Abstract:

Eutrophication of Lake Victoria is attributable^ the burgeoning human
from expanding urban, agricultural and industrial development. Paleolimnological and
nutrient status indicator's indicate excess P on a system scale. Excessive P has stimulated
phytoplankton biomass and promoted blooms of N-fixing cyanobacteria. High algal
biomass provides organic matter that contributes to extensive oxygen depletion in
hypolimnetic waters during the stratified period. Low oxygen concentrations cause a
complex suite of direct and indirect impacts including loss of aquatic animals and
changes in nutrient cycling. Anoxia may be contributing to fish kills upon upwelling in
Lake Victoria, and also causes release of materials bound to the bottom sediments
including P. This release of nutrients reinforces eutrophication during periods of deeper
and stronger mixing when dissolved nutrients are redistributed in the water column.
Data from this study suggest two patterns controlling nutrient status and
phytoplankton biomass production in the surface waters of Lake Victoria. The first is
seasonal alteration of N and P limitation and underwater light. Circumstantial evidence
indicates that thermal stratification led to better light conditions for increased
phytoplankton biomass, and increased P and N deficiency. Higher light during
stratification compensated for and lessened effects of N deficiency and hence maintained
higher algal biomass. Destratification and deeper mixing led to low underwater
irradiance and reduced algal biomass and nutrient limitation. Entrainment of nutrientrich
hypolimnetic waters with low algal biomass reduces the potential for nutrient
limitation during periods of stronger and deep mixing. A second pattern controlling
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population in its watershed. The lake is experiencing increasing anthropogenic P loads

nutrient status is a longitudinal pattern of increasing light limitation and decreasing
nutrient limitation, especially P. from inshore to offshore. Generally, light was the
principal factor limiting phytoplankton production offshore as the ratio Izj/Ir was often
below one and indicating light deficiency.
Thermal stratification and destratification influenced cyanobacterial species
composition. Relatively warmer and shallowly mixing epilimnion promoted elevated Nfixing
cyanobacteria and heterocyst biomass production which in turn led to elevated
rates of algal N-fixation and rapid N turnover during stratification. N-fixation was an
important source of N in Lake Victoria that resulted in increased total N as well as higher
particulate N:P ratios during the stratified period. N-fixation in Lake Victoria was
predictable from heterocyst abundances and light attenuation. Heterocyst abundances
can also be used to infer N-availability in Lake Victoria and as guide to water resource
management in the lake.
Overall, Lake Victoria is an example of a large ecosystem in which the
phytoplankton community is usually limited by light availability but seasonally limited
by nutrient availability. The ability to identify factors limiting phytoplankton
community is of considerable importance to water management practices of Lake
Victoria. Since both P and N are limiting, their reduction is essential in the control of
cyanobacterial blooms and eutrophication of Lake Victoria. Nutrient status also provides
a simple and yet an important tool for monitoring water quality.