Zooplankton Community Grazing Rates Study on the Natural Phytoplankton Assemblages in Lake Arsedi (Betemengist)
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Date
2006-06
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Addis Ababa University
Abstract
Zooplankton community grazing rates as percentage of algae cleared per day
(%G/d) were determined in Lake Arsedi by in-situ feeding experiments (with
changes in chlorophyll a concentration method) using 1 l incubation clear glass
bottles at two sampling sites (Littoral and open water). In replicated treatments,
zooplankton and phytoplankton size fractionation experiments were done and the
results indicate that invariably the larger zooplankton had higher % G/d than the
smaller-sized ones, and the smaller-sized phytoplankton were easily removed by
zooplankton than the larger-sized ones. Observations and experiments revealed
that zooplankton community composition; abundance and food density were
important factors determining grazing rates in this lake. Monthly variation in %
G/d was caused by changes in zooplankton and phytoplankton community
structures. During October 2005-February 2006 massive blooms of the
cyanobacteria, especially Microcystis aeruginosa reduced grazing impact,
whereas during March-April 2006, diatoms and small flagellates dominated the
phytoplankton and were highly vulnerable to grazing by both cylopoid and
calanoid copepods, although calanoid copepods had much stronger effect.
Increased copepod grazing during this period resulted in low phytoplankton
biomass (the clear –water period). These results suggest that variation in the
strength of grazing in eutrophic lakes such as Hora (Arsedi) result from changes
in zooplankton and phytoplankton densities and community structure. The
findings of this study are believed to lay a basis for further research and
consequently to solve the problem of eutrophication by reducing external nutrient
loading and controlling internal ecological processes.
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Biology