Influences of hydrodynamic conditions on the biomass of benthic diatoms in a natural stream

Studies on benthic algae and hydrodynamic characteristics are rarely conducted in natural streams. In this study, a natural stream was selected as a test field. First, we sampled the benthic diatoms on site. Then, we measured stream physicochemical characteristics in the laboratory. Finally, we estimated hydrodynamic characteristics using a numerical simulation. On these bases, the corresponding relations between benthic diatom biomass and stream velocity and shear stress at different sections were studied in the 1D case, and the corresponding relations between benthic diatom biomass and stream velocity and depth were studied in the 2D case. The results show that the suitable flow range for benthic diatoms is approximately 0.85-1 m/s and the suitable shear stress is approximately 24-28 N/m2 in the 1D case. In the 2D case, the suitable velocity and stream depth are 0.9-1.1 m/s and 0.40-0.48 m, respectively. Therefore, both along the stream and in cross-section, there is always a range with suitable hydrodynamic characteristics to support a relatively high biomass of benthic diatoms. Outside of this range, however, the biomass decreases regardless of whether the values of the hydrodynamic characteristics are higher or lower. The study conclusion may provide a reference for stream ecological assessment and restoration.