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. [KEYWORDS: Algal nutrient limitation; mineral limitation scenedesmus-acutus; varying quality; food quality; zooplankton; assimilation; phytoplankton; ingestion; release]
Original languageEnglish
JournalLimnology and Oceanography
Journal publication date1998

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