Effects of previous field-exposure history on the uptake of trace metals from water and food by the barnacle Balanus amphitrite

Rates of uptake from solution and assimilation efficiencies of trace metals were investigated in populations of the barnacle Balanus amphitrite with different metal contamination histories in Hong Kong coastal waters. The hypothesis under examination was that differential metal pre-exposure in the field would affect the assimilation efficiency (AE) of that metal and its rate of uptake from solution. There were no significant differences between the AE of Ag, Cd, and Zn of the different populations under the same experimental conditions, showing that previous field contamination history had no effect on the assimilation of the 3 trace metals examined. On the other hand, dissolved Cd and Zn uptake-rate constants did vary significantly between populations under the same experimental conditions, but for neither metal was there a significant correlation between metal uptake rate from solution and accumulated metal concentration, which is a surrogate measure of previous contaminant history. Thus, previous field-exposure history had no significant effect on the subsequent uptake of Zn or Cd from solution. Efflux-rate constants of accumulated Ag, Cd, and Zn in the barnacle are low in comparison to those of other benthic invertebrates, in a possible correlation with the large percentage of metal incorporated in insoluble form. A change in salinity from 33 to 15 caused an increase in the dissolved uptake of Zn and Cd in barnacles collected from a fully saline site, expected from physicochemical changes in the complexation of free Zn and Cd ions by chloride. The same salinity change caused an increase in dissolved Cd (but not Zn) uptake in barnacles from a low salinity site, suggesting some physiological acclimation. AE of Ag, Cd, and Zn did not differ at salinities of 15 and 33 in barnacles from the low salinity site. Increased exposure to metals in the laboratory or in extreme field conditions may nevertheless bring about changes in metal uptake rates in coastal invertebrate