Road Salt Alters the Ecological Effects of Atrazine–Cypermethrin Mixtures in Freshwater Microalgae

Abstract Freshwater ecosystems are increasingly exposed to multiple chemical stressors, such as pesticides and road salts, yet the role of salinity in modifying pesticide mixture toxicity remains poorly understood. Here, we investigated the individual and combined effects of the herbicide atrazine (50 µg L −1 and 100 µg L −1 ), the insecticide cypermethrin (50 µg L −1 ), and elevated salinity (NaCl, 860 mg L −1 ) on the freshwater microalga Chlorella vulgaris in controlled microcosms. We quantified biotic responses (chlorophyll, cell abundance, biomass) alongside abiotic parameters (total nitrogen, colored dissolved organic matter, dissolved oxygen, pH, electrical conductivity, and temperature). Salinity emerged as the dominant driver of algal response, with atrazine reducing chlorophyll at 50 µg L −1 , while cypermethrin alone showed no detectable effect. In contrast, the three-way mixture of atrazine, cypermethrin, and NaCl produced a synergistic increase in chlorophyll, demonstrating strong non-additive interactions under combined stress. Although cell abundance and biomass were not significantly altered, nutrient dynamics were highly sensitive to treatment: total nitrogen and colored dissolved organic matter exhibited pronounced non-linear, synergistic reductions under binary and ternary mixtures. In contrast, dissolved oxygen and pH remained stable across treatments. These results demonstrate that salinity can fundamentally alter both the direction and magnitude of pesticide mixture effects, shifting responses from inhibitory to stimulatory depending on stressor combinations. Our findings highlight the importance of incorporating realistic salinity regimes into mixture toxicity assessments and suggest that increasing freshwater salinization may reshape how chemical pollutants influence primary production and nutrient cycling.