Seasonal and Spatial Variations of Zooplankton in Relation to Physico-Chemical Variables and Phytoplankton Production in Lake Beseka (Metehara
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Date
2006-06
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Addis Ababa University
Abstract
Seasonal and spatial variations of zooplanktoR ·in relation to physico-chemical,
variables and phytoplankton production in Lake Beseka were studied from
October 2004 to September 2005. The measurements were made on monthly
basis. The lake shows temporal variations in temperature (24 -31°G),
transparency (0.42-0.63 m) and turbidity (27 -88 NTU). Conductivity (6332 -
7438jJScm,1), total alkalinity (31- 41 meq/l) and pH (9.1-9.5) are higher than
those of most other Ethiopian rift valley lakes. The present measurements of
phytoplankton production (166 -273 mg O2 m'3h(1) and biomass (7-17 mg chi a
m'3) suggest that the lake is not productive as other saline lakes. One factor
analysis of variance (A NO VA) and Pearson correlation were used to estimate the
level of interaction of physico - chemical and biological factors with zooplankton
dynamics. All major zooplankton groups showed temporal variations in
abundance, with highest densities during wet period (July to September) and
minimum densities during dry period (November to April). The cyclopid copepods
were the dominant group and made up almost 64% of the total zooplankton
abundance. Rotifers and cladocerans also contributed 19% and 13%,
respectively, of the major total zooplankton abundance of the lake. The results of
the Pearson correlation demonstrated that population dynamics, of the major
zooplankton groups in Lake Beseka appear to be mainly governed by biological
and environmental factors, their densities fluctuating along with primary
production, phytoplankton biomass and nutrients. Among environmental factors,
high temperature and rainfall appear to have promoted a higher density of egg
production and total zooplankton number, respectively. Turbidity affected the
cladoceran population negatively.
Key words: Lake Beseka, zooplankton, temp?ral dynamics,
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Keywords
Lake Beseka, zooplankton, temporal dynamics