Relative importance of competition with Daphnia (Cladocera) and nutrient limitation on Anuraeopsis (Rotifera) population dynamics in a laboratory study

1. In the laboratory, the growth and reproduction of Anuraeopsis fissa were measured when fed on Scenedesmus species grown in nutrient-sufficient, nit rogen-limited and phosphorus-limited media and in the presence or absence o f one adult Daphnia longispina per vial. 2. Poor food quality may reduce the effect of competition on rotifers. Comp etition from Daphnia was stronger with nutrient sufficient algae than with nutrient limited algae. P-limitation significantly reduced Anuraeopsis popu lation growth rate and fecundity. The effect of nutrient limitation on Anur aeopsis was stronger than that of competition with Daphnia. The Anuraeopsis population declined with P-limited food in both the presence and absence o f Daphnia. 3. Exploitative competition by Daphnia on Anuraeopsis was stronger in the n utrient-sufficient treatment than in the N-limited one. Density, fecundity and population growth rate of Anuraeopsis were negatively affected by Daphn ia in the nutrient-sufficient treatment, while only fecundity was reduced b y Daphnia in the N-limited treatment. Consequently, in the N-limited treatm ent, mortality should be lower in the presence of Daphnia. This result coul d suggest that Anuraeopsis lives longer when short of nitrogen. 4. Nutrient limitation may affect to the competitive interactions between z ooplankton species. P-limitation decreased the quality of algae as food for Anuraeopsis while N-limitation decreased the susceptibility of this rotife r species to exploitative competition by Daphnia.