Accumulation of Cyanobacterial Hepatotoxins by Daphnia in Some Egyptian Irrigation Canals

In this study, microcosm experiments were run in the laboratory to test the possibility of feeding of Daphnia parvula on toxic Microcystis aeruginosa in some Egyptian irrigation canal at Sohag city. The results demonstrated that Daphnia has a priority of feeding on green algae and the diatom Melosira granulata over toxic M. aeruginosa during the first 10 days. Thereafter, when the green algae and diatom were depleted from the water, Daphnia started to feed on toxic Microcystis. This presumably indicates that Daphnia feeds facultatively on toxic cyanobacteria under the conditions of depletion of edible food. Additionally, the results indicated that Daphnia accumulates the Microcystis toxins “microcystins” in its body with a level of 1.78 μg toxin/25 daphnids. No release of toxin into the water was detected during the experimental period. This emphasizes that the disappearance of toxic Microcystis was due to the feeding by Daphnia, not to death or cell lysis. Such an accumulation of cyanobacterial hepatotoxins in the primary consumers (Daphnia) should be taken into consideration when zooplankton are used in the biomanipulation of toxic phytoplankton.