Zooplankton Community Grazing Rates in a Small Crater Lake: Lake Kuriftu, Ethiopia

Abstract

Lake Kuriftu is a small eutrophic lake exposed to human impacts such as irrigation, resorts and livestock watering. It was studied for zooplankton community grazing rates using different size classes and concentrations of both zooplankton and phytoplankton from July 2005 to April 2006. The food removal method or changes in chlorophyll-a concentration measured before and after grazing was used to quantify zooplankton grazing rates. Grazing rates ranging from 18.3% to 135.6% per day were recorded during the study period. Increasing zooplankton density at two to four times the ambient density was found to decrease grazing rates, indicating that grazing was at optimal level in the lake. Grazing rates were higher for larger zooplankton (>250 µm) than smaller ones (<250 µm). Phytoplankton with size of <10 µm were found to be more easily removed than microplankton (up to 63 µm). Increasing the natural food density decreased the rates while diluting the algal concentration by one-fourth increased grazing rates. The implications of the size and density manipulations on zooplankton grazing are discussed with regards to using different management options for algal control in Lake Kuriftu. It is concluded that regulation and control of external nutrient inputs into the lake should be given priority to prevent algal productivity in the lake since grazing by zooplankton does not control the common large and filamentous cyanobacteria in Lake Kuriftu.