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Eutrophication of Lake Victoria Ecosystem

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LVBC Online IWRM Library

Eutrophication of Lake Victoria Ecosystem

Eutrophication of Lake Victoria Ecosystem



Gikuma-Njuru et al

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Lake Victoria, Eutrophication, Nutrients, Nitrogen,

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 µg/l and 39.6 - 92 µg/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 21.1µg/l) than the Mwanza and
Napoleon Gulfs (54.8 to 68.7µg/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
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)
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 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 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.

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