Significance of trophic transfer in predicting the high concentration of zinc in barnacles

Barnacles are known to accumulate Zn to a phenomenal concentration, but physiological processes governing Zn accumulation are poorly defined. We determined the assimilation efficiency and efflux rate constant of Zn in barnacles (Balanus amphitrite) using radiotracer technique. Assimilation efficiency of Zn from ingested food ranged between 76 and 87% for the diatom diets and between 86 and 98% for the zooplankton preys. These AEs were the highest measured among aquatic invertebrates. Varying distribution in the soft tissues of zooplankton did not account for the variability of Zn AE observed among different zooplankton preys. Most Zn was distributed in the guts of the animals, presumably associated with the numerous granules beneath the gut epithelium. The efflux rate constant was 0.003 d^-1, and the calculated biological retention half-time was about 230 days. Using a simple bioenergetic-based kinetic model, we demonstrated that trophic transfer can account for such a high Zn concentration in barnacles. The predicted Zn concentrations in barnacles (2610-11 560 μg g^-1)were directly comparable to the concentrations measured in Hong Kong coastal waters (3100-11 000 μg g^-1). The high Zn concentration is related to its very efficient assimilation in barnacles coupled with a very low efflux rate. Biological variability must be fully appreciated before barnacles can be designated as an appropriate biomonitor of Zn contamination in coastal waters. Our study suggests that metal concentration in aquatic animals can be predicted only when both physiological and geochemical processes are considered.