species of Atyidae, Chironomidae and Physidae identified during the study period.

Table 6 .
Spatial dynamic of abundances (relative abundances) of larvae and different species of copepods identified during the study period.