Changes in average activities of superoxide dismutase (SOD) (A) and glutathione peroxidase (GPx) (B), and average concentration of total glutathione (GSH) (C) in larvae of E. danica (n = 10) at four investigated localities (SK1, SK2, SK3, and SK4) along the Skrapež River during different sampling seasons (spring, summer, autumn, and winter). Samples at different localities taken in the same season that are significantly different are marked with different numbers (1, 2, 3, 4). Samples from different seasons taken at the same locality that are significantly different are marked with different letters (s, su, a, w). Statistically significant difference (P < 0.05) of values at downstream localities compared to the values at the control locality (SK1) is marked with an asterisk (*), and compared to the values at SK2 locality with a plus (+). (D) Effect of changes in SOD activity during the investigation period on the concentration of GSH at the investigated localities on the Skrapež River. The solid line represents the linear fit of the data (R = 0.862, P < 0.001).