Network structure, wastewater treatment plants and land cover control algal and nutrient dynamics in rivers

Understanding eutrophication at the river network scale is essential for sustainable and effective river water-quality management, yet most assessments have focused on local drivers and neglected the inevitable interactions between river network structure and spatial nutrient loading across stream orders. To address the knowledge gap, this study investigates how catchment form, land-cover distribution, and nutrient loads discharged from wastewater treatment plants (WWTPs) affect the coupled dynamics of nutrient and algal communities across river networks. As the proof-of-concept activity, we employ artificial river networks with contrasting geometries (elongated, rectangular, and square) but identical total area, generated on the basis of the Optimal Channel Network theory and constrained to realistic morphometric properties. These networks are coupled with archetypical land-cover configurations that impose equal total phosphorus (P) loads but differ in spatial organization, including homogeneous, random, and upstream- or downstream-clustered anthropogenic inputs from urban and agricultural areas as well as point source discharge from WWTPs. The coupled dynamics of nutrient P and algae are elaborated through each configuration-based simulations of the CⁿANDY model, which is a parsimonious mechanistic river-network-scale model for pelagic and benthic algae competing for light and phosphorus under steady hydrologic conditions. By comparing the different loading configurations, the study examines the implications of focusing management measures on headwaters versus downstream reaches, as well as the potential role of WWTP relocation, and end-of-pipe solutions. Overall, our findings are expected to support a more network-aware perspective on eutrophication assessment and management at the entire catchment scale.