The carbon (C) and phosphorus (P) kinetics were studied in Daphnia magna using ¹⁴C and ³³P as radiotracers. The potential effects of food quantity, food quality (in terms of phosphorus content), life-stage, and algal species were tested. At food concentrations of 0.03-0.3 mg C 1^-1 (C:P ratio=90 in molar), the C dietary assimilation efficiencies (AEs) were 34-70% and were independent of life-stages, while the P AEs were 38%-85% in the adults and 66-89% in the juveniles at food concentrations of 2-40 μg (C:P =90) P 1^-1. A negative correlation was found between food concentration and C AE, while the P AE decreased exponentially with an increase in food concentration. The C AEs were comparable between two algal species Chlamydomonas reinhardtii and Scenedesmus obliquus. The P AE increased u...[ Read more ]

The carbon (C) and phosphorus (P) kinetics were studied in Daphnia magna using ¹⁴C and ³³P as radiotracers. The potential effects of food quantity, food quality (in terms of phosphorus content), life-stage, and algal species were tested. At food concentrations of 0.03-0.3 mg C 1^-1 (C:P ratio=90 in molar), the C dietary assimilation efficiencies (AEs) were 34-70% and were independent of life-stages, while the P AEs were 38%-85% in the adults and 66-89% in the juveniles at food concentrations of 2-40 μg (C:P =90) P 1^-1. A negative correlation was found between food concentration and C AE, while the P AE decreased exponentially with an increase in food concentration. The C AEs were comparable between two algal species Chlamydomonas reinhardtii and Scenedesmus obliquus. The P AE increased under low food quality (C: P ratio > 300). The C turnover rate was 0.12-0.16 d^-1 and 0.32-0.35 d^-1 in the juveniles and adults, while the P turnover rate was 0.096-0.185 d^-1and 0.182-0.298 d^-1in the juveniles and adults, respectively. The turnover rates of C and P were independent of food quantity, however, the P turnover rate decreased significantly under low food quality. C was lost through different compartments including respiration, excretion, molting, and reproduction from D. magna during the efflux period, among which DOC and reproduction were the most important. In contrast, molting was the most important route for P loss from the animals. Allocation of C loss but not P loss varied between the juveniles and adults. The relative allocations of C and P were independent of food quantity except for respiration or DP in adults. Under low food quality (C:P >300), the mass specific loss rates of P through all the compartments decreased.

Stoichiometric regulation in D. magna under P-limitation was explored. With the increase of P-limitation, the C and P AEs and C turnover rate increased while the P turnover rate decreased significantly. The specific C loss rates from respiration, dissolved release, and molting all increased, while the specific P loss rates from all the responsible compartments decreased, strongly suggesting the overall stoichiometric regulation in D. magna.

In comparison of the importance of inefficient feeding and post-ingestive process (excretion and feces leakage) of D. magna in contributing to DOC production, the role of direct excretion of animals was highlighted on DOC production in this study. The relative DOC production (DP/ingestion) was affected by both food quality and grazer density but not by algal species and food quantity.

In conclusion, this study constructed the integrated carbon and phosphorus budgets and demonstrated the overall stoichiometric regulation of C and P budgets under P-limitation in Daphnia. Furthermore, this study revealed that DOC excretion by Daphnia may make a great contribution to the overall DOC production in a phytoplankton-herbivore system.