Water quality in Lake Victoria has declined greatly in the past few decades, owing chiefly to eutrophication arising from increased inflow of nutrients into the lake. Scientists advance two main hypothesis for these extensive changes. First, the introduction of Nile perch as an exotic species some 30 years ago has altered the food web structure; second, nutrient inputs from adjoining catchments are causing eutrophication.
Nutrient inputs have increased two to three-fold since the turn of the century, mostly since 1950. Concentrations of phosphorus have risen markedly in the deeper lake waters, and nitrogen around the edges. Stimulated by these and other nutrients, there has been a five-fold increase in algal growth since 1960, with a shift in composition towards domination by blue-green algae. Increased primary production is causing de-oxygenation of the water, there is an associated increase in sickness for humans and animals drawing water from the lake, clogging of water intake filters, and increased chemical treatment costs for urban centers. Aside from the near-total loss of the deepwater species, the de-oxygenation of the lake's bottom waters now poses a constant threat, even to fish living in shallower portions of the lake, as periodic upwelling of hypoxic water causes massive fish kills. The increased nutrient loads have also probably exacerbated the water hyacinth infestations.
Some areas of the rivers feeding the lake and the shoreline are particularly polluted by municipal and industrial discharges. Millions of liters of untreated sewage and industrial waste flow into the Lake Victoria every day from Kisumu, Kenya's third largest city, and from Mwanza in Tanzania. Watershed degradation and agricultural runoff contribute chemicals, nutrients, and sediment. And from Rwanda came the grisly addition of some 40,000 human bodies, war casualties that floated down the Kagera River in May 1994. Nor is the perch the lake's only alien species problem.
Some information has been collected by local and national authorities on the scale and location of polluting industries, and there are a number of basic industries that are common to most of the major urban areas, for example, breweries, tanning, fish processing, agro-processing (sugar and coffee) and abattoirs. Some of these have implemented pollution management measures but in general the level of industrial pollution control is low. Small-scale gold mining is increasing, in Tanzania in particular, and this is leading to some contamination of the local waterways by mercury, which is used to amalgamate and recover the gold. Some traces of other heavy metals, such as chromium and lead, are also found in the lake, although the problem has not yet reached major proportions.
Finally, the lake basin is used as a source of food, energy, drinking and irrigation water, shelter, transport, and as a repository for human, agricultural and industrial waste. With the populations of the riparian communities growing at rates among the highest in the world, the multiple activities in the lake basin have increasingly come into conflict. This has contributed to rendering the lake environmentally unstable. The lake ecosystem has undergone substantial, and to some observers alarming changes, which have accelerated over the last three decades.
In addition, massive blooms of algae have developed, and come increasingly to be dominated by the potentially toxic blue-green variety. The distance at which a white disc is visible from the surface, (a transparency index measuring alga abundance), has declined from 5 meters in the early 1930s to one meter or less for most of the year in the early 1990s. Water-born diseases have increased in frequency. Water hyacinth, absent as late as 1989, has begun to choke important waterways and landings, especially in Uganda. Over fishing and oxygen depletion at lower depth of the lake threaten the artisanal fisheries and biodiversity (over 200 indigenous species are said to be facing possible extinction.)
Thus, although the lake and its fishery show the evidence of the dramatic changes in the lake basin over the past century, changes within the lake are not the only source of the problems that have arisen in the surrounding basins through intensified human activity. For many years, the severity of the problems went largely unrecognised amid the major social upheavals taking place in East Africa.
Fisheries on Lake Victoria Next Section
Page Author: Dr Rick T Leah, Univ of L'pool - Contents last reviewed15/12/2005