Spatio-temporal dynamics of zooplankton communities (Rotifers, Cladocerans, Copepods) and water quality of Lake Léré (TCHAD)

Lake Lere is situated in the Mayo-Kebbi locality southwest of Chad, it is the site for RAMSAR since 2001 hosting manatee (Trichechus senegalensis). This lake has been under study with the aimed of determining the structure of zooplankton community from February 2016 to April 2017. For this study, three sampling stations were chosen and divided into 2 to 3 layers from water surface towards the bottom. A total of 49 zooplanktonic species were identifiedin LakeLere. This was dominated by Rotifers community with more than 75% (38 species).Rotifers and Copepodsdominated withabundance proportions of 78.10 and 60.04% respectively in the dry season. This was relatively higher compared to the rainy season (21.89 and 39.95%). In the Cladocere communities, a higherabundance was observed in the rainy season of 85.39% and 14.6% in the dry season. Therewere no significant differences between the physicochemical parameters, the sampling stations and between the different sampling levels.Thehigh values of Sorensen similarity index, shows the homogeneity of the waters of the lake and justifies the absence of significant differences in the specific richness between stations. The specific richness of zooplankton community in Lake Lere and its physico-chemical variables lead to a conclusion of the mesotrophic state of the lake's waters.


INTRODUCTION
Nowadays the low proportion of fresh water in the biosphere, the rampant demography and the accelerated urbanization, makes this water, an increasingly precious commodity. In fact, 97% of the biosphere is in the form of salt water, in the sea and the oceans (Serra, 1999) while fresh water is distributed in bodies of water reservoirs and groundwater, only represents 0.57% (Vikram Reddy, 2005). According to UNESCO (2003), industrial effluents, chemicals waste materials and agricultural waste constitute the main part of the waste discharged into waters bodies and this waste represent about two million tons per day. The discharge of domestic waste inwater is also an important The results of an enrichment of a hydrosystem with nutrients,favors the growth of primary producers (Othoniel, 2006) and leading in a long term eutrophication (Zébazé Togouet, 2011). The latter is manifested by an excessive proliferation of plants, a degradation of the water quality, a deoxygenation of the environment, a modification of the water profile and a decrease in biodiversity (Lemoalle et al., 2006;Zébazé Togouet, 2008 and2011). Biodiversity being the engine of the functioning of an ecosystems (Hooper et al., 2005), its imbalance leads to the dysfunction of the ecosystem, hence the need to maintain the quality of surface water which offers several services to the society. A physico-In Chad, as in most African countries andparticular in the Sahel, the preservation of aquatic ecosystems is of paramount importance for sustainable socio-economic and cultural development. In addition to the harmful effects of anthropogenic activities, there are problems of drought and accelerated degradation of natural resources since the beginning of this decade (El hadj Sene et al., 2006).
Among the aquatic ecosystems of Chad, Lake Léré, located in the Binder-Léré wildlife reserve has been a RAMSAR site since 2001. This lake is home to several protected species like the Lamentin (Trichechus senegalensis) (Beakgoubé, 2011). In addition to being a fish tank exploited by the whole locality through fishing, Lake Léré offers many services in the agricultural field through the use of its waters for irrigation and the exploitation of its watershed. These activities undoubtedly influence the state of health of this ecosystem, which can cause an imbalance at long term. The latest hydrobiological studies carried out on this ecosystem concern the work of Pourriot (1971) and Gras and Saint-Jean (1971). An assessment of the state of health of this ecosystem is therefore essential to understand its evolution and ensure a sustainable management and maintenance. The study of the spatial heterogeneity of zooplankton has other important implications in the structure and functioning of hydrosystems (Pinel-Alloul, 1995).
The objective of this work is therefore to determine the specific richness and spatial structure of zooplankton in relation to the physicochemical characteristics of the waters in Lake Léré 46 years later.

Study site
Located in the West region of Mayo-Kebbi, the Léré subdivision is between latitude 9th and 10th degrees north and between longitude 14th and 15th degrees east (Pabong Dagou et al, 2002). The climate is the Sudano-Sahelian type, with a long dry season which extends from October to April, and a short rainy season from May to September (Djonfoné, 2003). Rainfall varies between 700 and 1100 mm with an average of 834 mm per year (Pabong Dagou et al, 2002), and a maximum in August (Lévêque, 1971).
Lac Léré proper is locatedbetween latitude 9 ° 38 North and longitude 14 ° 13 East (Pabong Dagou et al, 2002), at an altitude of 231 m (Lévêque, 1971), on the border between Cameroon and Chad along the coast of the Mayokebbi which connects the Chadian basin to the Niger basin. This lake has a surface area of 40.5 km 2 at low water and is presenta large flat-bottomed basin oriented from East to West, about 13 km long and 5 km wide (Lévêque, 1971). Its maximum depth is 8 m with an average depth of 4.5 m at low water. It is fed by river Mayo-Kebbi, after it has crossed Lake Tréné.
This crossing of the lake by river Mayo-Kebbi, justifies the choice of the two sampling stations located at the entrance and the other at the exit of the lake. This will help us better understand the physico-chemical and biological quality of the lake, the samples were also collected at a third station which is located between the first two. At each station, samples were collected at different depth. Thus, the sampling stations for this study are shown in Figure 1:--Station S1, is located at the entrance to the lake about 0.95 km from the shore, consists of two sampling points with one at the water surface and the other 2.5 m deep; -Station S2, located 4.23 km from station S1 and it is made up of three sampling points: surface, middle (2.5 m deep) and depth (4.5 m deep) -Station S3, located at the exit of the lake at 4.47 km from station S2, made up of three sampling points: surface, middle (2.5 m deep) and depth (4.5 m deep).
Sampling was also done at the banks of the lake along the vegetation (herbarium) between the different sampling stations. This gave us a more precise idea of the specific richness of zooplankton.

Sampling and analysis of samples
The samples were collected from February 2016 to April 2017, water for physicochemical analyzes where collected for a duration of 15 months while the biological saples were collected for 13 months covering all the seasons. Sampling was carried out on a monthly basis, between 11 a.m. and 1 p.m. The water was sampled on the surface, using a 20 L bucket, and in depth using a 3L Van Dorn bottle. For the physico-chemical analyzes, the samples were stored in 1L polyethylene bottles filled to the brim, placed in a refrigerated enclosure and sent to the laboratory no more than 4 hours after collection. For biological analyzez, 20 L of water was filtered through a sieve of 0.64 µm mesh opening, then stored in 100 cc flasks and fixed in 5% formalin for courting and identification in the laboratory.
The physicochemical variables were determined in the field and in the laboratory. The transparency of water, expressed in m, was measured in situ using a 30 cm diameter white Secchi disc, weighted and attached to a graduated cord. The temperature, TDS, hydrogen potential (pH), electrical conductivity, dissolved oxygen contents of the water were measured in situ using an OAKTON brand multiparameter; the results are expressed respectively in ° C, mg / L, Conventional Units (U.C.), μS.cm -1 and percentage of saturation. Water color and turbidity expressed respectively in Platinium-Cobalt Unit (U.Pl-Co) and Formazine Turbidity Unit (FTU), the nitrogen forms and orthophosphates, expressed in mg.L -1 were evaluated in the laboratory by colorimetry using the Palintest 7100 spectrophotometer, following the methods of APHA (1998) and Rodier et al (2009). CO2, expressed in mg / L, was determined by volumetry.
For biological variables, identification and counting were performed on the fixed sample under a WILD M5 binocular magnifier at 500X magnification.Identification of the rotifers is based on the morphology, the shape of the chitinous shell, the number and arrangement of spines and the number of toes. The sodium hypochlorite tissue digestion technique (Sanoamuang 1993, Sergers 1995 a) allowed the study of Rotiferous mastas whose identification was not possible with a binocular microscope by observing their morphologies. These maxtas characteristic of each species, were mounted between slid and slid plate cover and observed under the Olympus CK2 UL WCD 0.30 microscope. With regard to cladocera, the identification was made on the observation of morphological characters such as body shape, cephalic region (cephalic crest, expansions in points of lateral edges, invagination of posterior edges), carapace, detailed examination of the post abdomen, appendages. The identification of copepods was done following the shape of the body, lateral ornaments of the segments ofthe abdomen, the shape of the antennae and antennae, the position of the ovarian bags. In some cases, identification was done after dissection. Thus, with the aid of minutiae, the individuals placed on a blade are dissected in order to be able to locate the fifth pair of legs, the shape of which is characteristic of each species. This dissection is carried out under an Olympus CK2 ULWCD 0.30 inverted microscope.
Counting of the organisms was done on the fixed sample. 10 mL of this previously homogenized sample was extracted using a pipette and placed in a 30 mm diameter petri dish squared of 3 mm sides. The Petri dish squared prepared prevents any repetition of counting (Gannon, 1971). All the zooplanktonic organisms contained in this volume were counted at 500X magnification under a binocular magnifying glass of brand WILD M5. For a fixed sample of 100 mL, the counts were made each time on 10 mL until 400 individuals were obtained or until the sample was exhausted.
When identification was not possible under a magnifying glass, the individual was mounted between slid plate and slid plate cover for observation under an optical microscope. The density of individuals was calculated using the formula D = (nx 1000) / V where D is the density (expressed in individuals per liter), n is the number of individuals found in the volume of water analyzed under the microscope and V is the volume of water filtered in the field (20 L).
Certain physico-chemical data enabled the calculation of the organic pollution index (IPO). It is obtained using BOD5, nitrites, phosphates and ammonium values. In the absence of BOD5 values, the values of ammoniacal nitrogen (NH 4+ ), nitrites (NO 2-) and orthophosphates (PO4 3-) made it possible to calculate the pollution index (IPO) during this study. The principle of this calculation is to distribute the values of these three polluting elements (ammoniacal nitrogen, orthophosphate nitrites) each of the five classes contain on Table 1 has an ecological significance. The average of the class numbers of each parameter gives the IPO values divided into 5 pollution levels ( Table 2).  Various indices allowing the characterization, composition and the evolution of the zooplankton made it possible to analyze the biological data -The specific richness which is the total number of species in a sample; -The SHANNON-WEAVER diversity index, which made it possible to characterize the structure of the stands. Its formula is H' = H' = - Pi  log2Pi where Pi represents the relative abundance of taxon i; -The frequency of occurrence (F) expressed as a percentage which is the ratio of the number of samples where this species is present by the total number of samples taken. It provides information on the consistency of a species or a taxon in a given habitat without any indication of its quantitative importance (Dajoz, 2000). According to their percentage of appearance, five categories of species are distinguished (Dufrêne & Legendre, 1997). : This index is based on the presence / absence matrix and is calculated according to the relationship: F = (P_i × 100) / P_t Where Pt = the total number of samples and pi = the number of samples where the species i is present.
-The redundancy analysis (RDA) made it possible to determine the abiotic factors which influence the abundance of zooplanktonic species in the different groups. The result of this analysis is presented by a graph on which the abiotic variables, the species and the sampling sites are projected. The Redundancy Analysis was performed using CANOCO for Windows software version 4.5

Physico-chemical characteristics
The transparency of Lac Léré varied from 0.5 to 1.1 at station S1, and from 0.5 to 1.5 at stations S2 and S3. The minimum value was recorded in the rainy season in the months of May 2016 at stations S1 and S2 and in June 2016 at station S3. In the dry season, transparency was greater ( Figure

Fig.2: Seasonal variations of transparency (A), temperature (B), color (C), turbidity (D), TDS and electrical conductivity (F) during the period of study
The pH of the water is more or less neutral and fluctuated from 5.3 (depth of S1) to 8.6 CU (area of S1) with an average of 7.3 ± 0.53. In the rainy season, the waters of Lake Léré were more basic. Dissolved oxygen values fluctuated between 35 (depth of S3) and 109% (surface of S1) for an average of 64.8 ± 18.7. Carbon dioxide (middle of S2), fluctuated between 7 and 105.6 mg / L (area of S1) for an average of 29.5 ± 20.9 mg / L.     The H test of Kruskal-Wallis carried out for all the physico-chemical parameters shows no significant difference, neither between the stations, nor between the different sampling points of the stations. However, the seasonal variations of certain physico-chemical parameters notably colour, turbidity, pH and dissolved O2, were significantly higher in the dry season than in the rainy season (the test U of Mann-Whitney; p ≤ 0.05). The Mann-Whitney test showed no significant differences in the different forms of nitrogen from one season to the next.

Specific richness
During this study, 49 species of zooplankton were collected from the waters of the lake. Among these species, there are 38 Rotifers belonging to 10 families, 5 Cladocera belonging to 3 families and 6 Copepods all from the same family (Table 5). At the surface of the three sampling stations, 27, 22 and 19 species of rotifers were identified in S1, S2 and S3respectively. At the depths of the lake, 22 species of rotifers were recorded at station S1, 26 at S2 and 25 at S3. While in the middle sampling stations, S2 and S3 recorded 25 species of Rotifers. The family Brachionidaewas the most abundant in all the sampling stationsand was closely followed by Trichocercidae and Testidunellidae.

Spatial and seasonal variations in abundance
Rotifers have been the most abundant group of zooplankton. They contributed 79.16, 70.18 and 86.45% abundances respectively in S1, S2 and S3 at the surface. In depth of the water, their percentages in terms of abundance were 73.09% at S1, 74.48% at S2 and 77.17% at S3. While in the middle, S2 and S3, showed 70.63 and 78.39% rotifers abundances respectively.
As for crustaceans, at surface of S1, the abundances were 30 ind / L or 1.35% and 433 ind / L or 19.48% respectively for Cladocera and Copepods with all stages of development considered. At the depth of water, their abundances were 88 ind / L or 2.36% for Cladocera and 912 ind / L or 24.53% for Copepods. In S2, the abundance percentages of Cladocerae at the surface were 5.9, 3.52 and 1.73% respectively. Copepods from all stages of development contributed 23.88% on the surface, 25.83% in the middle and 23.78% in depth of the water. In S3, as in the first two stations, Cladocera were the least abundant. Their proportions were 0.90% on the surface, 1.12% in the middle and 4.15% in the depth. The Copepods contributed 12.63%, 21.60% and 24.93% from the surface to the depth.
Among the Rotifers, the family Brachionidae, Testitudinellidae and Trichocercidae showed the highest abundances. Among Cladocerae, the family Moinidae distinguished themselves from the others while Copepods were only represented by the family Cyclopidae.
Seasonally, rotifers and copepods were more abundant in the dry season than in the rainy season, unlike cladocerans, which had considerable abundance in the rainy season. The Man-Whitney U test indicates a significant difference between the two seasons in the abundance of zooplanktonic species (p = 0.29).

Population structure
The diversity index of Shannon and Weaver (H ') per sampling station varied between 4.13 (S2 dep) and 4.51 (S1 surf), as for the Pielou equitability index (E), it varied between 0.75 (S1 surf) and 0.94 (S3 surf) in the three stations ( Figure 5A The values of the Sorensen similarity index (table 6) between the stations S1 and S2, S1 and S3 and the stations S2 and S3, were very high, indicating that there is a very great resemblance between the three stations The redundancy analysis ( Figure 6) highlights the physicochemical parameters influencing the abundance of zooplanktonic species. Axis I isolates in its positive part the species Keratella tropica, Keratella

3-1-1-Physico-chemistry of the waters of Lake Léré
The temperature values were higher on the surface of all the stations; this could be explained by the sunshine on water surface. It was slightly higher in 2016 during the months of February, March and April, compared to 2017. The rise in temperature is more significant if we consider the work of Lévêque (1971) on Lac Léré, as well as those of Palou (2005) who underlines the fact that, the climate of the Mayo-Kebbi basin experiences very strong variations in temperatures with maxima which exceed 40 ° C and minima which are above 25 ° C. The absence of a significant difference in the water temperature between the different sampling levels as well as between the seasons suggests the absence of thermal stratification within the lake.
The dissolved oxygen contents are higher at the surface than at depth, this can be explained by the high light intensity, thus causing a fairly significant photosynthetic activity compared to the depths of the lake which are not necessarily anoxic. Conversely, the CO2 content is higher at the depth of the stations.
The average pH of 7.29 obtained corresponds to a neutral pH. This value is in the range of those of natural waters which must not be less than 6 (Ramade, 2005). The value recorded in 1970 was 8 at all levels of the sampling stations (Lévêque, 1971). There is therefore a slight acidification of the waters of Lake Léré.
In the rainy season, color and turbidity had strong values; which would be due to the phenomenon of erosion of the watershed which leads to an excessive supply of dissolved matter by runoff as highlighted by Passinring (2016). In addition, the highest values were recorded at depth from all stations; this would reflect a concentration of deep water in suspended particles.
The value of electrical conductivity recorded in 1970 (89 µS.cm -1 ) is low compared to the average recorded during this study period (95.74 ± 13.77), this could be as a result of erosion of the watershed from 1970 to the present days (statistical test). The values of these parameters are lower than those found in eutrophic lakes such as the Yaoundé Municipal Lake (Zébazé Togouet, 2008), Lake Nkolbisson (Ndjama et al., 2017). Within these lakes, the minimum values recorded were 152 µS.cm -1 and 90 µS.cm -1 respectively and maximum values were 437 µS.cm -1 and 260 µS.cm -1 . However, they are higher than those recorded in oligotrophic lakes such as Lake Ossa (Nzieleu et al., 2012) where the average recorded was 27 µS.cm -1 .
The high content of orthophosphates in the rainy season is could be due to runoff from the watershed during the rainy season. Ryding and Rast (1994), stipulates that a lentic ecosystem is engaged in an accelerated eutrophication process when the PO4 3contents of its waters are greater than 100 μg. L -1 , for this purpose, Lake Léré can therefore, be called a eutrophe. The nitrates contents were higher during the dry season, at low water contain. This is due to evapotranspiration in the dry season, resulting in a high concentration of nitrates compared to the rainy season. These nitrate contents are similar to those obtained by Mama et al. (2011) in Lake Nokoué (Benin). The NH 4+ contents (0 to 0.36 mg / L) were low, they are far lower than those recorded in the Municipal Lake of Yaoundé, a lake qualified as eutrophe (Zébazé Togouet, 2008 and 2011).
Ultimately, the result of the physicochemistry, shows a homogeneity of the waters of Lake Léré and a slight evolution of these waters, mainly due to the erosion of the watershed. However, apart from the temperature, the other physico-chemical parameters mark the seasonality of the lake waters. These waters are less oxygenated, less turbid in the dry season, and rich in orthophosphates in the rainy season. Taking into account the Lewis classification (1980), Lac Léré can be classified as a mesotrophic lake.

Lake Léré Zooplankton
The species identified during this study period consisted of 38 Rotifers, 5 Cladocera and 6 Copepods, whereas in 1970, only 22 species of rotifers were recorded, with practically the same number of Cladocera and Copepods. However, the absence few species during this study period that werereported in 1970, particularly at the herbarium, could be explained by the disappearance of their different micro-habitats. Indeed, Passinring (2006) highlighted that, the elimination or reduction of possibility of plant fixation could be due to exploitation of quarries on the watershed of the lake. This fact was noted by Zébazé Togouet (2008) in the Yaoundé Municipallake where the decline in specific diversity was due to the disappearance of the herbarium as a result of the disappearance of a large number of microhabitats.
However, the works of Pourriot (1971) and Gras and Saint-Jean (1971) was only carried out during the month of February with a single sampling campaign, whilesamplingthis work was done for 13 months taking into account all seasons of the year and a number of sampling stations were fairly represented in the lake. This implies that a number of species could not be identified in 1970 due to the short sampling period. The sampling effort, as well as the duration of the study, makes it possible to consider these results to be closer to the reality (Zébazé Togouet et al., 2005). According to Balvay, 2009 long-term studies with a certain sampling frequency are therefore likely to improve knowledge of the composition of planktonic biocenosis and its seasonal and long-term variations. However, some species identified by Pourriot, Gras and Saint-Jean (1971) that were absent during our study period should not be considered extinct from the lake. However, some species may be temporarily absent because they are not abundant, or not sought in their preferred biotope (Balvay, 2009).
The specific richness obtained is higher than those obtained by Adandedjan et al. (2017) in Lake Nokoué (Benin) and Fofana et al. (2019) in Lake Kaby (Côted'ivoire) but it is lower than those obtained by Oueda and Guenda (2011) in the Bagré dam lake (Burkina-Faso) and Tchapgnouo et al. (2012) in the Ossa lake complex (Cameroon).
In comparison with the work of Pourriot (1971), Gras and Saint-Jean (1971), the results obtained during this study reveal an increase in the specific zooplankton richness of Lac Léré. Physico-chemical parameters, in particular orthophosphates, conductivity, nitrates, indicate a mesotrophic state of the waters of Lake Léré.
With regard to rotifers, the Brachionidae family is the most represented with 11 species, followed by Trichocercidae and Testitudinellidae with 6 and 5 species respectively. This family succession is similar to that observed by Nzieleu Tchapgnouo (2016) in Lake Ossa, Zébazé Togouet et al. (2005) and Zébazé Togouet (2011) in Yaoundé Municipal Lake which are eutrophic in nature. However, this two lakes host 14 and 17 species of Brachionidae respectivelywhich are greater compared toto the species identified in Lake Léré. Although the richness of a lake in Brachionidae had already been noted as a characteristic of eutrophic environments and of strong eutrophication of the environment (Lair et al., 1998), but that of Lake Léré cannot be conferred on the status of a eutrophic lake. Moreover, Lake Léré with its 8 species of the genus Brachionus is similar to Lake Oxbow (Kar Sulata and Kar Devashish, 2013), an oligotrophic lake, whose zooplankton fauna is dominated by Rotifers with 6 species of the genus Brachionus.
The specific richness of rotifers in Lac Léré is higher in the dry season than in the rainy season. The fairly large quantity of nutrientsis due to the high values of nitrates during this period, could explain the proliferation of Rotifers, which are mainly herbivores. This result concur with those of Okogwu (2009)  Among the crustaceans, Cladocères, Chydoridae and Macrothricidae, were the most represented in Lake Ossa followed by family of Bosminidae (Nzieleu Tchapgnouo, 2016). Consisted essentially of littoral and periphytic species, Chydoridae and Macrothricidae presented the same number of species during the two seasons at the stationS1, which is the shallowest point. At the two other stations S2 and S3, the number of species is generally higher in the rainy season than in the dry season. This could be explained by the fact that in the rainy season, these periphytic species would be carriedin runoffcurrentto pelagic environments. This demonstrates the multitude of micro-habitats that the herbarium offers to zooplanktonic species. In Lake Chad, the presence of pelagic species belonging to these families is believed to be due to the reduction in the water level of the lake (Rey and Saint-Jean, 1980). Likewise after the big rainy season, there is a modification of the lake ecosystems by the supply of nutrients, lowering the water temperature (favorable for the hatching of eggs) with a rise in the level of the lakes water which lead to the flooding of coastal areas rich in eggs (Dejen et al., 2004;Mergeay et al., 2006). The flood contributes positively to the growth of cladocera populations by bringing new nutrients and mixing the native nutrients present in the different strata of the lakes (Tchapgnouo et al., 2012) thus favoring the production of phytoplankton and, consequently, that of zooplankton. Like in the Yaoundé Municipal lake (Zébazé Togouet, 2008), the large predatory cladocerans were not observed during our study period as well as in 1970.
Among the Copepods, all identified species belong to the Cyclopidae family. They were not represented in the various stations throughout the study period. Gras and Saint-Jean (1971) underline the fact that the rarity of the adult forms of crustaceans is due to the strong predation exerted by the larvae of Chaoborus which are abundantly present in the bottom. During our study, the adult forms were identified mainly during the transition between the two seasons in most of the sampling stations. This can be explained by the abundance of nutrients in this period. Indeed, their developmental cycle depends on the presence of food in the environment (Drira et al., 2007), as well as on the temperature (Moison, 2009). According to Zébazé Togouet (2000), the copepod developmental cycle lasts less than a week in tropical regions where the sunshine is higher compared to the temperate zone.
The specific richness in Lake Léré during the study period confirms the assertion of Dumont (1994) concerning the increase in the number of studies in tropical waters which will undoubtedly result in an increase in the number of species described. Zébazé Togouet et al. (2005) also underlines that the paucispecificity of Rotifers is due to the scarcity of hydrobiological works in tropical waters. The species encountered are mostly cosmopolitan. This same observation was noted by Pourriot (1970).
In terms of abundance, rotifers were the most abundant zooplankton group during the entire study period, at all sampling stations. They represented 76.55; 72.27 and 79.20% of the abundances, respectively at stations S1, S2 and S3. This is a characteristic of tropical lakes (Bidwell & Clarke, 1977;Okogwu, 2009;Zébazé Togouet, 2011). The numerical dominance of Rotifers over other zooplankton groups in tropical environments can also be explained by their opportunistic nature allowing them to better resist variations in environmental conditions, as well as their greater competitiveness in these environments and their food plasticity vis-à-vis the available resources, but also because of their small size which makes them less vulnerable to the pressure of predation (Dumont 1977, Matsumura-tundisi et al., 1990. The abundance of Rotifers being greater in the dry season compared to the rainy season with 78.1 and 21.89% respectively could be explained by the increase in phytoplankton biomass with increase concentration of nitrates in the dry season.These nutrients (phosphates and nitrates) lead to an increase in the phytoplanktonic biomass of the environment (Akodogbo et al., 2014). Moreover, the growth of rotifers is favored by an increase in nutrient levels (Jalal et al., 2005). Chiali and Cherifi (2019) also stated that, the difference in the abundance of zooplankton is linked to the conditions of the environment favorable to its development such as nutrients and phytoplankton is considered as an important source of food. As the abundance of Cladocera Crustaceans is preponderant in the rainy season,their predation on Rotifers becomes considerable, hence the low abundance of Rotifers during this same period. Cladocerans represent the least abundant zooplankton group in Lake Léré during the study period with an abundance of 1.90; 3.18 and 2.07% respectively at the stations S1, S2 and S3. This low abundance of Cladocerans is a characteristic of most tropical lakes (Dumont, 1994;Fussmann, 1996;Moss, 1998). Cladocerans in Lake Léré was mainly represented by Moina brachiata species. This result is comparable to that of Pont (1977) which shows that the abundances of Moina brachiata can reach 98% in the rice fields of Camargue. This species is characterized by the rapidity of its development. Its optimum growth temperature varying between 24 and 31 ° C (Rottmann, 1992) which approaches the average temperature recorded in Léré Lake. Likewise, it adapts to variations in oxygen concentration (Rottmann, 1992).
The abundance of Copepods is mainly due to the larval stages. The low proportion of adult copepods could be explained by several factors, namely predator pressures, currents (Zébazé Togouet, 2008), the ecological logic of food webs (Odum, 1959;Frontier 1977) or the lack of suitable habitatin the biotope for their development (Shiel et al., 1998;Lougheed & Chow-Fraser, 1998).
Analysis of the seasonal dynamics of zooplankton in Léré Lake makes it possible to distinguish three periods. At the end of the rainy seasons, during the low water period, there is a drastic decline in the population of cladocera in favor of rotifers. These rotifers, which are more adapted to environmental changes and organic pollution, would be less affected by certain phenomena such as high temperatures and deterioration in water quality, unlike other organisms (Onana  et al., 2014). During the advanced low water period, the low volume of water and the low nutrient in the environment lead to a relative decrease in abundance and specific richness, which stabilizes until the start of the floods. During the rainy season, the abundance of cladocerans is gradually increased. In fact, the significant supply of nutrients leads to an increase in the specific richness and abundance of zooplankton. The rainy seasons contribute positively to the growth of cladoceran populations by bringing new nutrients and by mixing the old nutrients present in the different strata of the lakes, thus favoring a strong production of planktonic organisms and the drainage of new species of cladocerans bordering other aquatic environments (Tchapgnouo et al. 2012).
This dynamics of zooplankton populations is governed mainly by the volume of water but also certain physico-chemical parameters such as nitrates, ammonium ions, electrical conductivity, turbidity, water temperature and pH. Maintaining these parameters at more or less stable values which correspond to the requirements of the zooplankton communities is necessary for maintaining the diversity and stability of the stands.

Spatial and seasonal structure of zooplankton
The values of the Shannon and Weaver diversity and Pielou fairness indices suggest that the different stations contain a more or less equal number of species distributed in an equitable manner. This explains the high values of Sorensen's correlation coefficient which indicates a great similarity between these three stations. Similar observations were made by Saint-Jean (1983), Okogvu and Ugwumba (2006). These observations are however contradictory to those of Adandedjan, (2017) and Fofana (2019) who point out an instability in the structure of the zooplanktonic community respectively in Lake Nokoué and Lake Kaby, due to the variability of conditions of the environment along the water.
The absence of significant differences in the physico-chemical parameters as well as the specific richness between the different sampling levels justifies the homogeneous distribution of zooplanktonic organisms. It also shows the homogeneity of all the waters of Léré lake.

IV. CONCLUSION
Physico-chemical analyzes of the waters of Lake Léré have shown values indicating on one hand that Lake Léré is a mesotrophic lake and on the other hand the absence of stratification within the lake, hence the homogeneous distribution of species. The species newly cited in this study could be due to the sampling effort or to a significant enrichment in nutrients of Lake Léré since 1970 to the present day. Indeed, the population around the lake is estimated at 84,652 inhabitants (Florence & Solkem, 2016) carries out a less intense traditional agro-silvo-pastoral activity, this implies a less anthropogenic impact, not leading to a rapid enrichment of the lake. The physico-chemical parameters govern the zooplankton dynamics. The most resistant rotifers in the group were the most abundant, especially in the dry season than in the rainy season. Cladocera have a dynamic linked to water movements. Thus their abundance is improved in the rainy season. The higher abundance of Copepods in the dry season is mainly due to the larval stages.