Spatiotemporal patterns and driving factors of low-oxygen conditions in highly urbanized river basins in China

Low-oxygen conditions have become an increasing concern in aquatic systems worldwide, yet their spatial distributions and driving mechanisms remain insufficiently explored. Here, we analyzed monthly surface water monitoring data (2018-2023) from 149 sections in Guangdong Province to assess the temporal and spatial variations in dissolved oxygen (DO) and identify the primary factors driving low-oxygen conditions. Our results indicated an overall improvement in DO levels, with a 9.4% reduction in low-oxygen events from 2021 to 2023 compared with those from 2018 to 2020. Seasonally, low-oxygen conditions were more prevalent in the wet season, with a 17.5% greater occurrence than those in the dry season. Spatially, the DO concentrations were the lowest in the East Guangdong River Basin, West Guangdong River Basin, and Pearl River Delta River Basin. Across monitoring sites, low-oxygen conditions followed four major patterns. Persistent low-oxygen (Pattern I), occurring year-round with a 78.9% frequency, was primarily driven by continuous pollutant loading. Wet-season low-oxygen (Pattern II), predominantly from April to October, was linked to increased rainfall-driven non-point pollution. The summer low-oxygen (Pattern III), concentrated from June to September, was largely temperature-driven (56.0% contribution). Dry-season low-oxygen (Pattern IV), which peaked from October to December during the dry season, was influenced mainly by reservoir discharge and pollution accumulation. The mitigation of low-oxygen conditions requires targeted interventions based on pattern-specific drivers: controlling key pollution sources for Pattern I, accelerating rainwater-sewage separation for Pattern II, implementing ecological restoration to increase DO levels for Pattern III, and optimizing reservoir flow regulation and upstream pollution management for Pattern IV. By integrating multiple analytical approaches, this study enhances our understanding of the dynamics and drivers of low-oxygen conditions in highly urbanizing river basins, emphasizing the need for comprehensive water quality management strategies.