Taxonomic Diversity and Structure of Benthic Macroinvertebrates of Taabo Lake (Basin of Bandama; Ivory Coast)

The taxonomic diversity and structure of benthic macroinvertebrates of Taabo Lake (Ivory Coast), from November 2017 to October 2018 were evaluated. Water quality parameters, such as temperature (T °C), water transparency (cm), dissolved oxygen (mg.L-1), pH, turbidity (NTU), electrical conductivity (EC, μS.cm-1) were measured in situ using a multi-parameter probe HANNA, from four sampling points (littoral and sub-littoral zones) in Taabo Lake. Benthic macroinvertebrates sampling were collected by using Van veen in stainless steel. A total of 63 benthic macroinvertebrates taxa belonging to 23 Families, 8 Orders, 2 Classes and 2 Phylums (Molluscs and Arthropods) were identified. The most abundant of benthic macroinvertebrates were Mesogastropoda (66%) represented in all sampling points. They were followed by Diptera (25%), Odonata (3%), Ephemeroptera (3%), Basommatophora (2%), Heteroptera (1%), Lepidoptera and Coleoptera (less than 1%). The values of diversity indices of benthic macroinvertebrates collected in Taabo Lake showed no significant variations between sampling points.


Introduction
The construction of dams and reservoirs for hydropower production and flood control, water abstraction for irrigation, along with other water uses, has changed the natural hydrologic regimes of freshwaters worldwide (Dynesius and Nilsson 1994). These environments transform lotic in lentic environments, causing reduction of water flow and more deposition of fine sediment, increase of deep and water transparency, modifying also the physical and chemical conditions and the structure of biological communities (Tundisi 1986;Palmer et al. 2000). Within the aquatic communities, benthic macroinvertebrates represent one of the groups most affected by reservoir construction (Krzyzanek 1991;Krzyzanek and Kasza 1995).
Benthic macroinvertebrates are vital to the functioning of lakes by recycling autochthonous and allochthonous carbon and nutrients (Karlsson and Byström 2005 ;Berggren et al. 2010) and constituting an important resource pool for several aquatic top predators (Vander Zanden and Vadeboncoeur 2002;Vander Zanden et al. 2006), besides being important intermediate hosts of several parasite species present in lake food webs (Amundsen et al. 2003). Benthic macroinvertebrate communities are the most commonly used organisms for the assessment and monitoring of the ecological status of lakes and reservoirs (Bailey et al. 2001). Furthermore, they often show the strongest responses among bioindicators to water drawdown (White et al. 2008;Sutela et al. 2013). Their composition and abundance in lakes and reservoirs typically vary considerably with water depth, with species richness generally being higher near the shore than in deep water (Strayer 2008;Ngupula and Kayanda 2010).

Sampling of Benthic Macroinvertebrates
The major community which Klugh (1923) referred to as the lake bottom association has commonly been divided by limnologists into the littoral, sublittoral and profundal zones. These zones are defined as follows: (1) littoral, from the lake margin to a depth at which aquatic vegetation, through lack of light, can no longer grow; (2) sublittoral, extends from the lower edge of the rooted macrophyte zone to about the level of the upper boundary of the hypolimnion; and (3) profundal, roughly, the area of the bottom in contact with the hypolimnion, which consists of exposed fine sediment free of vegetation (Williams and Feltmate, 1992).
During this study, benthic macroinvertebrates were collected in all the seasons of the year beginning in November 2017 to October, along one transect parallel to the dam. On this transect, four sampling points were chosen (P1 to P4). The sampling points P1 and P4, are located in the littoral zones. These stations are characterized by the presence of macrophytes and receive influence from the predominant human activities (washing and bathing). The predominant substrata are sand and gravels.
The last two sampling points P2 and P3, are located in the sub-littoral zones. The vegetal covering is absent and the predominant substrata is mud.
The characteristics of Taabo sampling stations are given in Table 1. Sampling was carried out in all the seasons of the year, beginning in November 2017 to October 2018. Littoral and sub-littoral (from dephts ranging 6-9 meters) samples were collected using Van Veen sampler in stainless steel. At each point, five sediment samples corresponding to a total area of 0.25 m² were taken. All samples were sieved in situ through a 1mm mesh. The organisms retained were packed in plastic bags, labelled, fixed immediately in 70% alcohol and transported to the laboratory for processing. In the laboratory, the collected material was washed on a mesh of 0.25 mm, and sorted in Petri dishes, using stereomicroscope. The organisms were identified until the less possible taxonomic level, using identification keys of Diomandé et al. (2000), Tachet et al. (2010) And De Moor et al. (2003.

Physical and Chemical Variables of Water
Simultaneously with benthic macroinvertebrates sampling, the water temperature (T°C), electrical conductivity (EC, μS.cm -1 ), hydrogen-ion concentration (pH; pH unit), dissolved oxygen (mg.L -1 ) turbidity (NTU) were measured using multi-parameter probe HANNA HI 9146 and HANNA HI 99 1001. The transparency (cm) was measured with a Secchi disc.

Data Analyses
The benthic macroinvertebrate composition and structure were evaluated using: rarefied taxonomic richness, relative abundance, diversity (Shannon-Wiener index), evenness (Pielou index) and density. The frequency of occurrence was determined for each taxa: rare (< 25%), frequent (25-50%), and very frequent (> 50%) (Dajoz 2000). Before performing the comparison test, variations in environmental variables and biotic index were determined using Shapiro test. A significance level 0.05 was considered. This analysis was performed using the STATISTICA 7.1 computer package.
A Principal Focused Component Analysis (FPCA) was carried out using the R package (R Core Team 2013) to assess the main relationships between environmental variables and aquatic benthic macroinvertebrates.

Variations of Physicochemical Parameters of Taabo Lake
The spatial variations of physicochemical water parameters are illustrated in Figure 2. Temperature had highest value (35° C) in point P1 and lowest value (26.5°C) in point P4. Concerning the pH, its value ranged from 5.1 (P3) to 9.98 (P1). Conductivity had highest value (110.5 µS.cm -1 ) recorded at point P3 and lowest value (60 µS.cm -1 ) at point P2.
The concentration of dissolved oxygen recorded the higher value (8.5 mg.L -1 ) at point P3, and lower value (0.2 mg.L -1 ) at point P2. The higher value of turbidity (100 NTU) was observed at point P3 and the lowest value (20 cm) was recorded at this point. Turbidity had lowest value (5 NTU) at point P1, while the higher value of transparency (100 cm) was obtained at point P1. The values of abiotic variables of Taabo Lake showed no significant variations between sampling points (Mann-Whitney test; p > 0.05).

Taxonomic Composition of Benthic Macroinvertebrates
A total of 63 benthic macroinvertebrates taxa belonging to 23 Families, 8 Orders, 2 Classes and 2 Phylums (Molluscs and Arthropods) were identified during this study ( Table 2). The phylum of Molluscs includes 14 taxa divided between 7 Families and 2 Orders (Basommatophora and Mesogastropoda) all belonging to the class of Gasteropods. In this class, the taxa Melanoides tubercula was found in all sampling points.
The Arthropods are represented by the class of Insects. This class includes 63 taxa belonging to 6 Orders (Ephemeroptera, Heteroptera, Coleoptera, Lepidoptera, Odonata and Diptera) and 16 Families. Among theses families, Chironomidae were the most diversified with 12 taxa.
The anlysis of the taxonomic richness showed that station S1 is the most diversified with (53 taxa), followed by point P4 (38 taxa), and point P3 (7 taxa). Point P2 (3 taxa) had the lowest taxonomic richness (

Structure and Distribution of Benthic Macroinvertebrates
The global abundance of the benthic macroinvertebrates collected from each sampling point are shown in Figure  3. The most abundant of benthic macroinvertebrates were Mesogastropoda (66%) represented in all sampling points. They were followed by Diptera (25%), Odonata (3%), Ephemeroptera (3%), Basommatophora (2%), Heteroptera (1%), Lepidoptera and Coleoptera (less than 1%) (Figure 3). Concerning the spatial abundance, the sampling points P1 and P4 located in littoral zones were dominated by Insects belonged to Diptera order, while the point P2 was characterized by a high abundance of Molluscs Gasteropods ( Figure 4A).
The Shannon index respectively reached high and low values at P1 (2.340) and P3 (than less 1). As the rarefied taxonomic richness, the highest value was obtained at P1 (2.580) and lowest at P2 (less than 1). Concerning the Equitability index, the highest value (2.580) were recorded at P1 and the lowet (less than 1) were obtained at P2. The values of diversity indices of benthic macroinvertebrates collected in Taabo Lake showed no significant variations between sampling points (Kruskal-Wallis test; p > 0.05). (Table 3).  (Figure 5).
The Diptera Chironomidae Chironomus imicola and Polypedilum sp. are positively correlated with conductivity. The taxon Baetis sp. belonging to the order Ephemeroptera, is positively correlated with conductivity. This insect is negatively correlated with water transparency.

Discussion
Analysis of the physical and chemical parameters of Taabo Lake revealed that the abiotic parameters (temperature, hydrogen potential, dissolved oxygen, transparency, conductivity) shown no significant variation among sampling points. However, the high values of turbidity would be linked to effluents resulting from numerous human activities, namely the intensification of farming, washing and fisheries. Similarly, high values of conductivity are an indirect measure of the concentration of pollutants. Since conductivity is a numerical expression of the capacity of water to conduct electric current, thus depending on the ionic concentrations and temperature it indicates the amount of existing salts in the water column (Wetzel 1993). Detergents, which probably also come from sewage have phosphorus in their composition, which in excess leads to water eutrophication (Valente et al. 1997).
The settlement of collected benthic macroinvertebrates includes 63 taxa divided into 23 families, 8 orders and 2 classes. Taabo Lake taxonomic richness obtained was lower than that obtained by Kouamé et al. (2010; in the same Lake. In terms of diversity, these authors found 68 and 43 taxa of macroinvertebrates respectively. They worked on the macroinvertebrates associated to macrophyte habitats. Their results showed the importance of macrophyte habitats on macroinvertebrates taxonomic diversity. Generally, the more diversified substrates provided by macrophytes contain more ecological niches than homogeneous systems, which can allow more species to coexist. Macrophyte beds also offer refuges against predation by fish and other macroinvertebrates (Padial et al. 2009).
In addition, macrophytes provide more surface area for attachment by periphytons, which are a major component in the diet of macroinvertebrate primary consumers. According Cai et al. (2010), macrophytes provide more surface area for attachment by periphytons, which are a major component in the diet of macroinvertebrate primary consumers. Thiaridae (Melanoides tuberculata) were the most dominant and abundant Mollusca across all the sampling points. This family (Thiaridae) dominance may be attributed to the competitive advantage they have over the other snails of rapidly increasing their populations (Kouamé et al. 1989).
In this study, Insecta represent the most diversified class with 49 taxa. The insects colonize all the aquatic environments owing to their dispersal capacity with with strong preference for freshwaters (Tachet et al. 2003). Insecta is dominated by Chironomidae family (Chironomus sp., Polypedilum sp.). Lentic waters tend to promote the reproduction of algae and zooplankton and thus the Chironomid finds enough food for their energy needs (Ouattara et al. 2001 ;Diomandé et al. 2013), and sot they can become very abundant organisms for tolerating large climate, hydrological and limnological variations (Pamplin et al. 2006 ;Jorcin and Nogueira 2008). According to Helson et al. (2006) and Vander Vorste (2010), Chironomidae larvae are both taxonomically and functionally diverse in aquatic systems, represent most feeding and habit guilds, and have a wide range of tolerance values to varying environmental conditions. Anderson et al. (2012) argue that Chironomidae larvae are known to thrive in polluted environment probably due to possession of hemoglobin a pigment that transport and store dissolved oxygen. The presence of these indicator species suggests organic pollution from anthropogenic source.
The high densities (individuals.m -2 ) at S1 and S4 (littoral zones) let us suppose that these stations are characterized by an important development of algae. According to Salmoiraghi et al. (2001), for most lakes and reservoirs, both the abundance and the number of benthic taxa are highest in the littoral zone and decreases as the depth increase.
Regarding the principal focused component analysis (FPCA) showing the influence of environmental factors on the benthic macroinvertebrates, only temperature, conductivity and water depth are the abiotic parameters which explain macroinvertebrates distribution along Taabo Lake. This situation could attributed to water fluctuations in lakes and reservoirs. Water-level fluctuations are more frequent in lakes in regions where raine vents are strongly seasonal and occur in an irregular precipitation regime (Geraldes and Boavida, 2005). According Mc Ewen and Butler (2010), such fluctuations may often cause spatial and temporal differences in water chemistry, which in turn, can affect the diversity, density and overall resilience of reservoir biota.

Conclusion
A total of seventy-height (63) taxa were recorded in the Taabo Lake in this study. Our study up to date the benthic macroinvertebrate of this ecosystem and showed that the Taabo Lake's benthic macroinvertebrate was composed mainly by taxa commonly found in tropical ecosystems. Benthic macroinvertebrate community is marked by the numerical dominance of Gastropoda (63% of total abundance) and by the qualitative dominance of insect (37% of the total taxa). Distribution of aquatic macroinvertebrates of the Taabo Lake was best explained by Temperature, conductivity, dissolved oxygen, transparency, nitrates, and pH. The different indices http://journal.julypress.com/index.php/jess Journal of Environmental Science Studies Vol. 3, No. 1, 2020 studied, revealed that the ecological state is less alarming. It is therefore necessary to protect this.

Recommendations
For a better characterization of the entomological population, the dominant zoological group of macroinvertebrates from the Taabo reservoir, a deepening of their system is necessary, particularly for the family of Chironomidae. In addition, work should be carried out for a thorough knowledge of the diet of insects in the lake of the Taabo dam.