Strategies for the co-existence of zooplankton with the toxic cyanobacterium Planktothrix rubescens in Lake Zurich

Since the cyanobacterium Planktothrix rubescens, which dominates the phytop lankton community in Lake Zurich, is generally considered toxic to zooplank ton, we addressed the question whether co-occurring zooplankton species hav e developed adaptive responses. Artificially shortened filaments (<30 mu m in length) of P.rubescens significantly reduced survival of Thamnocephalus platyurus (Crustacea, Branchiopoda, Anostraca) naturally occurring in tempo rary ponds. In contrast to Thamnocephalus, the survival of co-existing zoop lankton was unaffected (Eudiaptomus gracilis) or enhanced (Daphnia hyalina and Cyclops abyssorum). High sensitivity to the microcystins of Plnnktothri x was coupled to strict food avoidance in Eudiaptomus, but not in Thamnocep halus. Daphnia and Cyclops exhibited higher physiological resistance to cya nobacterial toxins, and ingested Planktothrix. For the lake zooplankton spe cies, the feeding rates on high-quality algae were not significantly reduce d in the presence of Planktothrix. In order to separate the effects of mech anical interference (filament length) versus toxins, clearance rates on Pla nktothrix filaments were compared to clearance rates on filaments subjected to toxin extraction. The results show that microcystins are important feed ing deterrents against grazing by Daphnia since feeding rates on Plankrothr ix increased significantly after an aqueous-methanolic extraction of the ma jor part of microcystins. On the other hand, copepods persisted in food avo idance, but exhibited high clearance rates on Planktothrix after a more Lip ophilic extraction was applied. Both microcystins and a lipophilic, unident ified toxin may contribute to the avoidance behaviour of copepods. For both Daphnia and copepods, the grazing resistance of Planktothrix is mediated b y chemical defences rather than by the large size and the rigidity of the f ilaments.