Pulsed Exposure to Environmentally Relevant Concentrations of 6PPD-Q Elicits Concentration-Dependent Delayed Mortality and Prolonged Behavioral Impairment in Rainbow Trout

Abstract

The tire rubber transformation product N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine-quinone (6PPD-Q) is acutely toxic to select salmonids, yet current risk thresholds are derived exclusively from constant-concentration exposure tests that fail to represent the intermittent, rainfall-driven pulses patterns of urban runoff to surface waters. Here, we exposed rainbow trout to four 8-h pulses of 6PPD-Q at environmental relevant concentrations (0.125–1.95 μg/L), followed by a 14-d recovery period. The resulting multiple-pulsed LC₅₀ (0.28 μg/L) was significantly lower than thresholds from standard constant-exposure bioassays. Notably, substantial delayed mortality occurred during recovery, and environmental pulses of 6PPD-Q frequently reached or exceeded levels triggering such delayed mortality. Distinct lethal phenotypes were observed across the four successive pulses and the recovery period, suggesting differential disposition and/or toxicity pathways influence each stage. Furthermore, surviving fish exhibited significant behavioral impairments even after the 14-d recovery period, indicating prolonged sublethal impacts. Collectively, our findings demonstrated that environmentally realistic pulsed exposure regimes and delayed toxicity are critical yet overlooked components of ecological risk for 6PPD-Q, and should be considered to support more environmentally realistic assessment of 6PPD-Q hazards in urban watersheds.