Biological and Ecotoxicological Responses of Daphnia pulex to 2,4-D and Malathion-Contaminated Laboratory Microcosms

Industrialization, urbanization, and agricultural practices have substantially increased the accumulation of contaminants in the environment, often adversely affecting nontarget aquatic species. Cladoceran species are considered valuable bioindicators in ecotoxicity studies because they are easy to culture, have transparent bodies, and are highly sensitive to chemical pollution. This study aimed to determine the acute and chronic toxicity of two selected pesticides, namely Malathion 50 EC (a.i., Malathion) and 2,4-D ethyl ester 38% EC (a.i., 2,4-D), to Daphnia pulex. The 24- and 48-h median lethal concentration (LC₅₀) values for 2,4-D were 3.5 and 2.3 mg L⁻¹, respectively, and those for Malathion were 0.76 and 0.25 µg L⁻¹, respectively. Malathion reduced clutch size, heart rate, population density (individuals per liter), and body length in D. pulex. As the 2,4-D concentration increased, heart rate and clutch size decreased significantly compared with those in the control group. However, body length and population density were not significantly affected at lower concentrations of 2,4-D during chronic exposure. Approximately 20% of the daphnids in the exposure groups switched to producing ephippia, resulting in fewer newborn individuals. The study suggests that D. pulex is a reliable bioindicator of toxicity caused by sublethal doses of Malathion and 2,4-D. However, its sensitivity varied over the 11-day exposure period. The findings indicate that the tested pesticides significantly affected the evaluated parameters of D. pulex.