Role of sediment particle size in cypermethrin toxicity to <i>Chironomus dilutus</i> and <i>Hyalella azteca</i>: insights from bioavailability and exposure pathways

Variation in sediment particle properties gives rise to diverse settling velocities and distances, leading to distinct distributions of particle sizes in field sediments, while the impact of particle size on sediment toxicity is scarcely known. We investigated the variations in spiked-sediment toxicity across different particle sizes (<20 μm, 63-180 μm, 180-500 μm, and original) with cypermethrin, one of the main toxicity contributors in sediment from South China, as the target compound and two invertebrates <i>Chironomus dilutus</i> and <i>Hyalella azteca</i> as the testing species. Cypermethrin in fine sediments was more toxic than in coarse sediments, as evidenced by lower LC50 values, even though the desorption rate from fine sediments was slower than that from coarse sediments. The LC50 and LC25 (μg per g OC) values of cypermethrin for <i>C</i>. <i>dilutus</i> and <i>H</i>. <i>azteca</i> differed by 5- and 4-fold, respectively, across sediments. The variation in toxicity was only partially reduced when the LC50 values were based on bioavailable concentrations derived from biomimetic extraction methods, implying that bioavailability was not responsible for the enhanced toxicity of fine particle sizes. The ingestion of fine sediment particles bound with cypermethrin may significantly contribute to sediment toxicity to benthic and epibenthic invertebrates, underscoring the importance of evaluating multiple exposure pathways in sediment toxicity assessments.