EFFECTS OF TOXIC CYANOBACTERIA AND PURIFIED TOXINS ON THE SURVIVAL AND FEEDING OF A COPEPOD AND 3 SPECIES OF DAPHNIA

Acute toxicity experiments with purified toxins show that four species of zooplankton differ markedly in their physiological sensitivity to cyclic peptide hepatotoxins from Microcystis aeruginosa (microcystin-LR) and Nodularia spumigena (nodularin). The copepod Diaptomus birgei was most sensitive (48-h LC50 for microcystin-LR ranged from 0.45 to 1.00-mu-g ml-1), Daphnia pulicaria was least sensitive (48-h LC50), 21.4-mu-g ml-1), and Daphnia hyalina (48-h LC50, 11.6-mu-g ml-1) and Daphnia pulex (48-h LC50, 9.6-mu-g ml-1) were intermediate in sensitivity. Survivorship in the presence of toxic Microcystis was strongly influenced by both physiological sensitivity and feeding behavior. Relatively good survivorship by D. pulicaria was associated with low sensitivity to purified toxin and rapid feeding inhibition in the presence of toxic cells. In contrast, very poor survivorship by D. pulex was associated with greater physiological sensitivity and nearly uninhibited feeding on toxic Microcystis. Intermediate survivorship by D. birgei was associated with food selection capabilities on the one hand and high physiological sensitivity and uninhibited feeding on the other. Our results suggest that zooplankton have evolved both physiological and behavioral adaptations which enhance their abilities to coexist with toxic cyanobacteria.