Parsimonious Mechanistic Modelling of Dam Effects on Coupled River Algae-Nutrient Dynamics

Algae play a core role in sustaining river ecosystem health as primary producers, but excessive nutrient inputs can trigger uncontrolled growth and algal blooms, posing serious ecological risks. Hence, identifying vulnerable river segments is of importance to assess eutrophication risk and prioritize management actions, which in turn requires representing algal spatial distribution across entire river networks rather than isolated reaches. Nonetheless, achieving such network-scale characterization is particularly challenging in regulated river systems, where hydraulic structures modify flow regimes, disrupt longitudinal connectivity, and alter nutrient transport, thereby reshaping spatial patterns of algal communities. To address this challenge, we develop a parsimonious mechanistic model that explicitly reflects dam-induced hydrological alterations while retaining a minimal set of state variables and parameters. The model builds upon the Coupled Complex Algal–Nutrient Dynamics (CⁿANDY) model, a parsimonious process-based model that simulates interactions between pelagic and benthic algae competing for a single limiting nutrient and light along river networks. We extend the original CⁿANDY model by incorporating additional modules describing the physical effects of dams and associated reservoir characteristics, resulting in the CⁿANDY-dam model, which enables prediction of algal dynamics under hydraulic regulation at the river-network scale. As a baseline validation step, the original model is first applied to the Gyeongan River watershed (~506 km²), an unregulated sub-basin of the Han River, the largest river basin in South Korea, to evaluate model behavior under flow conditions unaffected by upstream regulation. River network structure, including Horton–Strahler stream order and hydraulic geometry, is derived from geomorphic observations. To approximate steady-state conditions, monthly mean runoff (March-November) and phosphorus (P) inputs are estimated using observations from 2017-2022, with non-point source P loads derived from land-use data and point source P loads obtained from wastewater treatment plant records. The original model reproduces stream-order-dependent patterns of algal dominance, with benthic algae prevailing in low-order streams and pelagic algae dominating in higher-order reaches, driven by differences in hydraulic geometry features and nutrient uptake. Building on this validated model, the extended CⁿANDY-dam model is applied to a regulated sub-basin of the Han River to demonstrate its workability under hydraulic regulation. The application of the CⁿANDY-dam model is expected to confirm its capacity to represent algal–nutrient dynamics under hydraulic regulation at the river-network scale.

Acknowledgements
This work was supported by the Creative-Pioneering Researchers Program through Seoul National University and by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. RS-2025-00523350).