Downsizing plastics, upsizing impact: How microplastic particle size affects Chironomus riparius bioturbation activity.

Microplastic contamination in freshwater systems poses serious ecological risks, yet the role of particle size in shaping these impacts remains underexplored. This study investigates the influence of microplastic size on bioturbation activities of Chironomus riparius larvae, a process essential for sediment dynamics and nutrient cycling. Employing luminophore sediment profile imaging (LSPI), we tracked the vertical distribution of polyethylene particles within sediment layers, focusing on two distinct size ranges: small (53-63 μm) and large (250-300 μm) spherical particles. Microplastics (MPs) were introduced at a 0.076% sediment ratio to reflect natural exposure scenarios. Initial findings reveal that both particle sizes undergo downward transport, though with different patterns. Notably, smaller particles demonstrated a more pronounced effect on larval behaviour, appearing frequently in digestive tracts and suggesting increased bioavailability. Temporal analysis showed distinct reworking dynamics for each particle size, with larger particles exhibiting a delayed reworking time compared to the smaller particles. This highlights the critical influence of particle size on the fate and behaviour of MPs in freshwater systems, with smaller particles potentially posing a greater ecological risk due to their quicker and more active incorporation into sedimentary processes. This study provides critical insights into size-specific interactions between MPs and freshwater organisms, enhancing our understanding of their impacts on ecosystem health and sedimentary processes.