Kinetics of phosphorus in Daphnia at different food concentrations and carbon : phosphorus ratios

We examined the assimilation efficiency, excretion, and efflux of phosphorus (P) in adults and juveniles of Daphnia magna under different food levels (2-40 μg 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 μg 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. © 2007, by the American Society of Limnology and Oceanography, Inc.