Dynamique phytoplanctonique et niveau trophique de quelques étangs de la ville de Bertoua (Est-Cameroun)

Abstract

The physicochemical and phytoplankton variables studied in five ponds (Mokolo,Mopa, Côte d'azur, Ngaikada and Kpokolota) in the city of Bertoua, with two sampling stations (surface and depth) per pond and data collected monthly from March 2016 to April 2017 (14months) allowed assessment of the spatial and seasonal dynamics of phytoplankton in relation to anthropogenic disturbances. Physicochemicals analysis were carried out according to standard methods, while the phytoplankton organisms were collected by direct sampling and analysed by the Utermöhl method. Bathymetry shows that each pond has its own morphological and topographical soil characteristics. The abiotic variables of the ponds show that they are highly anthropised with relatively high temperature (> 23°C), low water transparency (< 70 cm) despite the shallow depth of the ponds (< 170 cm), oxygenation relatively on the average (> 50%) with hypoxia (<35%) recorded in the rainy season, high mineralization of water, high levels 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 strong organic pollution of the water with nitrogen as a limiting factor of eutrophication. These characteristics make it possible to classify the water bodies in the category of hypereutrophic ponds. The measures to be implemented for the management of eutrophication should focus on controlling the flow of nitrogen compounds into the ponds.The phytoplanktonic organisms identified during this study were 336 species belonging to 139 Genera, 86 Families, 46 Orders and 8 Phytoplanktonic classes (Chlorophyceae,Chrysophyceae, Cryptophyceae, Cyanophyceae, Diatoms, Dinophyceae, Euglenophyceae and Xanthophyceae). Côte d'azur pond was the most diversified with 159 species identified,dominated by the taxa Azpeitia Africana (Diatoms), Eremosphaera gigas (Chlorophyceae) and Trachelomonas acanthophora (Euglenophyceae). Mopa Pond (147 species) is most represented by the Diatoms Pinnularia gibba and Stauroneis phoenicenteron and the Chlorophyceae Pleurotaenium trabecula. Ngaikada Pond (143 species) is mainly represented by the Diatoms Azpeitia Africana and Pinnularia gibba. Mokolo Pond (138 species) is dominated by the Chlorophyceae Volvox tertius and Chlorella vulgaris while Kpokolota Pond (136 species) is more represented by the taxa Phacus orbicularis (Euglenophyceae) and Azpeitia Africana (Diatoms).The specific richness of the phytoplankton classes was dominated by Diatoms (38%) and Chlorophyceae (31%), followed by Euglenophyceae (14%), Cyanophyceae (11%), Dinophyceae (4%), Chrysophyceae, Cryptophyceae and Xanthophyceae (1%). The Diatoms Class was most represented in Mopa and Ngaikada ponds with 52 % and 43 % of the total density, respectively. Euglenophyceae represented 32 % in Côte d'azur and Kpokolota ponds while Chlorophyceae (46 %) were most abundant in Mokolo pond. The specific richness and densities of surface are higher in all ponds and at all seasons, confirming the hypothesis that light intensity, an indispensable element for photosynthesis, which can vary with depth, is the main regulator of phytoplankton dynamics in these ponds. The high phytoplankton biomasses reflect, high photosynthetic activity in ponds and consequently a high primary productivity due to the absorption of nutrients which are the factors controlling phytoplankton growth. The abiotic typology classifies ponds into three groups according to the major origin of the pollutants while the biotic typology classifies them into three groups according to the tolerance of the phytoplankton species to nitrogen levels. These phytoplanktonic organisms are quite diversified, but not in equilibrium because of the dominance of a small group of saprophils and polluotolerant species constituted essentially of taxa Azpeitia africana and Stauroneis Phoenicenteron (Diatoms), Eresmophaera gigas Chlorophyceae), Microcystis aeruginosa and Aphanocapsa incerta (Cyanophyceae), characteristics of the hypereutrophication of the ponds studied. The high presence of rare taxa (80 %) demonstrates a permanent renewal of phytoplankton species that are very sensitive indicators of environmental aggression. We note an ageing of aquatic ecosystems with phytoplankton stands at the end of ecological succession and similarities on the average demonstrating intrinsic characteristics specific to each pond, depending on the nature and type of pollutant present. The analysis carried out during this study made it possible to decide on the trophic level of the ponds and to determine the role of environmental factors on the dynamics of phytoplankton populations. This approach is a necessary prerequisite for the restoration of ponds, the resumption of aquaculture activities and the proactive management of phytoplankton blooms, responsible for the eutrophication of ponds.