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

Barnacles are known to accumulate Zn to a phenomenal concentration, but phy siological processes governing Zn accumulation are poorly defined. We deter mined the assimilation efficiency and efflux rate constant of Zn in barnacl es (Balanus amphitrite) using radiotracer technique. Assimilation efficienc y of Zn from ingested food ranged between 76 and 87% for the diatom diets a nd between 86 and 98% for the zooplankton preys. These AEs were the highest measured among aquatic invertebrates. Varying distribution in the soft tis sues of zooplankton did not account for the variability of Zn AE observed a mong different zooplankton preys. Most Zn was distributed in the guts of th e animals, presumably associated with the numerous granules beneath the gut epithelium. The efflux rate constant was 0.003 d(-1), and the calculated b iological retention halftime was about 230 days. Using a simple bioenergeti c-based kinetic model, we demonstrated that trophic transfer can account fo r such a high Zn concentration in barnacles. The predicted Zn concentration s in barnacles (2610-11 560 mu g g(-1))were directly comparable to the conc entrations measured in Hong Kong coastal waters (3100-11 000 mu g g(-1)) Th e high Zn concentration is related to its very efficient assimilation in ba rnacles coupled with a very low efflux rate. Biological variability must be fully appreciated before barnacles can be designated as an appropriate bio monitor of Zn contamination in coastal waters. Our study suggests that meta l concentration in aquatic animals can be predicted only when both physiolo gical and geochemical processes are considered.