The Razdolnaya (Suifen) River is a transboundary river flowing from northeastern China into Amur Bay, Sea of Japan. It delivers substantial amounts of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) from an intensively cultivated watershed to the estuarine and coastal environment. Excessive nutrient inputs have contributed to eutrophication, recurrent phytoplankton blooms, and seasonal bottom-water hypoxia, causing ecological and economic impacts on regional fisheries and mariculture.

Recent observations indicate that the hydrochemical regime of the estuary is influenced by both anthropogenic nutrient inputs from the watershed and strong interannual variability in river discharge associated with monsoon precipitation and typhoon events. Understanding how these factors affect nutrient transport and phytoplankton productivity is essential for predicting ecosystem responses in this transboundary coastal system.

To investigate these processes, we analysed data from three hydrographic and hydrochemical surveys conducted under contrasting hydrological conditions (2018, 2020 and 2025). Measurements included temperature, salinity, dissolved oxygen, chlorophyll‑a and nutrient concentrations. Satellite-derived chlorophyll‑a data from the ESA Ocean Colour Climate Change Initiative (OC-CCI) product were used to reconstruct phytoplankton bloom dynamics. Additional nutrient measurements from the Chinese part of the watershed were obtained through collaboration with the Second Institute of Oceanography (SIO, MNR, China).

Results revealed strong spatial gradients in nutrient concentrations, along with phytoplankton blooms. Nutrient measurements from the Chinese part of the basin showed elevated DIN concentrations and, compared to the estuary, lower DIP concentrations, resulting in extremely high DIN:DIP ratios (78–161). In contrast, the most intense phytoplankton bloom was observed during the low-discharge year of 2025, suggesting an increasing role of internal nutrient recycling and favourable light conditions. Hypoxic waters were recorded in both estuarine and marine environments and were associated with strong stratification and restricted vertical mixing.

Our results demonstrate that the ecological state of Amur Bay is controlled by the interaction between transboundary nutrient inputs, river discharge variability, water-column stratification, and internal nutrient recycling. These findings highlight the importance of considering both watershed processes and estuarine dynamics when assessing eutrophication in coastal ecosystems.

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