Urban-industrial contamination drives developmental toxicity in Danio rerio early life stages in a tropical river system.

The Ipojuca River in northeastern Brazil receives untreated domestic, industrial, and agro-industrial effluents along its course, yet the sublethal effects of this complex mixture on aquatic vertebrates remain poorly characterized. This study evaluated surface water quality across nine sites spanning intermittent to perennial stretches of the Ipojuca River using Danio rerio early life stage bioassays, physicochemical analysis, water quality indices, and quantification of ammonia and polycyclic aromatic hydrocarbons (PAHs). A gradient of toxicity correlated with land use was observed. Samples from urban-industrial zones, particularly downstream of Caruaru city (IP49), caused severe teratogenic effects, including uninflated swim bladder (∼100%), incomplete yolk sac absorption (∼100%), and circulatory stasis, coinciding with larval mortality exceeding 85%. Un-ionized ammonia (NH3) concentrations reached up to 1.15 mg L-1 at urban sites and were strongly correlated with blood stasis and swim bladder non-inflation, with concentrations below the reported LC50, suggesting that additional contaminants contributed to the observed lethality. ΣPAH concentrations increased fivefold from upstream (155.2 ng L-1) to downstream of Caruaru (772.8 ng L-1), with diagnostic ratios indicating a petrogenic origin from urban runoff and sewage. Sites with lower pollutant concentrations and no acute lethality still induced significant sublethal effects, including reduced swimming speed (up to 50% reduction) and developmental delay, demonstrating that mortality-based assessments alone are insufficient to capture the full ecotoxicological burden of these waters. Rainfall events were associated with increased toxicity, likely due to mobilization of contaminants from surrounding agricultural areas. Our findings demonstrate that D. rerio sublethal endpoints are sensitive, effect-based tools for detecting developmental and cardiorespiratory toxicity in multi-stressor tropical watersheds and should be integrated into routine water quality monitoring programs.