Assessment of waterborne and dietary exposure pathways in the toxicity of a flumioxazin-based herbicide to Daphnia magna

Flumioxazin-based herbicides are widely used in agriculture due to their high effectiveness against weeds. However, their use raises concerns about potential risks to aquatic ecosystems. Despite evidence of direct toxicity in water, little is known about the indirect effects mediated by contaminated primary producers consumed by zooplankton, key organisms in aquatic food webs. This study aimed to evaluate the acute toxicity of the commercial formulation Sumysin 500 SC® to Daphnia magna and to investigate chronic effects through trophic exposure using contaminated microalgae as food, thereby integrating multiple exposure routes into the ecotoxicological assessment. Acute immobilization tests with D. magna showed EC₅₀ values of 30.61 mg L ^− 1 (24 h) and 29.59 mg L ^− 1 (48 h), indicating moderate sensitivity of the species to the flumioxazin-based formulation. In feeding assays, the microalga Raphidocelis subcapitata was pre-exposed to 0.852 µg L ^− 1 flumioxazin, equivalent to the reported EC ₅₀ for this species. The ingestion of contaminated algae by D. magna resulted in severe sublethal effects, including a 100% inhibition of egg production, a reduction in lifespan of up to 13 days, and developmental delays at all ontogenetic stages. Biochemical analysis showed increased carbohydrate levels in the algae after exposure, which may have intensified negative effects on the consumers. The combined results highlight the high risk potential of flumioxazin to aquatic invertebrates and emphasize the need to incorporate food pathways into risk assessment frameworks. These findings underscore the ecological implications of herbicide contamination and the vulnerability of aquatic food webs to indirect exposure pathways.