Spatial and Temporal Variation Characteristics of Nitrogen and Phosphorus in the Mudong River Basin of Karst Wetlands in Southwest China

This study investigates the spatiotemporal patterns and driving factors of surface water chemistry in the Mudong River Basin, Guilin, combining hydrological monitoring, principal component analysis (PCA), and nitrate dual-isotope tracing. Results indicate that: (1) Total nitrogen (TN) is the primary pollutant, with a 70.6% exceedance rate, showing significantly higher concentrations in the dry season. Spatially, the northern recharge area exhibited the highest NO₃⁻-N/TN ratio, while total phosphorus (TP) accumulated along the flow path in the western discharge area. (2) PCA identified three key dimensions governing water quality: hydro-physical status (water temperature vs. dissolved oxygen), nutrient load (TN/TP), and hydrodynamic conditions. (3) Crucially, a fundamental seasonal shift in nitrate sources was revealed Bayesian source apportionment: sewage manure dominated in the dry season (37.9%), while soil organic-N (became the primary source in the rainy season (63.1%). These findings highlight the seasonal dynamics of pollution sources and are vital for formulating targeted water management strategies.