Zooplankton Community Grazing in Lake Kuriftu
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Date
2006
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Addis Ababa University
Abstract
Studies on zooplankton community grazing rates were done in Lake Kuriftu from July
2005 to April 2006 using food removal method with chlorophyll ‘a’ as algal food
indicator. The study considered the main factors that affect zooplankton community
grazing rates such as density and size of both grazers and phytoplankton. Mean
grazing rate at the natural density of zooplankton was 59.3% per day during the study
period. Grazing rate values ranged from 18.3 % to 135.6% per day at both sites and
all months. Increasing zooplankton density at two to four times ambient density was
found to decrease grazing rates. Grazing rates were higher for larger zooplankton
(>250mm) than smaller ones (<250mm). Pico and nanoplankton (up to 20mm)
especially those of size <10mm were found to be more easily removed than
microplankton (up to 63mm). Increasing the natural food density decreased grazing
rates while diluting the algal food concentration increased rates, even at ¼ dilution
factor. The Cyclopoid Thermocyclops consimilis dominated the zooplankton
community during the study time followed by the rotifer Brachionus sp. The results
suggested that increasing the density of zooplankton in Lake Kuriftu will reduce
grazing rates; and on the other hand the natural food concentration should be diluted
to increase zooplankton impacts through grazing. Top-down control with large-sized
zooplankton can bring the reduction of chlorophyll ‘a’ concentration in the lake only on
smaller and non-cyanobacterial phytoplankton (<10μm). The control of larger
phytoplankton and nuisance cyanobacteria may be of limited use using the existing
zooplankton community. Controlling external nutrient inputs into the lake should be
given priority in controlling the algal productivity of the lake, since bottom-up route is
significantly important in controlling larger and filamentous cyanobacteria. However
top-down approaches should not be ignored since large-sized zooplankton grazers
have contribution in removing smaller-sized phytoplank
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Biology