In Situ isotope tracing for metal bioaccumulation in open waters: Implications for interpreting biomonitoring data and safe seafood aquaculture

Understanding the toxicokinetics of metal bioaccumulation in natural environmental conditions is crucial for translating biomonitoring data into accurate environmental risk assessments. However, the lack of effective in situ tracing techniques has limited our ability to effectively assess and predict metal bioaccumulation within the environment. We developed a novel isotope tracing approach that combines laboratory isotope-labeling with field transplantation of organisms. Mussels (Perna viridis and Mytella strigata) were labeled with enriched stable isotopes (⁶¹Ni, ⁶⁵Cu, ⁶⁸Zn, ¹⁰⁷Ag, ¹¹³Cd, and ²⁰⁶Pb) and then transplanted in an estuary. Temporal changes in isotopic concentrations were modeled using a one-compartment toxicokinetic model. In-situ metal efflux rate constants (k_e^field) were found 1.4-12 times greater than the laboratory-derived values (k_e^lab) for different metals, indicating a faster metal processing rate in the field. Metal influx through aqueous and dietary routes was modeled using non-labeled metal concentrations. The in-situ aqueous uptake rate constants (k_u^field) closely aligned with laboratory values (k_u^lab), with a bias of 20 % or less, while the in-situ dietary uptake rate constants k_f^field, determined indirectly, exhibited higher uncertainty. Our model predicted maximum safe concentrations of Cd and Pb in water for cultured mussels (P. viridis) at 0.9 and 0.03 µg L^-1, which are lower than the China "Water Quality Standards for Fisheries" for Pb (50 µg L^-1) and Cd (5 µg L^-1). This study extends the utility of stable isotope tracing beyond laboratory conditions, improving metal biomonitoring data interpretation for risk assessments in complex environments.