Centennial-scale water level fluctuations driven by land-sea interactions: diatom evidence from a coastal lagoon in...

Regional sea-level changes pose challenges to coastal environmental management, yet long-term records on such changes remain scarce for many coastal areas. Coastal lagoons, as dynamic interfaces between terrestrial and marine systems, are particularly sensitive to sea-level fluctuations. In this study, we aimed to reconstruct historical water depth changes in Pinqing Lagoon, a typical subtropical coastal lagoon in South China, using sedimentary diatoms as bioindicators. Statistical analysis of surface diatom communities and environmental parameters identified water depth as the primary driver for diatom distribution. A weighted averaging partial least squares (WA-PLS) transfer function (R² = 0.82; RMSE = 0.09) was developed and applied to a 150-year sediment core PQ2 to reconstruct past water level changes. The reconstruction revealed three distinct phases: (1) water depth declined from 2.85 m in the 1860s to 1.06 m in the 1940s (mean 2.31 m), driven by sandspit development and progressive semi-enclosure of the lagoon; (2) a rapid rise of water level in the 1940s-2010s (mean 2.50 m) was consistent with regional sea-level trends; (3) the lagoon was being shallower due to siltation and pollution in post-2010 phase. This study provides the first quantitative diatom-based water depth reconstruction for coastal lagoons in South China. Beyond documenting local hydrological changes, our findings offer palaeoenvironmental insights into regional sea-level dynamics, historical land-sea interactions, and human-environment feedbacks over the past century. The results also support long-term environmental monitoring and provide valuable guidance for adaptive coastal management under the combined pressures of sea-level rise and human disturbance.