Spatial-temporal distribution differences of nitrogen and phosphorus nutrients and potential eutrophication assessment in the Pearl River Estuary.

Abstract

Estuaries serve as critical land-sea interfaces with high ecological and socioeconomic value, yet they are increasingly threatened by excessive anthropogenic nutrient inputs due to rapid urbanization and industrialization. These inputs have intensified eutrophication, harmful algal blooms, and hypoxia in estuarine systems globally. The Pearl River Estuary (PRE), southern China's largest estuary, receives substantial nutrient loads from both riverine discharge and intense human activities in the highly urbanized Pearl River Delta. Using long-term monitoring data (2017-2024), this study systematically investigated the spatiotemporal distributions of dissolved inorganic nitrogen (DIN), reactive phosphate (PO₄-P), and dissolved oxygen (DO) in the PRE and assessed the eutrophication status. Results show a persistent nutrient regime characterized by nitrogen enrichment and phosphorus limitation. DIN distribution shows a strong correlation with river discharge, which appears to be an important factor controlling its large-scale land-sea decreasing gradient, with no pronounced east-west differentiation. In contrast, PO₄-P showed an "east-high, west-low" pattern, linked to coastal anthropogenic inputs and regional hydrodynamics. DO dynamics are regulated by phytoplankton photosynthesis, organic matter mineralization, water temperature, and freshwater runoff, resulting in seasonal and spatial variability. Heterogeneity was observed in nutrient limitation and eutrophication status across seasons and regions, where eutrophication in the inner estuary has been alleviated yet remains severe. This study systematically synthesizes eight years of spatiotemporal dynamics of nutrients and DO across the PRE. The integrated findings provide scientific support for developing regionally differentiated nutrient management frameworks, and have important guiding implications for alleviating eutrophication and seasonal hypoxia in large estuary ecosystems.