Trophic transfer of heavy metals from freshwater zooplankton Daphnia magna to zebrafish Danio reiro

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The trophic transfer of metals along the food chain has been recognized as an important issue in the study of water quality in recent years. Feeding experiments were conducted to examine the assimilation of three metals (Cd, Cr and Zn) by the zebrafish Danio reiro feeding on the freshwater zooplankton Daphnia magna. The zooplankton were exposed to radiotracers from both the aqueous and dietary phases for different duration, and then pulse-fed to the zebrafish for measurements of metal assimilation efficiency (AE). The calculated AEs were 3–8% for Cd, 2–39% for Cr, and 17–36% for Zn in the zebrafish. For Cd and Zn, there was no statistically significant difference between the two different radiolabeling routes (aqueous and dietary exposure). For Cr, the AEs were higher when it was accumulated by D. magna from the dietary source than when it was accumulated from the aqueous phase. The gut passage time (GPT) was 6–10 h for all metals, with less variation for Zn among the different treatments. There was no obvious relationship between metal GPT and metal AE, presumably due to the narrow range of variation of metal gut passage. About 5–36%, 20–31%, and 8–30% of the total Cd, Cr and Zn was found in the soft tissue of D. magna after the radiolabeling. A much higher fraction of Cd and Zn was found in the soft tissue of D. magna when the metals were accumulated from the dietary phase. No significant relationship between the metal AE and the metal distribution in the soft tissue of D. magna was however documented in this study. Our results demonstrated that there was major difference in metal AE in freshwater fish among different metals. Metal localization in prey organisms and GPT appear to have little influence on metal assimilation by the zebrafish.