Influences of metal concentration in phytoplankton and seawater on metal assimilation and elimination in marine copepods

Radiotracer experiments were conducted to examine the influence of the concentration of Cd, Se, and Zn in ingested phytoplankton (dinoflagellate Prorocentrum minimum and diatom Thalassiosira weissflogii) and in ambient seawater on metal assimilation and elimination efficiencies of three marine copepods, Acartia spinicauda, Paracalanus aculeatus, and Calanus sinicus. The assimilation efficiencies (AEs) decreased by 1.7 to 2.0 times, 1.4 to 4.1 times, and 1.3 to 2.2 times in the copepods with an increase in metal concentration in ingested algae by 16 to 84 times, 14 times, and 45 to 153 times, for Cd, Se, and Zn, respectively. However, the physiologic turnover rate constant was relatively independent of the metal concentration in copepods. No evidence was found of any interaction between Cd and Zn in their assimilation by copepods. Assimilation efficiencies of Cd were higher in copepods feeding on the dinoflagellate P. minimum, whereas the AEs of Zn were higher in copepods feeding on the diatom T. weissflogii. Differences in metal distribution in algal cytoplasm at different ambient metal concentrations may be partially responsible for the observed influence of metal concentration in algal cells on metal assimilation in copepods. However, metal desorption within the gut of the copepod may have little influence on metal assimilation, as a result of the short gut residence time of food particles and the neutral gut pH. Our study also indicated that the ingestion rate of copepods was reduced by a higher concentration of Cd and Se, but was not affected by Zn concentration in the food particles. Consequently, partial regulation of metal trophic transfer in response to increasing metal contamination may be achieved by a change in metal assimilation efficiency and the ingestion activity of the copepod, but not by changes in metal turnover rates from the animals.