Kinetics of phosphorus in Daphnia at different food concentrations and carbon:phosphorus ratios
He, Xuejia, and Wen-Xiong Wang
Limnol. Oceanogr., 52(1), 2007, 395–406

We examined the assimilation efficiency, excretion, and efflux of phosphorus (P) in adults and juveniles of Daphnia magna under different food levels (2–40 ΅g P L^-1) and dietary carbon : phosphorus (C : P) ratios (90–930 in molar) with Chlamydomonas reinhardtii as food. The P assimilation efficiencies calculated by regression analysis were 38–85% and 66–89% for adults and juveniles, respectively, and were constant at food concentrations >24 ΅g P L^-1, but increased significantly when the diet shifted from P-sufficiency to P-deficiency. The mass-specific excretion rate of adults and juveniles was 1.1–33.2 ng P mg dry weight (DW)^-1 h^-1 and 3.0–63.4 ng P mg DW^-1 h^-1, respectively, and was influenced by the food concentration and decreased with an increase in dietary C: P ratio. The efflux rate constants of the adults and juveniles were 0.182–0.298 d^-1 and 0.096–0.185 d^-1, respectively. Food concentration did not affect the efflux, but an increase in dietary C: P ratio reduced the P efflux, suggesting stoichiometric regulation. Among the different routes involved in P loss from Daphnia, molting was the most important, contributing 44–75% of the total loss for the juveniles and adults. The mass specific loss rates were 13–54 ng P mg^-1 h^-1 and 45–110 ng P mg^-1 h^-1. The relative and absolute P loss from each compartment (except the dissolved P release in adults) was independent of food concentration. Increasing the dietary C: P ratio decreased the mass-specific release rates by molting, dissolved P release, and reproduction, indicating the animals’ endeavor to maintain P stoichiometric homeostasis.