Individual responses of trace-element assimilation and physiological turnover by the marine copepod Calanus sinicus to changes in food quantity

The assimilation efficiencies (AEs) of 3 trace elements (Cd, Se, and Zn) from ingested algal food (the diatom Thalassiosira weissflogii and the dinoflagellate Prorocentrum minimum) in individuals of the marine copepod Calanus sinicus were measured. In general, the metal AEs were comparable in copepods feeding on T. weissflogii and P. minimum. They decreased by 1.4 to 2.0×, 1.2 to 1.5× and 2.2 to 2.7× for Cd, Se, and Zn, respectively, after a 20× increase in food concentration (0.054 to 1.076 mg C l–1 for T. weissflogii, 0.174 to 3.470 mg C l–1 for P. minimum). The physiological turnover-rate constant was, however, independent of food density. There was no evidence of any effect of starvation on metal AEs. The AE of Cd in the copepods was significantly correlated with its distribution in the algal cytoplasm, whereas no such relationship was found for Se and Zn. This study also indicated that the metal AEs were positively related to the gut transit-time of food particles and the gut passage-time of metals in the copepods, implying a more efficient digestion and absorption when the food particles and metals were retained longer in the digestive tract. The gut physiology of copepods may thus considerably affect metal assimilation in diverse food environments. Most unassimilated Cd and Zn was lost by copepods through feces egestion, whereas excretion contributed substantially to the loss of unassimilated Se from the copepods. Excretion was a major route by which assimilated metals were depurated and turned over from the copepods. The relative contribution of fecal egestion to the total metal loss from copepods increased with increasing food concentration. Physiological responses of metal uptake by marine copepods in response to phytoplankton blooms may thus influence the fate of metals in aquatic systems.