Polyethylene microbead distribution in sediment toxicity test system with estuarine amphipod Grandidierella japonica

Microplastics (MPs) are ubiquitous in the environment and may pose risks to aquatic ecosystems. Although sediment toxicity tests are crucial for the risk assessment of MPs, exposure concentrations of MPs in sediment toxicity tests have not been quantified well yet. In this study, we developed and demonstrated a practical method to quantify MP number across all compartments of a typical sediment toxicity test system. Fluorescent polyethylene microbeads (30 and 100 µm) were introduced into formulated and field-collected sediments and monitored in the presence or absence of the estuarine amphipod Grandidierella japonica. Under all conditions, the majority of microbeads were retained in the sediment; however, MP distribution varied with sediment type, particle size, and the presence of amphipods. For example, field-collected sediment transferred a higher proportion of MPs to the water surface (8.1% ± 2.8% of added MPs) compared with formulated sediment (0.8% ± 0.7%). Furthermore, 100 µm beads were more abundant on the water surface, lid, and beaker walls than 30 µm beads, whereas 30 µm beads were predominated in the overlying water. The presence of amphipods resulted in many numbers of both 30 and 100 µm beads in nonsediment compartments, likely due to their bioturbation activity. Notably, the accumulation of 100 µm beads on the beaker walls is significant (up to 7.7% of added MPs), indicating a potential decrease in MP exposure to benthic organisms. These findings suggest that assessing MP exposure based solely on nominal and bulk sediment concentrations may underestimate risk. In addition, the presented workflow offers a practical framework to obtain compartment-specific MP counts and can be applied to MPs of other polymers and shapes for more realistic toxicity assessments.