The insecticide fipronil selectively reduces the net emergence of aquatic microinvertebrates from neotropical lake sediment egg banks.

Freshwater environments rely on the sediment dormant invertebrate stages for recolonization and the maintenance of biodiversity after environmental stressors. However, while these resting structures are resilient to natural physical extremes, the effects of anthropogenic contaminants on them remain poorly understood. So, we evaluated the effects of the insecticide fipronil (Regent® 800 WG) on the emergence of dormant invertebrate stages. In a microcosm experiment, we exposed 100 g of dry sediment containing dormant stages to a control and three fipronil concentrations: T1 (41 µg/L), T2 (186 µg/L), and T3 (1,390 µg/L), with five replicates per treatment. We chose these concentrations to investigate aquatic invertebrate toxicity thresholds (T1), peak concentrations reported in Brazilian surface waters near sugarcane fields (T2), and a worst-case scenario (T3). The microcosms were maintained at 25°C under a 12h:12h light/dark photoperiod for 28 days, with sampling every 6 days to assess invertebrate abundance and species richness. Microcrustaceans, ostracods, rotifers, tardigrades, and mites hatched from the resting structures. Fipronil significantly suppressed the emergence of specific taxa when compared to control, particularly microcrustaceans and tardigrades. Treatments T2 and T3 significantly reduced the emergence of Moina minuta, Macrothrix spinosa, and Ostracoda, whereas Ilyocryptus spinifer and tardigrades showed significant inhibition only in T3. Conversely, fipronil had no significant negative effects on total invertebrate abundance, mites, or rotifers. Overall, these findings indicate that fipronil, at initial measured concentrations within (41 and 186 µg a.i./L) and above (1,390 µg a.i./L) the peak values reported for Brazilian surface waters, can reduce the net emergence of microcrustaceans and tardigrades from Neotropical sediment egg banks, with taxon-specific sensitivity. Our results highlight the need to integrate egg bank responses into ecotoxicological risk assessments and standardized toxicity testing protocols.