Zooplankton Community Grazing Rates Study on the Natural Phytoplankton Assemblages in Lake Arsedi (Betemengist)

No Thumbnail Available



Journal Title

Journal ISSN

Volume Title


Addis Ababa University


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.