Mechanism of eutrophication in a semi-enclosed bay: Insight from dual water isotopes

Under the dual pressures of global change and human activities, semi-enclosed bays, as critical zones of land-sea interaction, are experiencing increasingly severe eutrophication and significant ecological risks. Current understanding of the causes of bay eutrophication predominantly focuses on environmental factors such as terrestrial nutrient inputs, often overlooking the regulatory role of hydrodynamic processes. In this study, we employed dual water isotopes (δD and δ¹⁸O), hydrographic and other physicochemical parameters from field observations across spring, summer, autumn, and winter of 2020 to explore the seasonal water dynamics and their interrelationship with eutrophication in Zhanjiang Bay, a semi-enclosed aquaculture bay in South China. The results showed that EI exhibited pronounced spatial and seasonal variability, and the upper bay consistently exhibited higher EI values than the lower bay. Summer season showed peak eutrophication among the four seasons, followed by the autumn, whereas eutrophication was lower in winter and spring. Dual isotope analysis combined with hydrographic data revealed a significant seasonal variation in the contribution of water masses, with high-salinity seawater intrusion dominating in summer and terrestrial freshwater dominating in autumn. Both of these processes can intensify salinity front within the bay, hindering the export of terrestrial nutrients and causing them to accumulate. By contrast, during the dry season (winter and spring), the reduction of terrestrial freshwater input and high-salinity seawater intrusion weakens the salinity gradient in the bay. This allows nutrients to diffuse toward the lower and outer bay, thereby decreasing the eutrophication load. The findings provide critical insights for coastal management and nutrient mitigation strategies in such tropical semi-enclosed bay systems.