Physical Controls on Coastal Hypoxia: Insights from a Novel Dual-Model Framework

Effectively managing coastal hypoxia requires disentangling physical influences from biogeochemical factors. We developed a novel dual-model framework for the Pearl River Estuary (PRE) to isolate these mechanisms, pairing a coupled physical-biogeochemical model with a simplified oxygen model that uses satellite chlorophyll climatology to prescribe fixed biogeochemical rates. By comparing the responses of the two models to identical forcing perturbations, this approach quantitatively separates the physical controls exerted by stratification and substance transport on hypoxia. Results show that physical transport, by redistributing nutrients and organic matter, amplifies stratification-driven hypoxia changes, contributing 54-70% of the wind-induced hypoxic area variability off the PRE during the summer season. For river discharge variations, nutrient loading variability can either amplify or counteract stratification-driven hypoxia changes: when loads scale with discharge (e.g., nonpoint-source-dominated systems), hypoxia responses are amplified approximately 4-fold; when loads remain fixed (e.g., point-source-dominated systems), the expected stratification effects are counteracted. These findings advance our quantitative understanding of substance transport and its synergistic interaction with stratification, and how nutrient loading variability modulates these physical controls. Our framework offers a portable methodology for systematic hypoxia attribution in coastal systems; the satellite-derived model provides an accessible alternative for hypoxia simulation, particularly for large-scale, long-term applications.