Competitive ability of Daphnia under dominance of non-toxic filamentous cyanobacteria

It is generally assumed that Daphnia is more susceptible to the inhibitory effects of filamentous cyanobacteria than small cladocerans since daphnids have a larger gape size and filtrate the filaments, whereas small cladocerans do not. This study addresses the question whether food limitation has the potential to modify this scenario of cladoceran response to dominance of non-toxic filamentous cyanobacteria. Daphnia galeata was grown under limited (0.1 mg C l(-1)) and unlimited concentrations (1.0 mg C l(-1)) of high-quality food algae both in the absence/presence of non-toxic filamentous Aphanizomenon flexuosum. As the effects of these cyanobacteria on D. galeata were positive under food limiting conditions and negative at the high food density, it was concluded that D. galeata was mainly affected by nutritional quality due to its ability to ingest the filaments, while mechanical interference with food collection was not important. In competition experiments between D. galeata and Bosmina longirostris, D. galeata was the dominant species at regular additions of food (1.0 mg C l(-1)) in the absence of Aphanizomenon. In the presence of these cyanobacteria, D. galeata was inhibited during the first days of the experimental period. However, the negative effect at the initially high food density was outweighed by nutrition at food limiting conditions and the outcome in competitive dominance was not changed. The results demonstrate that the ability of D. galeata to ingest large-sized non-toxic cyanobacteria can be considered as advantageous under food limiting conditions.