This is the first study to document the seasonal dynamics of oxidative and antioxidative parameters in Sadleriana fluminensis (Küster, 1853). Sadleriana fluminensis mostly inhabits spring biotopes, which are threatened by anthropogenic activities. We reported changes in the malondialdehyde (MDA) content, activities of superoxide dismutase (SOD) and catalase (CAT), total antioxidative capacity (TAC), and the qualitative and quantitative composition of the soluble proteins in S. fluminensis. We collected samples from four sites along the longitudinal profile of the Krupa River in three seasons (autumn, winter, and spring). The amount of total soluble proteins and the qualitative protein composition varied seasonally at all sites. Detected protein bands were in the range of 103.8–5.7 kDa in all three seasons, whereas protein bands in the range of 105–60 kDa and 5–10 kDa were not detected in autumn or in spring. In all three seasons, the highest level of MDA was observed upstream at site 1. The values of TAC varied with site and season and were positively correlated to the soluble protein content. SOD activities were highest in winter, followed by autumn and spring; CAT activities were highest in spring, followed by winter and autumn. Principal component analysis based on oxidative and antioxidative parameters revealed marked differences between seasons. The results of this study enhance our understanding of the seasonal dynamics in protein composition and cell redox status within S. fluminensis and will aid future studies. The approach used in this study could also be applied more generally in other ecosystems for the monitoring of bioindicators.