Biodiversité et structure de la communauté zooplanctonique (Rotifères, Cladocères, Copépodes) du lac Léré au Tchad.

Abstract

To preserve the quality of surface water, it is essential to ensure efficient planning and management of aquatic ecosystems, these requires an assessment of the water state through physicochemical and biological analyzes. Zooplanktons are among the aquatic organisms that are use in the monitoring of hydrosystems since there play a multiple roles and occupy a prominent place in the aquatic feeding web. The present study is carried out in lake Léré in Chad, which is part of the Binder-Léré wildlife reserve, named Ramsar site since 2001, and which hosts protected species such as the manatee. This work aimed at determining the diversity and spatial structure of zooplankton community in relation to the physicochemical characteristics of the water in this lake. This study was conducted over a period of 15 months from February 2016 to April 2017, with sampling done on monthly basis. Three stations each consisting of different depth levels (surface, 2.5 m and 4.5 m deep) were chosen. Water samples (for physicochemical analyzes and filtering of zooplankton) were collected using a 20L bucket at the surface and a 3L VanDorn bottle at depth. In addition, zooplankton samples were collected from the herbarium and from the water colon of the lake. The physico-chemical analyzes were carried out both in the field and in the laboratory following standard methods. For the sampling of zooplankton, 20 L of water was filtered through plankton net having a mesh size of 64 µm. The samples were preserved using 5% formalin which facilitate the identification and counting of the zooplankton species, under a WILD M5 binocular magnifying glass with the help of appropriated identification documents and keys. From a physico-chemical point of view, the water of Lake Léré have a pH close to neutrality (7.29 ± 0.53). It has average temperatures of 28.22 ± 2.50oC. There was no significant difference between sampling stations and sampling levels, this implies that there is no thermalstratification within the lake. The dissolved oxygen contents expressed in % saturation are higher at the surface (with average values of 73.4 ± 20.53 for S1 ; 65.8 ± 19.58 for S2 and 64.53 ± 19.44 for S3) where as at depth, these contents are slightly low with mean values of (65.06 ± 18.54 ; 61.93 ± 17.81 and 60.86 ± 18.48 respectively for station S1, S2 and S3). This thus permit to delimited a trophogenic zone of the surface 4.5 m deep and a tropholytic zone 4.5 m deep to the bottom of the lake. The mean concentrations of orthophosphate were 18.27 ± 14.86 mg / L during the study period. This is as a result of an excessive input by runoff. These concentrations increased in the rainy season (26.73 ± 19.64) than the dry season (14.04 ± 10.69). Nitrates, showed an average mean of 1.73 ± 1.38 mg / L, with the highest concentration in the dry season (1.95 ± 1.53). The absence of significant differences between the physicochemical parameters at the different sampling levels leads to the conclusion of the homogeneity of the lake’s waters as well as the absence of thermal stratification. In addition, the values of the physico-chemical parameters make it possible to classify Lake Léré as being a mesotrophic lake following Lewis classification. Concerning biologically analyzes, a total of 49 zooplankton species were identified in the different sampling stations. These include 38 species of Rotifers belonging to 10 families, 5 species of Cladocerans distributed into 3 families and 6 species of Copepods belonging to a single family. The Brachionidae family was the most represented among Rotifers with 11 species. The total rotifer densities was 4476, 6126 and 7125 ind./L respectively at sampling stations S1, S2 and S3. As for microcrustaceans, Cladocerans and Copepods showed respectively abundances of 118 and 1345 ind./L at station S1, 270 and 2080 ind./L at station S2 and 187 and 1683 ind./L at station S3. Spatial variations indicate that zooplankton abundances increase from the surface to the depth of the water, especially for Rorifera and Copepods. Seasonal variations show a high abundance in the dry season with 16,996 ind / L (corresponding to 72.60%) against 6414 ind / L (representing 27.40%) in the rainy season. Rotifers and Copepods were more abundant in the dry season than in the rainy season unlike the Cladocerans. The occurence frequency of the different taxa made it possible to distinguish the Rotifera species such as Asplanchna sieboldi, Brachionus urceolaris, Brachionus angularis, Keratella tropica, Keratella germinata, Lecane bulla, Polyarthra vulgaris, Trichocerca pussila, Trichocerca chattoni, Rotolaria sp. Filinia terminalis as characteristic species of Lake Léré, as well as the Cladoceran Moina brachiata. The presence of these species considered as indicators of the evolution aquatic milieu from oligotrophic to eutrophic level confirms the mesotrophic state of Lake Léré. For a long-term protection of Lake Léré, regular monitoring of the water quality and raising awareness among citizens of the importance of its preservation would be necessary.