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Abstract

Laboratory growth and feeding experiments were used to study the utilization of nutritionally deficient resources separately and in mixed diets by four Daphnia species and Simocephalus vetulus. All five daphnids exhibited reduced growth and reproduction on pure diets of the cyanobacterium Synechococcus elongatus or a P-deficient green alga, Scenedesmus acutus, in comparison to the same concentration (0.5 mg C/L) of high-quality resources. Growth rates on Synechococcus declined markedly over time in 6- and 7-d trials. D. magna fared the best on the cyanobacterium and S. vetulus the worst. In agreement with stoichiometric models, D. pulicaria, the species with the lowest specific P content of body tissues (i.e., highest C:P ratio), was the best species on P-deficient Scenedesmus, whereas S. vetulus, the species with the lowest C:P ratio, was the worst. No evidence was found for a trade-off between maximal growth rate and sensitivity to P limitation. The outcome of competition among daphnids could be influenced by their abilities to utilize the poor-quality resources.

All five species showed improved growth and reproduction when small quantities (5-20%) of Synechococcus were mixed with P-deficient Scenedesmus. Since Synechococcus is rich in P, these results support the mineral-limitation hypothesis. Moreover, these results are contrary to both the fatty-acid-limitation hypothesis and the digestion-resistance hypothesis. The addition of a phosphate-rich medium rapidly increased the specific P content of P-deficient Scenedesmus, but growth rates of the five daphnids remained 15-23% lower than growth rates with P-sufficient Scenedesmus. Thus, mineral limitation may not entirely account for reduced growth with P-deficient Scenedesmus.

Experiments with dual-labeled (¹⁴C/³²P) P-sufficient and P-deficient Scenedesmus revealed no effect of food quality on the ingestion rates of D, pulicaria or S. vetulus. However, in agreement with stoichiometric models, P deficiency enhanced the incorporation of P relative to C. The importance of resource C:P ratios was tested directly by labeling one resource, P-rich Synechococcus or P-deficient Scenedesmus, and altering the C:P ratio of the diet with the alternative, unlabeled resource. P-deficient diets again enhanced P incorporation, whereas mixed diets enhanced C incorporation. Estimates of gross growth efficiencies (growth/ingestion) and P release rates provide further evidence for efficient P incorporation and reduced C incorporation with P-deficient diets. Both the growth and feeding experiments support stoichiometric models for mixed diets of complementary resources.