Effects of phosphorus-deficient diets on the carbon and phosphorus balance of Daphnia magna

We used laboratory growth and feeding experiments to study the balance of carbon (C) and phosphorus (P) in Daphnia magna. Daphnia were fed high-concentration mixtures of P-sufficient and P-deficient green algae (Scenedesmus acutus; molar C:P 80 and 900, respectively) or mixtures of P-deficient Scenedesmus and a P-rich cyanobacterium (Synechococcus elongatus; C:P = 60). P-deficient diets resulted in rapid declines in the growth rates of Daphnia and unexpected declines in Daphnia's P-to-dry-weight ratio. P-deficient Scenedesmus and P-rich Synechococcus supported poor growth as sole foods but improved growth in mixtures. A 2-d experiment with a dietary C: P gradient from 120 to 900 revealed a linear relation between Daphnia's final P-to-dry-weight ratio and its growth rate (r(2) = 0.92). Growth in mass (mean +/- SE) ranged from 0.54 +/- 0.01 to 0.17 +/- 0.02 d(-1) whereas final specific P content ranged from 1.47 +/- 0.01% to 1.08 +/- 0.02%. Supporting stoichiometric theory, experiments with dual-labeled (C-14/P-32) Scenedesmus showed that C assimilation efficiency declines as the C:P ratio of the diet increases. Adding unlabeled P-deficient algae reduced the C assimilation efficiency for labeled P-sufficient algae, whereas adding unlabeled P-rich algae improved the C assimilation efficiency for labeled P-deficient algae. C gross growth efficiency (production/ingestion) steadily decreased with declining dietary P. In contrast, P gross growth efficiency exhibited a unimodal pattern with reduced values for both P-rich and strongly P-deficient resources. The unexpected decline with P-deficient diets was apparently due to low but consistent P-release rates by strongly P-limited Daphnia. Mass-balance calculations confirmed that Daphnia was a strong sink for P when resources exhibited intermediate levels of P deficiency. Our results support stoichiometric theory but show that P-deficient diets lead to significant declines in Daphnia's P content.