Refining marine metal biomonitoring through insights into allometry in green mussels

Size-dependent variability in metal accumulation complicates the use of bivalves as biomonitors, potentially leading to inaccurate contamination assessments. While some biomonitoring programs attempt to account for size-related variability, existing methods may pose practical challenges. This study investigates how size influences metal accumulation in Perna viridis and explores the underlying toxicokinetic mechanisms using an improved double stable isotope method. Our results reveal metal-specific allometric patterns. The concentrations of non-essential metals (Pb, Ni, and Cd) decrease significantly with increasing mussel size, whereas Zn, an essential metal, remains invariant. Toxicokinetic analysis reveals that the uptake rate constants (k_u) for all four metals decline with size. However, Zn differs from the non-essential metals in that its elimination rate constant (k_e) also decreases with size, offsetting lower uptake and resulting in size-independent accumulation. In contrast, for Pb, Ni, and Cd, the effect of size on k_e is minor, making k_u the primary driver of metal concentration scaling. To improve biomonitoring accuracy, we propose metal-specific thresholds for determining when size correction is necessary and introduce two correction approaches. By elucidating metal-specific toxicokinetic mechanisms and establishing a threshold-based framework, this study advances the standardization of bivalve biomonitoring, ensuring more robust contamination assessments.