Importance of Main Stream/Tributary Interface for Nutrient Dynamics and Microbial Communities in Upper River Subcatchment

Aims: A significance of tributary confluence for dynamics of nutrients and microbial communities in main river channel was studied in the Iskar River upper subcatchment (Bulgaria).
Methodology: The fluctuations of physicochemical parameters, nutrients, abundance of total bacteria, oligotrophes and coliforms in main stream and tributary were estimated seasonally by standard methods in small spatial scale (<2 km).
Results: The P-PO4 concentrations in river and tributary waters were higher in cold seasons (between 0.14÷0.53 mg/L) compared to values observed in the other seasons. The quantity of nitrate nitrogen in tributary waters showed a statistically significant difference in comparison with the concentration in site upstream from the inflow but not in the site downstream from the inflow. Similar differences were observed in the nitrite dynamics even though this nitrogen form was less abundant. Ammonium nitrogen concentrations were generally in the range 0.03-0.6 mg/L in main stream waters and 0.15-1 mg/L in tributary waters. There were not statistically significant differences among sites but the seasonal profiles indicated the difference in ammonium concentrations during the autumn.
Conclusion: The results showed that tributary effect is more significant during the autumn and winter. The water quality of study area was disturbed in these seasons and the content of total suspended solids, phosphorus, ammonium and bacterial indicators for fecal contamination might be increased to a critical significance in sites with higher anthropogenic impact.