Eutrophication of the Lake Victoria Ecosystem

Abstract

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 (NO 3 - N) and phosphate phosphorus (PO 4 - P) concentrations ranged between 16.2 - 87.9 μg/l and 39.6 - 92 μg/l respectively and were both higher in the northeast. Silica (SiO 2 - 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 PO 4 - P concentrations (16.2 to 21.1μg/l) than the Mwanza and Napoleon Gulfs (54.8 to 68.7μg/l) but registered higher SiO 2 - Si concentrations (4.5 to 5.2 mg/l) than th e other two gulfs. NO 3 - N concentration in the gulfs ranged between 25 and 93 μg/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) range d between 0.53 and 0.83 mg/l. The TN:TP ratio (<20) in the main lake indicate d 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 wit h 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 pelag ic waters. Comparison with Tall ing’s 1961 values, SiO 2 - 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 μg/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 compar ed 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 fisher y - 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.