UTILIZATION OF A CYANOBACTERIUM AND A PHOSPHORUS-DEFICIENT GREEN-ALGAAS COMPLEMENTARY RESOURCES BY DAPHNIDS

Laboratory growth and feeding experiments were used to study the utili zation of nutritionally deficient resources separately and in mixed di ets by four Daphnia species and Simocephalus vetulus. All five daphnid s exhibited reduced growth and reproduction on pure diets of the cyano bacterium Synechococcus elongatus or a P-deficient green alga, Scenede smus acutus, in comparison to the same concentration (0.5 mg C/L) of h igh-quality resources. Growth rates on Synechococcus declined markedly over time in 6- and 7-d trials. D. magna fared the best on the cyanob acterium and S. vetulus the worst. In agreement with stoichiometric mo dels, 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-defi cient Scenedesmus, whereas S. vetulus, the species with the lowest C:P ratio, was the worst. No evidence was found for a trade-off between m aximal growth rate and sensitivity to P limitation. The outcome of com petition among daphnids could be influenced by their abilities to util ize the poor-quality resources. All five species showed improved growt h 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, th ese results are contrary to both the fatty-acid-limitation hypothesis and the digestion-resistance hypothesis. The addition of a phosphate-r ich medium rapidly increased the specific P content of P-deficient Sce nedesmus, but growth rates of the five daphnids remained 15-23% lower than growth rates with P-sufficient Scenedesmus. Thus, mineral limitat ion may not entirely account for reduced growth with P-deficient Scene desmus. Experiments with dual-labeled (C-14/(32)p) P-sufficient and P- deficient Scenedesmus revealed no effect of food quality on the ingest ion rates of D, pulicaria or S. vetulus. However, in agreement with st oichiometric models, P deficiency enhanced the incorporation of P rela tive to C. The importance of resource C:P ratios was tested directly b y labeling one resource, P-rich Synechococcus or P-deficient Scenedesm us, and altering the C:P ratio of the diet with the alternative, unlab eled resource. P-deficient diets again enhanced P incorporation, where as mixed diets enhanced C incorporation. Estimates of gross growth eff iciencies (growth/ingestion) and P release rates provide further evide nce for efficient P incorporation and reduced C incorporation with P-d eficient diets. Both the growth and feeding experiments support stoich iometric models for mixed diets of complementary resources.