Toxic effect of cyanobacterial blooms on the grazing activity of Daphnia magna Straus

The investigations aimed to determine the impact of cyanobacteria Microcystis aeruginosa (K¨utz.) K¨utz. and Aphanizomenon flos-aquae (L.) Ralfs ex Bornet et Flah., both toxic algae, on the grazing intensity of Daphnia magna Straus. In order to determine the parameter permitting the quantitative determination of the grazing intensity of herbivorous organisms, methods based on the following techniques were applied: microscopy, to determine the degree of gut fullness; spectrophotometry, to determine the levels of chlorophyll a and its degradation products in the food composition; high-performance liquid chromatography (HPLC) to determine the content of exogenous and endogenous carotenoids. Each of these methods confirmed that the tested algae species inhibited grazing intensity in D. magna Straus. The most obvious effects were obtained when M. aeruginosa (K¨utz.) K¨utz. was used as food. With these cyanobacteria, the gut fullness indicator did not exceed 58%, and the chlorophyll a content in the digestive system of the tested D. magna was three times lower than that in the control organisms. It seems that the defensive reaction of organisms was a reversible process. However, the possibility of a long-term, sublethal influence of cyanobacteria on the physiology and internal processes of this species cannot be ruled out.