Findings From the Pearl River Basin: Dissolved Oxygen Dominates Functional Trait Filtering and Diversity Patterns in...

Urbanization of river systems introduces novel environmental stressors-such as industrial pollutants and domestic wastewater-that impose selective pressures on aquatic species and reshape fish community structure. This study investigates the effects of urbanization on fish community in the Pearl River Basin, comparing the highly urbanized Huadi River with the relatively natural Zengjiang River. We conducted field surveys of fish assemblages at 44 sampling sites along both urbanized and natural rivers during summer and winter-spring in 2024, measuring environmental variables and fish functional traits. Indicator value analysis was utilized to identify species linked to specific river types. R-L-Q analysis (RLQ) was performed on the relationships between environmental factors and functional traits, and structural equation model (SEM) was used to test causal pathways between key environmental factors, trait combinations, and community diversity. Results showed marked differences in fish community structure between urban and natural rivers. Urban rivers were dominated by a few tolerant species (e.g., Oreochromis niloticus), while natural rivers supported more diverse and balanced assemblages. RLQ and SEM results indicated that dissolved oxygen was the primary environmental filter in urban rivers, selecting for tolerant, low-oxygen-adapted, bottom-dwelling species and thereby driving the low-diversity pattern in urban river reaches. In contrast, natural rivers maintained higher diversity due to a broader range of environmental factors. This study demonstrates that urbanization, primarily through hypoxic stress, promotes trait convergence and biotic homogenization, while natural rivers preserve greater diversity. Targeted management to alleviate hypoxia and control pollution-tolerant invasive species is essential for restoring native ecosystem in urban rivers.