Nsengiyumva, EmileSous la direction de : Directeur : Dr. Pierre NtakiyirutaCo-directeur : Dr. Norbert Manirakiza2025-11-042025-11-042025-03https://repository.ub.edu.bi/handle/123456789/2107Mémoire présenté et défendu publiquement en vue de l’obtention du diplôme de master en sciences chimiques. Spécialité : Contrôle et Analyses ChimiquesDespite its rich biodiversity, Lake Tanganyika is continually undergoing degradation, one of the main causes of which is pollution. This pollution poses a perilous threat to certain species. These include physical pollution due to watershed erosion, chemical pollution due to wastewater, wastewater and chemicals from industries. This study assesses the influence of the pollutant load of the Ruzibazi River, one of its tributaries, on the abundance of zooplankton, which are considered as one of the indicators of pollution. This study was carried out in two seasons (dry season and rainy season) due to two campaigns for each season. Samples were taken upstream and at the mouth of this river. Some parameters such as pH, electrical conductivity, dissolved oxygen, TDS, temperature were measured in the field. Other parameters such as phosphates, different forms of nitrogen, organic matter, metals, abundance and diversity of zooplankton were analyzed in the laboratory using standard methods. In order to calculate the pollutant load, the flow rate of this River was also measured. The results of this study show that the concentration of total iron exceeds Burundian standard, reaching 4.183 mg/l against a limit of 1 mg/l, while manganese and nickel remain compliant. Turbidity increases considerably during the rainy season (102.925 ± 75.82 NTU against a threshold of 30 NTU), and fecal contamination remains high throughout the year, with levels of total coliforms, E. coli and Salmonella exceeding Burundian standards. Nutrient and heavy metal fluxes increase during the rainy season due to runoff, with significant increases in manganese (80.25 kg/d), iron (3926.1 kg/d), and nickel (27.06 kg/d). Discharges of BOD5 (5132.2 kg/d), suspended solids (3593 kg/d), oils and greases (641.05 kg/d), and TDS (15,853.7 kg/d) are also higher during this period. The zoopl ankton community is dominated by copepods, rotifers, ostracods, and cladocerans. Maximum abundance is observed in June and August, with a decrease in December. Mesocyclops leuckarti and nauplii are present year-round, while rotifers, mainly Lecane luna and Scaridium sp, peak in August. The Shannon diversity index ranges between 1.66 and 2.46 bits/ind, indicating maximum diversity in early December. The Piélou evenness index ranges between 0.64 and 0.87, reflecting an uneven distribution of species across seasons. These fluctuations demonstrate the influence of seasonal variations on zooplankton structure and dynamicsfr