Interactive Effects of Herbicide Atrazine and Road Salt on Freshwater Quality and Algal Physiology in Microcosms

Pesticides and road deicing salts are widespread contaminants in freshwater systems and frequently co-occur due to runoff and direct application. Atrazine, a triazine herbicide, and sodium chloride (NaCl), commonly used for road deicing, can independently alter water chemistry and affect aquatic primary producers. Atrazine can affect the freshwater algae, either inhibiting or stimulating their growth. The prolonged persistence of dissolved salts, ranging from decades to centuries, can lead to complex environmental scenarios characterized by multiple interacting stressors. Although many studies have examined each pollutant separately, their interactive effects have only recently gained scientific attention. This study investigated the individual and interactive effects of atrazine and NaCl on freshwater quality and algal physiological responses using laboratory microcosms. Two NaCl concentrations (120 and 230 mg/L) and two atrazine concentrations (1 and 5 μg/L) were tested using the green alga Selenastrum capricornutum. Water-quality parameters and algal endpoints, including chlorophyll concentration and cell abundance, were measured over the experimental period. Both atrazine and NaCl altered water chemistry, with nitrate concentrations increasing at higher atrazine concentrations and in mixture treatments. Combined atrazine–NaCl exposures resulted in elevated chlorophyll concentrations relative to controls, indicating a nonadditive physiological response, whereas cell abundance did not differ among treatments. These findings demonstrate that co-occurring chemical stressors can influence freshwater quality and algal physiology without proportional changes in population growth, highlighting the importance of considering mixture effects in water-quality assessment and management.