Water Quality and Phytoplankton Dynamics in Geffersa Reservoir/ Ethiopia
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Date
2010-06
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Addis Ababa University
Abstract
Dynamics of phytoplankton and physico-chemical parameters of water quality were
studied in Geffersa Reservoir from October, 2009 to June, 2010 using samples
collected at bi-weekly intervals. Secchi depth (vertical visibility) varied from a
minimum of 0.20 of the minor rainy season to a maximum of 0.66m during the dry
period. Turbidity, which fluctuated between 7.2 and 189 NTU, exhibited a seasonal
pattern, which was more or less similar to that of phytoplankton biomass measured
as Chl a (r= 0.649, r2= 0.415 at p= 0.0070). The concentrations of NO3-N, NH3 + NH4+-
N and Soluble Reactive Phosphate (SRP) varied from 10.0 to 300, 146 to 866 and 29
to 201 μg L-1 respectively while Molybdate-reactive silica ranged from 2.5 to 13.4 m L-
1. Turbidity, TDS, and other chemical parameters were found to be far below the
upper permissible limits for drinking water supply sources. The phytoplankton
community whose species diversity was found to be low was primarily constituted by
green algae, blue-green algae and diatoms, the contributions of the species-rich green
algae ranging from 1 to 95 % and with major species belonging to the genera
Crucigenia, Kirchneriella and Ankistrodesmus. The second and third most abundant
algal groups were the blue-green algae and diatoms, which were dominated by two
species of Microcystis, M. aeruginosa and M. panniformis. and by a species of the
filamentous genus Melosira, M. agassii, followed by Fragilaria capucina and
Rhopalodia gibberulla. Among the flagellate groups, dinoflagellates were the most
important, with Peridinium cinctum and P. bipes as the quantitatively most important
species. Among the cryptomonads, Cryptomonas marsonii and C. ovata were found in
large numbers, Chl a biomass of phytoplankton averaged 12.41 μg L-1 and varied
from a minimum of 2.29 μg L-1 to a maximum of 40.67 μg L-1 with its fluctuations
being correlated more with ammonia +ammonim (r =0.662, r2 = 0.44 at p=0.0045)
than with nitrate (r=-0.4093, r2= 0.1675 at p= 0.1028) or phosphate (r= 0.435,
r2=0.289 at p= 0.0800). Rotifers were the most abundant and species-rich
zooplankton group with mean percentage contributions of 66.82 while the
contributions from copepods and cladocerans were 24.51% and 8.67% of the total
abundance of the zooplankton respectively. The causal relationships among
physicochemical and biological variables are discussed and recommendations made.
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Biology