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 h igh-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 sup ported 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 l inear relation between Daphnia's final P-to-dry-weight ratio and its g rowth rate (r(2) = 0.92). Growth in mass (mean +/- SE) ranged from 0.5 4 +/- 0.01 to 0.17 +/- 0.02 d(-1) whereas final specific P content ran ged from 1.47 +/- 0.01% to 1.08 +/- 0.02%. Supporting stoichiometric t heory, experiments with dual-labeled (C-14/P-32) Scenedesmus showed th at C assimilation efficiency declines as the C:P ratio of the diet inc reases. 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-defici ent algae. C gross growth efficiency (production/ingestion) steadily d ecreased with declining dietary P. In contrast, P gross growth efficie ncy exhibited a unimodal pattern with reduced values for both P-rich a nd strongly P-deficient resources. The unexpected decline with P-defic ient 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.