Response of Bottom Salinity and Dissolved Oxygen to Spring-Neap Tidal Cycles in the Yangtze Estuary: Observations and Coupled Modeling Analysis

Estuaries are vital ecosystems with dense populations and developed economies. However, their complex hydrodynamics and intense human activities make it difficult to understand how the properties of the water vary spatiotemporally. As a typical zone of interaction between rivers and the ocean, the Yangtze Estuary suffers from severe hypoxia, and the spring-neap modulation of the properties of water masses remains unclear. Our research reveals significant spring-neap variability of Yangtze Estuary water masses with a distinct vertical bi-layered structure, contradicting traditional views that spring tide mixing enhances bottom DO. This study focuses on analyzing the coupled physical-biogeochemical mechanisms driving such variability, with a particular emphasis on tidal asymmetry and human activities.

Methodologically, we integrate long-term, high-frequency, in situ data (from a seabed cable system), satellite observations, and a high-resolution, coupled hydrodynamic-ecological model (SCHISM-CoSiNE). The model adopts unstructured grids of ≤1 km in key nearshore areas and optimized tidal parameterization in order to accurately capture the high-frequency spring-neap dynamics and biochemistry. Dynamical diagnoses and sensitivity experiments quantify the contributions of tidal asymmetry, advection, and human activities to water mass variations.

The key results demonstrate the distinct spring-neap variability of the water masses in the Yangtze Estuary with vertical structure: the salinity of the upper layer decreases by over 4 psu during spring tides. Reduced upper-layer salinity induces a shoreward pressure gradient that drives deep-ocean, high-salinity water towards the shore, increasing lower-layer salinity by up to 2 psu. Furthermore, satellite data confirm that there are corresponding variations in the concentrations of chlorophyll-a and particulate organic carbon in the sea surface. Interestingly, bottom dissolved oxygen (DO) levels decrease during spring tides, contrasting with traditional expectations. Dynamical diagnoses confirm that tidal current asymmetry (which modulates the ratio of freshwater to seawater) is the driving factor, and similar patterns are observed in the Mississippi River Estuary. Additionally, dams in the watershed alter variability hotspots by reducing sediment flux and causing tidal flat erosion. The coupled model effectively reproduces these characteristics, and improve the simulation accuracy of bottom DO and salinity.

This study advances the modelling of coastal systems by combining hydrology and ecology on a fine scale. Moreover, it establishes a scientific foundation for the ecological management of estuaries and provides guidance on assessing the impact of human activities on these ecosystems.