The daily dynamics of water chemistry were investigated in the polymictic and polyhumic Siemianówka Dam Reservoir (SDR), in NE Poland. The highest concentration of ammonia, nitrate and orthophosphate ions during the day and the highest values of conductivity at night were recorded. A maximum of dissolved organic carbon was noted during afternoon hours while the content of aromatic DOM compounds was smaller during the day than at night. Significant differences in water chemistry were noted between the morning, afternoon and night hours, especially in the spring season, this being affected by the activity of phytoplankton in the water of the eutrophic reservoir.