Contrasting Environmental Drivers of Temporal Dynamics and Spatial Variation in Riverine Nutrients: A Landscape Ecological Approach

Understanding spatiotemporal riverine nutrient patterns and their drivers is essential for sustainable watershed management. However, within a single watershed, controls on long-term temporal dynamics and spatial heterogeneity remain unclear, particularly under nutrient legacy effects. This study couples a 44-year historical record of nutrient concentrations at the Yong’an outlet to examine long-term trends, alongside synchronous observations from 24 nested sub-watersheds in 2023 to characterize spatial variability. Multivariate analysis and change-point detection were employed to identify environmental drivers governing nutrient variability and quantitatively derive the critical thresholds. Overall, total nitrogen, total phosphorus, and ammonium declined substantially, whereas nitrate (NO 3 - -N) continued to increase under legacy effects. For long-term outlet dynamics, landscape composition, landscape configuration, hydroclimate, and anthropogenic pressures collectively explained 75% of nutrient concentration variations, with landscape configuration alone accounting for 28%. Spatial heterogeneity among sub-watersheds was explained by the same four driver groups (67%), with a notable 22% shared contribution from landscape composition and anthropogenic pressures. Agricultural land-use metrics exhibited divergent trajectories when comparing long-term trends at the basin outlet against spatial configurations across the sub-watersheds. Over the study period, cropland coverage decreased from 15.1% to 11.9%, while impervious surfaces increased from 1.0% to 3.4% and became increasingly aggregated. Temporal patterns were mainly shaped by watershed-scale landscape reorganization and legacy nutrient transport, especially for nitrate, whereas spatial patterns reflected sub-watershed source heterogeneity. Nitrate has become the dominant nutrient pollutant. Threshold analysis indicated that NO 3 - -N surge risk increased when impervious surface cover exceeded ∼3% and aggregation exceeded ∼80, while sub-watersheds with agricultural land proportion >10% should be prioritized for non-point source pollution control. These findings provide a quantitative basis for watershed management prioritization and threshold-based regulation.