Spatial scale effects of riparian zone landscape patterns on river water quality in the Dongting Lake Basin of China

The riparian zone functions as a critical interface for regulating river water quality. However, the extent and scale at which riparian zone landscape patterns influence river water quality remain debated, which hinders the formulation of targeted water pollution control strategies rooted in riparian zone landscape optimization. Quantifying the spatial scale effects of riparian zone landscape patterns on river water quality, and identifying key landscape patterns indicators and optimal riparian buffer sizes are essential for effectively improving water quality. Accordingly, this study selects the Dongting Lake basin as the study area—a region experiencing prolonged and compounded pressures from intensive agriculture and rapid urbanization. Using high-resolution land use data and contemporaneous surface water quality monitoring data from 2023, this study integrated the recursive feature elimination-random forest model with redundancy analysis to identify key riparian landscape indicators and determine optimal riparian buffer sizes for specific and composite water quality variables. The results demonstrated that total nitrogen (TN) was identified as the predominant pollutant, with elevated concentrations primarily distributed in the Dongting Lake area and Chang-Zhu-Tan urban agglomeration, which is fundamentally driven by the combined effects of agricultural non-point source pollution and urban emissions. Second, terrain characteristics serve as a common foundational factor influencing all water quality variables, whereas human activity-dominated landscape types and their spatial configurations exert scale-dependent and nonlinear regulatory effects on specific water quality variables across different riparian zone scales. Third, the most significant influence of riparian landscape patterns on dissolved oxygen (DO) occurs at a buffer scale of 25 m. In contrast, the optimal buffer scale for influencing TN, total phosphorus (TP), and the chemical oxygen demand by manganese method (COD_Mn) is 200 m, while for ammonia nitrogen (NH₃-N), the most effective buffer scale is 100 m. Forth, the explanatory power of the landscape pattern of riparian zones to the composite river water quality demonstrated a nonlinear trend of initially decreasing, then increasing, and finally decreasing. With the maximum explanatory power of 41.87% observed at the 200 m buffer zone, the landscape pattern of this riparian zone is thus identified as an effective predictor of composite river water quality variations. Fifth, the dominant landscape indicator influencing composite water quality shifted distinctly across scales: patch density of cultivated land at 25 m, contagion index at 100 m, patch density of construction land at 200 m, and shannon's diversity index at 800 m. The 50 m and 400 m buffers were both characterized by the edge density of construction land. These findings indicate that the landscape patterns of cultivated land and construction land are key factors affecting composite river water quality. This study contributes to a deeper understanding of the mechanisms by which landscape patterns influence river water quality across different spatial scales within riparian zones. It is recommended that the 200 m riparian zone be designated as a core management unit, with landscape optimization strategies focusing on spatially precise regulation. By systematically reducing the proportion of cultivated and construction land , appropriately increasing the proportion of forest land, and significantly expanding the interface where land, construction land, and forest land intersect, a hierarchical regulatory framework integrating "source reduction–process interception–end purification" can be established. This will fully leverage the optimized landscape pattern of riparian zones to systematically mitigate the deterioration of river water quality.