Seasonal and scale-dependent regulation of dissolved organic matter by anthropogenic activities in a karst river

Dissolved organic matter (DOM) is a major carbon reservoir in aquatic ecosystems. While there is strong evidence that human disturbances can alter DOM, its response to anthropogenic activities remains poorly understood, particularly in ecologically sensitive karst rivers. Here, we examined the seasonal variability of DOM across multiple spatial scales using excitation-emission matrix spectroscopy with parallel factor analysis. A random forest model was applied to assess the relative importance of anthropogenic activities on DOM. Cropland consistently governed DOM abundance, while the population density, distance of roads, density of factories, animal farming facilities, and plantations showed seasonally shifting influences on DOM. Scale-dependent patterns were also evident: distance of roads, density of factories, animal farming facilities, and plantations shaped variability of DOM at reach scale, while cropland dominated DOM at riparian scale, and population density, nighttime light and cropland constrained DOM at the catchment scale. These results demonstrate that human-influenced DOM dynamics in the karst river are both seasonally variable and scale-dependent, highlighting the importance of explicitly accounting for multi-scale anthropogenic pressures rather than relying solely on land-use-based approaches to DOM dynamics. Our findings are expected to provide valuable insights into riverine DOM dynamics driven by anthropogenic activities.