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.