Accumulation of trace elements in a marine copepod

We measured assimilation efficiencies (AEs) from ingested algal food, uptake rates from the dissolved phase, and efflux rate constants of five trace elements (Ag, Cd, Co, Se and Zn) in the marine copepod Temora longicornis. AEs of Ag, Cd, Co, Se, and Zn from two diatom diets were 13, 35, 14, 59, and 61%, respectively. AEs of metals from ingested natural seston collected during the spring phytoplankton bloom in Long island Sound were comparable to AEs from diatoms. The assimilation of all trace elements within the T. longicornis gut took longer (4-15 h) than the gut passage time of ingested food particles. The rate constant of metal uptake from the dissolved phase was highest for Ag, followed by Zn > Cd > Co > Se. Efflux rate constants for all trace elements ranged from 0.08 to 0.30 d(-1), indicating that metals were regenerated at a very fast rate in these animals. Generally, the efflux rate of metals was higher following uptake from food than uptake from the dissolved phase. The retention half-times of trace elements in copepod carcasses were 0.6-3.3 d. A bioenergetic-based kinetic model was used to quantify the relative importance of metal uptake in copepods from different pathways. The model indicates that under conditions typically encountered by copepods in nature, >50% of Zn and >98% of Se in copepods are obtained from ingested food. For Ag, Cd, and Co, >50% accumulates from the dissolved phase. The relative importance of trace element uptake from the dissolved phase vs, particulate ingestion in the overall metal uptake depends greatly on the metal AE, the feeding rate of copepods, and the partition coefficient of metals in ingested food particles.