Zooplankton Community Grazing Rates in Some Lakes and a Reservoir in Ethiopia

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Addis Ababa University


Zooplankton community grazing rate as % of algae grazed per day was determined in five lakes and one public water-supply reservoir from March, 1993 to October, 1993_ Physical and chemical measurements (Temperature, Secchi depth, alkalin~y, salin~y etc_) were taken at the same time. Radio-labelling and change in chlorophyll A (chl.a.) methods were used. Zooplankton commun~y grazing rates ranging from 0-95% day'l were obtained. In size fractionated community grazing always, the larger size zooplankters showed higher %G.day'1. Increase in ambient density of zooplankton resulted in increased %G.day'l in ali cases except for Lake Bishoftu in which the reverse took place. Two regression models relating %G.day-l to temperature (Temp.). mean zooplankton biomass (SO), total zooplankton biomass (ZB), large size zooplankton biomass (UB) and total chlorophyll a (chl.a.) were tested. Backward deletion procedure resulted in three models which explained 27 - 64% of the total variation in %G.day'l. In both the regression models and the correlation matrices, chl.a. concentration was negatively related to %G.day'1. In the later case, temperature was negatively related to %G.day'l and ali types of zooplankton biomass. The regression coefficients were not significant at 5% significant level. Grazing results are compared with other works in temperate and tropical regions and discussed.