ALBERT

Description: Within the past five decades, it has been realized that people significantly influence the water quality, which, in turn, affects the fisheries. Since the 1960s, Lake Victoria and its basin have come under increasing and substantial pressure from a variety of human activities. These include deforestation, over-fishing, intense cultivation, animal husbandry and introduction of exotic fish species. Human activities have disrupted biogeochemical cycles with have consequences for the water quality and fisheries of Lake Victoria. Measurements and theoretical analyses have clearly connected observed changes in the watershed and water quality and conclusions have been drawn that aquatic biota including fisheries are strongly influenced by disturbances around and within Lake Victoria The Lake Victoria basin has clearly shown signs of population pressure and associated land use changes to and within the lake. Nutrient enrichment in the water column, sediment and in rainfall over Lake Victoria accelerated after the 1960s. Phosphorus concentrations have risen by a factor of 2 to 3. Atmospheric loads of nutrients, especially of nitrogen (N) and phosphorus (P) have increased due to biomass burning and deforestation in the catchment. These high nutrient concentrations support elevated algal primary production and algal biomass that have risen by factors of 2, and 6 to 8 respectively. Algal and invertebrates species composition have responded to changes in water quality. Algae are now dominated by the blue-greens with types that are potentially toxic. Elevated algal production probably supports the 4 to 5-fold rise in fish production in the lake since the 1950s. Increased fish production is good as more local people have turned to the lake for their livelihood and fish exports to international markets earn foreign exchange for the riparian states. But problems associated with nutrient enrichment (eutrophication) impacts of elevated algal biomasses and proliferation of the obnoxious water hyacinth (Eichhornia crassipes) threatens continued beneficial use of the lake resources. High algal biomass and increased sediment have contributed to a 4-fold reduction in the lake transparency since the 1960s. Reduced lake transparency is believed to have accelerated hybridization of some cichlids that are picky in choosing mates and use visual cues of bright male coloration to identify suitors of their own species. Deoxygenation of deep waters is another undesirable change that has precluded a stable demersal fishery. Seasonal bottom oxygen deficiency created by decomposition of algal biomass and aggravated by stronger thermal stability directly affects the distribution of organisms including invertebrates and fish. Low oxygen has led to loss of approximately 50% of aerated fish habitat since the 1960s, and also lowers potential fish production. Among the invertebrates, Caridina nilotica has become a keystone species as it is resilient to low oxygen conditions. Low oxygen conditions (anoxia) and algal blooms are often associated with fish kills in Lake Victoria.

Description: Published

Description: This chapter explores the impact of hypoxia on the fishery of Lake Victoria.

Description: Published

Description: Between 2000 and 2005 water quality and limnological studies were carried out in Lake Victoria in order to establish the eutrophication effects on ecosystem health. Comparison between littoral and pelagic areas of the lake showed marked spatial and temporal differences between and within the zones. Nitrate nitrogen (NO3-N) and phosphate phosphorus (PO4-P) concentrations ranged between 16.2-87.9 ~kg/l and 39.6-92 ~kg/l respectively and were both higher in the northeast. Silica (SiO2-Si) concentrations ranged between 0.525 and 0.902 mg/l and the values were higher in the northeast and southwest compared to mid-lake stations. Nyanza Gulf had lower PO4-P concentrations (16.2 to 21.1~kg/l) than the Mwanza and Napoleon Gulfs (54.8 to 68.7~kg/l) but registered higher SiO2-Si concentrations (4.5 to 5.2 mg/l) than the other two gulfs. NO3-N concentration in the gulfs ranged between 25 and 93 ~kg/l with Napoleon Gulf having higher values than the other two gulfs. Total phosphorus (TP) in the pelagic waters ranged between 0.078 and 0.10 mg/l and total nitrogen (TN) ranged between 0.53 and 0.83 mg/l. The TN:TP ratio (〈20) in the main lake indicated that phytoplankton growth in the lake may be nitrogen-deficient; a situation favoring dominance of nitrogen fixing Cyanobacteria. This low TN:TP ratio is probably associated with the increased phosphorus loading and selective nitrogen loss through denitrification aqs well as enhanced recycling of P associated with increased anoxic conditions in the deep pelagic waters. Comparison with Talling’s 1961 values, SiO2-Si concentrations in the lake have generally decreased by a factor of 3 and up to 8 at the Talling’s historical station of Bugaia (UP2). Chlorophyll a concentrations in the pelagic areas ranged between 3.6 and 11.7 ~kg/l and were generally higher in the littoral than to the pelagic areas. The phytoplankton community was dominated by Cyanobacteria (〉50%) especially the species Microcystis, Anabaena and Cylindrospermopsis in both the littoral and pelagic waters. Relatively high diatom biomass was recorded in the pelagic compared to the littoral areas, but Aulacoseira (Melosira), the formally dominant diatom species was rarely encountered. Compared to previous records, the invertebrate community composition has remained relatively stable despite drastic changes in water quality and fish stocks, but changes in abundance were evident. Zooplankton densities were generally higher in the littoral than pelagic zones. The abundance of Caridina nilotica, lake fly larvae, and other invertebrates have increased in the lake with the decline of haplochromine stocks. Comparison of present zooplankton density estimates with previous records indicates no marked differences in abundance patterns over the past 15 years suggesting a stable and dependable resource to sustain water quality and fishery-related functions. The OECD indicators of trophic status indicate that the pelagic waters range from mesotrophic to eutrophic and the littoral zones are hypertrophic. In order to stem further deterioration of lake water quality, management of phosphorus loading into the lake should be given urgent priority.

Description: Published

Description: Lake Victoria is the second largest lake in the world(69000km2) by surface area, but it is the shallowest (69mmaximum depth) of the African Great Lakes. It is situatedacross the equator at an altitude of 1240m and lies in ashallow basin between two uplifted ridges of the easternand western rift valleys (Beadle 1974). Despite theirtropical locations, African lakes exhibit considerableseasonality related to the alteration of warm, wet andcool, dry seasons and the accompanying changes inlucustrine stratification and mixing (Tailing, 1965; 1966;Melack 1979; Hecky& Fee 1981; Hecky& Kling,1981;1987; Bootsma 1993; Mugidde 1992; 1993).Phytoplankton productivity, biomass and species composition change seasonally inresponse to variations in light environment and nutrient availability which accompanychanges in mixed layer depth and erosion or stabilization of the metalimnion /hypolimnion (Spigel & Coulter 1996; Hecky et al., 1991; Tailing 1987). Over longer,millennial time scales, the phytoplankton communities of the African Great Lakes haveresponded to variability in the EastAfrican climate (Johnson 1996; Haberyan& Hecky,1986) which also alters the same ecological factors (Kilham et al., 1986). Recently, overthe last few decades, changes in external and or internal factors in Lake Victoria and itsbasin have had a profound inlluence on the planktic community of this lake (Hecky,1993; Lipiatou et al., 1996). The lake has experienced 2-10x increases in chlorophylland 2x increase in primary productivity since Tailing's observations in the early 1960s(Mugidde 1992, 1993). In addition to observed changes in the lake nutrient chemistry(Hecky & Mungoma, 1990; Hecky & Bugenyi 1992; Hecky 1993; Bootsma & Hecky 1993), the deep waters previouslyoxygenated to the sediment surface through most ofthe year are now regularly anoxic(Hecky et al., 1994).

Description: Worldwide, human activity in the watershed has beenfound to induce lake responses at various levels, includingat population and ecosystem scale. Recently, Carignanand Steedman (2000) reported on disruptions ofbiogeochemical cycles in temperate lakes followingwatershed deforestation and lor wildfire and Carignanet al., (2000 a, b) concluded that water quality and aquaticbiota are strongly influenced by disturbances in thewatershed. Similarly, Lake Victoria is no exception aspeople in its catchment have exploited it for the last hundred years or more, but have now begun to understand the extent to which they have thrown the lake into disorder and how their increasing activity in the watershed have driven some environmental changes within and around the lake.

Description: Relationships between nutrient concentrations and water hyacinth biomass and composition have been studied in the shallow inshore bays of lakes Victoria, Kyoga and Albert. Additional information was obtained from Victoria Nile, Albert Nile and Kagera River. In this section, seasonal changes in nutrients and oxygen concentrations are used to explain changes in water hyacinth composition, biomass and distribution in Lake Victoria. Lake Victoria is of particular interest because it experienced strong hyacinth infestations in 1995, a sink in 1998 and resurgence in 2001. The lake has also been extensively sampled and provides time series data in nutrient, oxygen, mixing and thermal stratification which provide an opportunity to relate water hyacinth distribution and biomass to environmental factors. The possible origins and impacts of nutrient loads into Lake Victoria are also discussed in relation to water hyacinth proliferation and distribution especially in relation to known 'hot-spots'.

Description: On title page: Draft 2

Description: The mobile water hyacinth, which was produced in growth zones, especially Murchison bay, was mainly exported to three sheltered storage bays (Thruston, Hannington and Waiya). Between 1996 and May 1998, the mobile form of water hyacinth occupied about 800 ha in Thruston bay, 750 ha in Hannington bay and 140 ha in Waiya bay). Biological control weevils and other factors, including localised nutrient depletion, weakened the weed that was confined to the bays and it sunk around October 1998. The settling to the bottom of such huge quantities of organic matter its subsequent decomposition and the debris from this mass was likely to have environmental impacts on biotic communities (e.g. fish and invertebrate), physico-chemical conditions (water quality), and on socio-economic activities (e.g. at fish landings, water abstraction, and hydro-power generation points). Sunken water hyacinth debris could also affect nutrient levels in the water column and lead to reduction in the content of dissolved oxygen. The changes in nutrient dynamics and oxygen levels could affect algal productivity, invertebrate composition and fish communities. Socio-economic impacts of dead sunken weed were expected from debris deposited along the shoreline especially at fish landings, water abstraction and hydropower generation points. Therefore, environmental impact assessment studies were carried out between 1998 and 2002 in selected representative zones of Lake Victoria to identify the effects of the sunken water hyacinth biomass.

Description: On title page: Draft 2

Description: The mobile water hyacinth, which was produced in growth zones, especially Murchison Bay, was mainly exported to three sheltered storage bays (Thruston, Hannington and Waiya). Between 1996 and May 1998, the mobile form of water hyacinth occupied about 800 ha in Thruston Bay, 750 ha in Hannington Bay and 140 ha in Waiya Bay). Biological control weevils and other factors, including localised nutrient depletion, weakened the weed that was confined to the bays and it sunk around October 1998. The settling to the bottom of such huge quantities of organic matter its subsequent decomposition and the debris from this mass was likely to have environmental impacts on biotic communities (e.g. fish and invertebrate), physico-chemical conditions (water quality), and on socio-economic activities (e.g. at fish landings, water abstraction, and hydro-power generation points). Sunken water.hyacinth debris could also affect nutrient levels in the water column and lead to reduction in the content of dissolved oxygen. The changes in nutrient dynamics andoxygen levels could affect algal productivity, invertebrate composition and fish communities. Socio-economic impacts of dead sunken weed were expected from debris deposited along the shoreline especially at fish landings, water abstractionand hydropower generation points. Therefore, environmental impact assessment studies were carried out between 1998 and 2002 in selected representative zones of Lake Victoria to identify the effects of the sunken water hyacinth biomass

Description: Lake Victoria Environment Management Project (LVEMP)