Beyond survival: Metabolomic profiling of midge (Chironomus tepperi) larvae reveals mechanisms of sanitary sewer influent toxicity under dry weather and intermediate wet weather conditions

Sanitary sewage influent released to the environment is one of the major sources of aquatic contaminants, yet their ecotoxicological impacts remain poorly understood. This study assessed the survival and sublethal biochemical responses of Chironomus tepperi larvae, exposed to increasing concentrations of unfiltered sanitary sewer influent (SSI) collected under dry weather (DW) condition and intermediate wet weather (IWW) condition associated with inflow and infiltration from a rainfall event. Water quality parameters revealed higher concentrations of ammonia, major ions, and trace metals in DW samples compared to IWW. Larval survival declined in a dose-dependent manner, with significantly lower survival observed in DW conditions than IWW condition. Significant reductions in larval survival were observed at 25 % unfiltered sewage influent under DW condition and 50 % under IWW conditions. Metabolomic and lipidomic analyses revealed widespread disruptions across multiple biological pathways, including amino acid, carbohydrate, lipid, energy, and nucleotide metabolism. Notably, alterations in lipid profiles suggest a dual impact of SSI contaminants: impaired lipid biosynthesis and increased energy demand to mitigate stress, resulting in the depletion of essential lipid reserves. These metabolic disruptions followed the same pattern as survival responses, with more pronounced effects at 25 % DW and 50 % IWW. This suggests that a spill of SSI under DW conditions has potential for a greater adverse ecological effect than a spill during wet weather when SSI has been diluted from inflow and infiltration of stormwater into the wastewater system and will be further diluted in receiving waters where stormwater inflow would be also occurring.