The Environment of Lake Victoria (East Africa): Current Status and Historical Changes

Abstract

Beginning in the mid-1980s Lake Victoria experienced severe eutrophication and it was suggested that deteriorating water quality might lead to a collapse of its fisheries. A series of lake-wide surveys carried out 1999-2001 and 2005-2009 revealed that the temperature of the lake had risen by > 1 °C since 1927, with more rapid warming of the deeper waters reducing the thermal gradient in the water column and thus weakening stratification and the extent and severity of deoxygenation. The chlorophyll a concentrations in open water decreased since the 1980s, while Secchi disc visibility increased, indicating a reduced severity of algal blooms. Chlorophyll a was higher and Secchi disc visibility lower in inshore waters but there has been no deterioration in these areas since the 1980s. The conductivity remained unchanged, although it was about 50% greater in the semi-enclosed Nyanza Gulf than in the open lake. The water quality of the lake has therefore improved considerably despite the fact than concentrations of plant nutrients have not decreased and the reasons why this may be the case are discussed. IntroductionLake Victoria is the world's second largest freshwater lake (area = 68,800 km 2 , mean depth = 35 m) and it supports one of the world's largest inland fisheries, yielding almost one million tons per annum (LVFO, 2009a). It has attracted much interest in recent years because of the extraordinary ecological changes that occurred in it over the last three decades, comparable in their magnitude to those in newly created man-made lakes. These changes were triggered in the mid-1980s by an introduced predatory fish, the Nile perch Lates niloticus (L.), and its subsequent destruction of the endemic haplochromine cichlids. These fishes, which formed a species flock with some 500 described forms (WITTE et al., 2007), accounted for over 80% of the fish biomass in the lake (KUDHONGANIA and CORDONE, 1974) but they were small fish thought to be of little commercial value. Nile perch were therefore introduced with the specific intention of converting haplochromines into a more * Corresponding author 210 L. SITOKI et al.

Nile perch distribution in south-east Lake Victoria is more strongly driven by abiotic factors, than by prey densities

“…The increasing eutrophication of Lake Victoria caused degradation of water quality, resulting in an increase in phytoplankton biomass. This led to changes in visibility and oxygen content of the water (Hecky, 1993(Hecky, , 1994, while global climate change resulted in an increase of water temperatures in Lake Victoria with more than 1°s ince the 1920s (Sitoki et al, 2010). In addition, it is hypothesized that prey density and availability may have profound effects on the size structure and distribution patterns of Nile perch (Goudswaard et al, 2011;Downing et al, 2013).…”

Microplastics in Inland African Waters: Presence, Sources, and Fate

“…Lake Victoria is the world's second largest freshwater lake by area (the largest being Lake Superior in North America) and has been described as eutrophic and polluted due to human influences within the catchment area [44]. The area surrounding the lake is among the most densely populated in the world, and this population growth is set to continue -by the year 2020, an estimated 53 million people will inhabit the lake basin [45].…”

Chromium Distribution and Spatial Variations in the Finer Sediment Grain Size Fraction and Unfractioned Surficial Sediments on Nyanza Gulf, of Lake Victoria (East Africa)

“…Water hyacinth [40] and eutrophic conditions [41][42][43][44][45] are also a common phenomenon in the lake, linked to ecological changes in the lake. Fish inhabiting shallow waters are also at risk from periodic upwelling of hypoxic waters [40].…”