Effects of River Hydrodynamics on Periphytic Algal Groups and Extracellular Polymeric Substances Based on a Field Investigation: Implications for Sustainable River Management

Periphytic algae and extracellular polymeric substances (EPS) shape river biofilms, but hydrodynamic relationships remain poorly resolved in mountain rivers. We surveyed 32 points in the Nanxi River, Zhejiang, China, in August and October 2025. Gaussian, linear, and Spearman analyses assessed velocity responses and algal–EPS relationships. Algal-group densities showed significant unimodal responses (p < 0.05), peaking at 0.62–0.63 m/s for diatoms, 0.14–0.20 m/s for cyanobacteria, and 0.23 m/s for green algae. Total EPS was unrelated to velocity; soluble EPS decreased, whereas bound fractions increased (p < 0.05). Total algae correlated with total EPS (ρ = 0.65–0.73, p < 0.01), and diatoms with bound fractions (ρ = 0.52–0.68, p < 0.05). These patterns suggest that flow may affect biofilms through linked pathways: direct scouring and EPS redistribution, and differential algal detachment and retention that alter community composition and, in turn, EPS production and partitioning. Group-specific density peaks provide a quantitative basis for determining site-specific near-bed velocity ranges for sustainable flow management. The findings provide field observational evidence for aquatic habitat conservation and sustainable water-resource management. However, sampling covered only two periods, and hydrodynamics were represented solely by near-bed velocity; long-term multiparameter monitoring is needed.