Etude comparée des communautés zooplanctoniques et des niveaux trophiques de quelques étangs à Bertoua (Est-Cameroun)

Abstract

In order to propose a strategy for improving fish farming in the city of Bertoua (East Cameroon Region). The physicochemical and zooplanktonic variables studied in five ponds (Mokolo, Kpokolota, Ngaïkada, Côte d'azur, and Mopa), with two sampling stations (surface and depth) were chosen for each pond, following a monthly step from March 2016 to April (14 months) allowed to make an analysis of environmental pollution on the zooplanktonic communities and trophic levels of ponds in the ecological region of Bertoua. The physicochemical parameters of the water considered were analyzed following standard methods The specific richness and the abundance of the collected zooplankton were evaluated following the classical methods. Bathymetry evaluation shows that each pond has its own morphological and topographical soil characteristics. Abiotic variables of the ponds indicate that they are highly anthropized with relatively high temperatures (> 23°C), low water transparency (< 70 cm), shallow depth of the ponds (< 170 cm), average oxygenation (> 50%) with hypoxia (< 35%) recorded in the rainy season, high mineralization of the waters, high concentration of nutrients, organic matter and photosynthetic pigments (> 30 µg/L). We note a homogeneous physicochemical quality of the water column of each pond and a high organic pollution of the waters. These characteristics allow us to classify the ponds in the category of hypereutrophic. One hundred and eighteen (118) species of zooplankton were identified in the ponds, divided into 64 Rotifera, 45 Cladocera and 9 Copepoda. The families of Brachionidae, Lecanidae, Asplanchnidae are the dominant groups in Rotifers, Chydoridae represent Cladocerans and Cyclopidae are those of Copepods. The characteristic species of these water bodies are: Rotaria rotatoria, Plationus patulus, Platyias quadricornis, Brachionus quadridentatus, Asplanchna herricki, Asplanchna priodonta, Lecane bulla, Habrotrocha sp., Alona monocantha, Chydorus eurynotus, Moina micrura, Moinadaphnia macleayi, Moina sp., Diaphanosoma brachyurum, Thermocyclops crassus. The abiotic typological approach classifies the ponds into three groups according to the major origin of the pollutants while the biotic approach classifies them into three groups according to the affinity between the abundances and the ponds in which they live. These zooplanktonic organisms are diversified, we note the dominance of a small group of species characteristic of the hypereutrophication of the studied ponds. The strong presence of rare taxa (80%) demonstrates a permanent renewal of zooplanktonic species which are very sensitive indicators of environmental aggression. We note an aging of the hydrosystems and relatively average taxonomic similarities demonstrating intrinsic characteristics specific to each pond, depending on the nature and type of pollutant present. The analysis carried out during this study has allowed us to determine the trophic levels of the ponds' waters, to determine the role of environmental factors and to make a comparative analysis of the zooplanktonic community. This approach is a necessary prerequisite for the restoration and management of the ponds. Decreasing order of pollution of the surveyed ponds allows us to classify them as follows: Kpokolota, Ngaïkada, Côte d'azur, Mopa and Mokolo.