Evaluation of bioaccumulation thresholds for Hyalella azteca and fish relative to different environmental protection...

Bioaccumulation assessment is a key component of chemical regulations, with fish-based bioconcentration factors (BCFs) serving as the standard metric. Because conventional fish BCF tests (OECD TG 305) require many vertebrates, alternative tests such as the amphipod Hyalella azteca BCF test (OECD TG 321, HYBIT) have been developed. However, physiological and ecological differences between fish and aquatic invertebrates affect chemical uptake and elimination, raising uncertainty when applying fish-based BCF thresholds directly to invertebrate data. In this study, empirical and model-based approaches were combined to compare bioaccumulation potential in H. azteca and fish. An evaluation of empirical BCFs showed consistent categorizations between H. azteca and fish for non-bioaccumulative chemicals (log KOW ≤ 4.5) and for recalcitrant organochlorines categorized as (very) bioaccumulative. In contrast, for biotransformed chemicals, H. azteca BCFs frequently exceeded fish BCFs and often surpassed regulatory thresholds, reflecting the amphipod's lower metabolic capacity. A food-chain bioaccumulation model was used to assess the influence of chemical (e.g.,, KOW, biotransformation rate constant) and biological (e.g.,, metabolic capacity) parameters. At log KOW 5, amphipod BCFs were lower than fish BCFs for poorly biotransformed chemicals. However, at log KOW ≥ 5, moderately to highly biotransformed chemicals showed higher BCFs in amphipods than both fish BCFs and biomagnification metrics (BMF, TMF), indicating potential overestimation of bioaccumulation potential. These findings highlight potential misalignment in bioaccumulation assessment outcomes when amphipod BCFs substitute fish data. Applying tiered, weight-of-evidence assessment frameworks that integrate multiple bioaccumulation metrics with biotransformation information can strengthen the scientific robustness of assessments and contextualize HYBIT results.