Environmental Changes in the Tanzanian Part of Lake Victoria

Abstract

Lake Victoria is known for its explosive speciation and recent time hybrid-ization, which is highly mediated by deterioration of water quality. This chapter sum-marizes the knowledge on change of water quality and environment of southern part of Lake Victoria, Tanzania. It analyses rainfall, air temperature and water quality data spanning 30 years (1985 to 2015). It also investigates changes in physical-chemical data sampled during and after the Lake Victoria Environmental Management Project I (LVEMP I). The chapter reviews some of the significant water quality changes that have occurred for the past 50 years. The results indicate no significant changes in annual rainfall variability. Nevertheless, trends of air temperature showed no clear pat-terns for Mwanza and Musoma, but trends of minimum and maximum air temperature in Bukoba increased significantly at annual rates of 0.19 °C and 0.14 °C, respectively. Water level in Lake Victoria has also declined significantly at an annual rate of about 5.5 cm from 1965 to 2004. These findings suggest that lake levels are determined by evapotranspiration rather than rainfall. It was also found that anthropogenic stressors are more important in explaining nutrients loading while thermal stratification explains hypoxia and reduction in water mixing. It is concluded that the current blooms of harmful algae and excess biomass in Lake Victoria will continue unabated unless nutrient loading, anoxia and high rates of denitrification are curbed. Appropriate mea-sures to improve land use management should therefore be taken, while deliberate 38 dumping of industrial, municipal and agricultural wastes into the lake should be controlled. 3.1 Introduction Lake Victoria is the largest lake by area in Africa and second largest freshwater lake in the world, second only to Lake Superior in North America (Hecky et al. 1994; Scheren et al. 2000). The lake is best known in the world for its faunal diversity, explosive speciation and recent time hybridization. The lake is extremely diverse and provides various ecosystem goods and services to over 30 million people who are estimated to either directly or indirectly depend on the lake for their sustenance. Lake Victoria supplies water for domestic and industrial consumption, agriculture and hydroelectricity production. It provides fish, which serves as a source of animal protein and income; an aesthetic value, which attracts tourists and other non-eco-nomic benefits; and a scientific value for academic and research purposes. However, these benefits are jeopardised by the current destructive human activities, such as unsustainable agriculture, urban development including unplanned and illegal set-tlements, overfishing, industrial development, and introduction of alien and invasive species (Hecky 1993; Msomphora 2005). Furthermore, the lake is a recipient of agrochemicals, domestic and industrial wastes, and mining effluents that are caus-ing havoc to the lake's environment, thereby jeopardizing the provision of ecosys-tem goods and services to the riparian communities. It has been polluted such that it reached a 'tipping point' and went into a permanent eutrophication phase since early 1980s. Some of the changes that caught attention of the scientific fraternity include pro-longed anoxia in deep waters (Ochumba and Kibaara 1989; Mugidde 1993; Hecky et al. 1994), which resulted into periodic massive fish kills, during the cool dry season's mixing events (Akiyama et al. 1977; Hecky et al. 1994). Periodic anoxia condition in deep waters of the lake is however not an uncommon phenomenon. It was reported by several studies even before eutrophication of the lake could be ascertained (Worthington 1930; Talling 1966; Akiyama et al. 1977). Since then, the water quality of Lake Victoria has thus changed towards the negative extreme of the spectrum. The change in water quality has been associated with unsustainable human activities such as increasing agricultural activities, clearing of forests, pasto-ralization, and domestic and industrial effluents, and mining wastes (Hecky 1993; Msomphora 2005). These activities are cause for excessive nutrients input into, and the eutrophication of, Lake Victoria. The lake is also facing the challenge of increasing algal blooms (Mugidde 1993), which have contributed to the 'new nuisance' and changing the odour of the lake water while rendering some parts of the lake unusable by affecting its aesthetic value. The occurrence of toxic algal blooms such as microcystin (Mbonde et al. 2004; Sekadende et al. 2005), and the introduction of non-native fish species such I.A. Kimirei et al.