Local freshwater fluxes determine salinity variations of the Bohai Sea: Based on 60 years of observations

Salinity, a fundamental characteristic of seawater, significantly influences the ocean's physicochemical properties and directly affects marine biodiversity and its spatial distribution. The semi-enclosed Bohai Sea is home to diverse marine organisms and serves as a critical area for human mariculture. Salinity variations in this region are crucial for the stability of the ecosystem and the sustainability of the marine economy. However, significant discrepancies have been observed between the practical salinity measurements of the Bohai Sea in winter 2007 and that recorded in widely used national and international marine atlases prior to 2000. To identify these discrepancies, this study systematically investigates the salinity variations and the regulatory mechanisms in the Bohai Sea. First, we constructed a climatological salinity dataset with a broader temporal scope based on in-situ salinity data from 1960 to 2020, confirming that the climatological salinity field of the Bohai Sea did not undergo a significant shift, but exhibited phase changes. Subsequently, using the principles of conservation of mass and salinity, we developed a box model to analyze annual salinity changes in the Bohai Sea. This model was optimized using in-situ hydrological measurements and reanalysis data spanning over 60 years, successfully revealing the central regulatory role of local freshwater fluxes in annual salinity changes in the Bohai Sea. Furthermore, we elucidated that the anomalously high salinity in the Bohai Sea in the winter of 2007 resulted from the cumulative effect of several consecutive years of negative freshwater fluxes. The results of this study not only enhance our understanding and prediction of salinity changes in semi-enclosed seas, providing a scientific basis for ecosystem protection and management, but also offer valuable insights for studying long-term salinity changes in other semi-enclosed seas.