Mechanisms influencing the impact of microplastics on freshwater benthic invertebrates: Uptake dynamics and adverse effects on Chironomus riparius.

Chironomids inhabit freshwater benthic ecosystems which are prone to microplastic contamination. This work aimed at understanding the factors and mechanisms influencing microplastic uptake and related adverse effects on Chironomus riparius, by exploring an extensive project database, conducting a literature review, and performing an agent-based model to explore trends in data. Results reveal that high concentrations of small microplastics fill the gut of fourth instar C. riparius (99.7 %). Ingested microplastics had an average size of 38-61 μm, presenting slower elimination rates than undigested organic or mineral particles. Ingestion rates of microplastics depend mainly on encounter rates, and therefore on available concentrations, until reaching a plateau corresponding to the maximum gut volume. Short-term toxicity of microplastics seems to result from damage to gut epithelium, with inflammatory reactions, production of reactive oxygen species, and a negative energy balance exacerbated by the lack of food (organic matter). Long-term toxicity is characterized by a reduction in larval body length and increase in mean time to emergence, seemly from increased energy costs rather than a decrease in nutrient absorption. Wild chironomids already present microplastics in their guts and environmental concentrations in hotspots may already exceed no effect concentrations. Therefore, environmental exposure to microplastics may induce adverse effects to wild C. riparius in freshwater benthic ecosystems, which could compromise their ecologic role as deposit-feeders (e.g., reducing their nutrient cycling ability) and key-stone species in aquatic food webs.