Status and changes of water quality in typical near-city zones of three East African Great Lakes in Tanzania

Tanzania is the only country bordering all three transboundary East African Great Lakes, i.e., Lake Victoria, Lake Tanganyika, and Lake Nyasa (Lake Malawi). This study investigated the spatiotemporal variability of basic physicochemical parameters of nearshore surface waters in Mwanza Gulf (Lake Victoria), Kigoma Bay (Lake Tanganyika), and Wissmann Bay (Lake Nyasa). Water quality was evaluated using the water quality index (WQI) method. Results showed that N and P nutrient pollution was relatively severe in central and southern parts of Mwanza Gulf owing to external agricultural emissions and internal release associated with physically disturbed sediment resuspension. External inputs from inflowing surface runoffs from the city of Mwanza typically enhanced N loading in northern parts of the gulf during the rainy season. Poor water quality was found in central and southern parts of Mwanza Gulf, especially in the rainy season. Algal blooms and NH4+-N (total P and total N) were the main factors driving water quality degradation in the rainy (dry) season. Kigoma Bay and Wissmann Bay both had good water quality, except in river mouth areas in Lake Nyasa during the rainy season. The degradation in water quality was caused primarily by increased land-based nutrient and turbidity inputs. To respond to challenges associated with climate change and local socioeconomic development, long-term monitoring of the lacustrine environment and systematic limnological studies will be required, not only in the three bays but also more widely throughout the three lakes and their basins.