CONTRASTING RESPONSES OF 2 CLADOCERANS TO CHANGES IN THE NUTRITIONAL-VALUE OF NANNOPLANKTON

SUMMARY. 1. The potential tor competition in the zooplankton of small Araucana Lake (Andes, Argentina) was tested experimentally. 2. Laboratory enrichment of lake water with nutrients resulted in an increased abundance of nannoplankton with the cyanobacteria Gloeocapsa minor predominating. 3. The dominant species of zooplankton showed contrasting responses to enrichment: the abundance of Bosmina longirostris increased, whereas that of Ceriodaphnia dubia decreased relative to the corresponding controls. 4. The clearance rates for various nannoplankton by these cladocerans were similar, resulting in high niche overlap as estimated by the Pianka index. 5. Life table experiments showed that when fed on a pure suspension of lake flagellates (103 cells ml−1) Ceriodaphnia had a significantly higher average intrinsic rate of natural increase (r) than on a suspension of Gloeocapsa (0.28v. 0.146); in the case of Bosmina no significant differences for the two foods were found (0.182 and 0.141, respectively). 6. The contrasting responses of the animals to enrichment are explained by the differing suitability of particular nannoplankton species as food for different cladoceran species. The seasonal succession in the species composition and food quality of nannoplankton may be important for the coexistence of competing zooplankters in lakes. Copyright © 1990, Wiley Blackwell. All rights reserved