Allocation and stoichiometric regulation of phosphorus in a freshwater zooplankton under limited conditions: Implication for nutrient cycling.

Because zooplankton is potentially limited by phosphorus (P) in freshwater, they may modify their body P distributions in different biochemical and anatomic components depending on the environmental P levels. In the present study, we quantified the distribution and regulation of P in a freshwater zooplankton Daphnia magna under P-limited conditions by using P as a radiotracer. We demonstrated that the P allocation patterns in D. magna were independent of the ontogenic development. Carapace accounted for 35-54% of total body P, followed by small molecules and nucleic acids (11-30%), whereas phospholipids represented only a minor P pool. The proportion of body P allocated into carapace decreased from 51.8% in +P adults to 16.5% in the -P adults, and a lower proportion of body P was also allocated to eggs in the -P adults than in the +P adults (3.8 vs. 16.5%). Meanwhile, no difference in allocation pattern was detected in the juveniles under +P and -P conditions, demonstrating an interaction between effects of P condition and ontogeny. Furthermore, the P turnover rates of nucleic acids and phospholipids in the -P juveniles were only half of those found in the +P individuals, suggesting a reduced metabolic rate under P-deficient conditions. However, the P turnover rate of small molecules, nucleic acids and phospholipids did not vary with the P condition in adults. It appeared that the adults could maintain their basic P metabolism by down-regulating the P allocation to carapace and eggs. Our results provide an insight into the tolerance of zooplankton to P-deficiency and bear implications on involvement of Daphnia in regulation of P cycling and availability in the epilimnion.