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

Since the cyanobacterium Planktothrix rubescens, which dominates the phytoplankton community in Lake Zurich, is generally considered toxic to zooplankton, we addressed the question whether co-occurring zooplankton species have 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 temporary ponds. In contrast to Thamnocephalus, the survival of co-existing zooplankton was unaffected (Eudiaptomus gracilis) or enhanced (Daphnia hyalina and Cyclops abyssorum). High sensitivity to the microcystins of Plnnktothrix was coupled to strict food avoidance in Eudiaptomus, but not in Thamnocephalus. Daphnia and Cyclops exhibited higher physiological resistance to cyanobacterial toxins, and ingested Planktothrix. For the lake zooplankton species, the feeding rates on high-quality algae were not significantly reduced in the presence of Planktothrix. In order to separate the effects of mechanical interference (filament length) versus toxins, clearance rates on Planktothrix filaments were compared to clearance rates on filaments subjected to toxin extraction. The results show that microcystins are important feeding deterrents against grazing by Daphnia since feeding rates on Plankrothrix increased significantly after an aqueous-methanolic extraction of the major part of microcystins. On the other hand, copepods persisted in food avoidance, but exhibited high clearance rates on Planktothrix after a more Lipophilic extraction was applied. Both microcystins and a lipophilic, unidentified toxin may contribute to the avoidance behaviour of copepods. For both Daphnia and copepods, the grazing resistance of Planktothrix is mediated by chemical defences rather than by the large size and the rigidity of the filaments.