Seasonal variation of dissolved oxygen in river water in an urbanized area: example of the Vladayska River

This study investigates the seasonal variation of dissolved oxygen (DO) concentrations in urbanized rivers, with a focus on the Vladayska River in Sofia, and examines key influencing factors such as temperature, biological oxygen demand (BOD5), chemical oxygen demand (COD), precipitation and river runoff. The waters of the Vladayska River, subjected to significant anthropogenic pressure, show significant fluctuations in DO levels, with the lowest concentrations recorded in summer and autumn. These periods coincide with high water temperatures, low river discharge, and elevated BOD5 and COD levels, indicating increased oxygen consumption by organic and chemical pollutants. The results show that DO levels in the Vladayska River fall below critical thresholds (5 mg/L) in summer and autumn, representing an ecological risk and reducing river waters’ self-purification capacity. In spring and winter, higher DO levels suggest a seasonal recovery associated with cooler temperatures and increased river flow volume. This study accentuates the seasonal decline in dissolved oxygen levels in urbanized rivers due to both natural processes and intensive anthropogenic activity, highlighting the importance of continuous water quality monitoring.