Water-quality responses and management-oriented spatial thresholds in a complex river-estuary continuum

The Pearl River Delta is a densely urbanized river-estuary continuum with one of the world's most complex lowland networks. Its highly branched, strongly tidal, and salinity-stratified setting under intensive regulation complicates the role of network configuration in governing water quality. A limited understanding of spatiotemporal variations, multi-driver attribution, and environmental impact thresholds hinders the shift from uniform to targeted management. We analyzed 2019-2023 daily water-quality records from 45 stations alongside network metrics and environmental drivers. Network geometry exhibited distinct spatial differentiation, with Dendritic Connectivity Index (DCI) showing a clear north-south gradient. With increasing buffer radii, reach, node, and development coefficient (Kω) stabilized at 600-800 m, indicating diminishing returns beyond ∼800 m. Spatial heterogeneity revealed network regulation of water quality via flow connectivity and material transport. Western stations (with higher water-surface ratios, Wp) influenced by Xijiang carbonate weathering had higher pH; eastern electrical conductivity (EC) hotspots reflected saltwater intrusion and constrained estuarine exchange; turbidity peaked at embayments and bifurcation mouths (with lower Kω), while nutrients were concentrated in the eastern urbanized estuary. LightGBM identified 400-800 m as the most robust explanatory distance, with dissolved oxygen, turbidity, and ammonia nitrogen linked mainly to hydrochemistry, EC to hydrology, and nutrients to socioeconomic drivers. Among structural metrics, DCI, river density, and Wp contributed most to EC and turbidity. Mantel analysis confirmed human activity and hydrodynamic connectivity are the primary drivers of water-quality variation, particularly at intermediate distances (≥ 600 m). An 800 m buffer provided the clearest river-estuary separation for zoned water-quality management across the urban agglomeration.