| The efficiencies with which a variety of elements (Ag, Am, C, Cd, Co, P, S, Se, Zn) were assimilated in copepods (Acartia tonsa, A. hudsonica, Temora longicornis) and bivalve mollusc larvae (Crassostrea virginica and Mercenaria mercenaria) fed uniformly radiolabeled phytoplankton (Thalassiosira pseudonana and Isochrysis galbana) ranged from 0.9% for Am to 100% for Se. To determine the assimilation of selenium, a dual-label radiotracer ratio method was developed using two gamma-emitting radioisotopes, {dollar}sp{lcub}75{rcub}{dollar}Se and {dollar}sp{lcub}241{rcub}{dollar}Am. Zooplankton fed phytoplankton uniformly labeled with both isotopes assimilated Se with very high efficiencies (97-100%). A 1:1 relationship between assimilation efficiencies in these herbivores and the cytoplasmic content of the phytoplankton food was observed for all elements tested. The results suggest that both copepods and bivalve larvae, which have short gut passage times, utilize digestion strategies in which only the easily mobilized, cytoplasmic fraction of ingested phytoplankton cells is assimilated.; Element assimilation efficiencies in mature bivalve molluscs (Crassostrea virginica and Mercenaria mercenaria) generally reflected the fractionation of elements in ingested phytoplankton, but for some elements assimilation efficiencies were significantly influenced by the animal's digestive physiology. Silverside fish (Menidia spp.) fed uniformly radiolabeled copepods (A. tonsa) assimilated 50-60% of ingested C, P, and S, but less than 6% of ingested Cd, Co and Zn. Assimilation efficiencies in the silversides were directly related to the proportion of each element in non-exoskeleton copepod tissues.; Based on the retention half-times of trace elements and protein in copepod (Anomalocera patersoni) carcasses ({dollar}le{dollar}10 d), herbivore (Crassostrea virginica and Mercenaria mercenaria) feces ({dollar}le{dollar}10 d), and sediment trap particulate matter (38-87 d), significant amounts of trace elements could be delivered to the deep sea via sinking biogenic debris depending on particle abundances, sinking speeds, and the extent to which particles are consumed during their descent. Protein appeared to influence the turnover rates of the slowly exchanging pools of Se and Cd in sediment trap particulate matter. Those elements found to be appreciably incorporated into algal cytoplasm and assimilated in zooplankton are generally recycled in surface waters and have longer oceanic residence times than elements bound to cell surfaces or crustacean zooplankton exoskeletons which are egested as sinking fecal matter. |
