Soil water salinity indicates vertical hydrological connectivity in intertidal wetlands: A case study in the Yellow River Delta, China.

While vertical hydrological connectivity (VHC) plays a critical role in material exchange and ecosystem functionality of intertidal wetlands, it remains poorly understood compared to horizontal connectivity. This knowledge gap hinders the development of effective restoration strategies for degraded coastal wetlands. Here, we used isotopic tracers (δH and δO) to characterize VHC patterns in an intertidal wetland of the Yellow River Delta, China. Our results revealed that the vertical patterns of soil water salinity are controlled by vertical hydrological processes including seawater-precipitation mixing and evaporation, and exhibit a strong correlation with the wetland geomorphological zonation along the elevation gradient. According to our understanding that VHC conditions lead to the heterogeneity of vertical soil salinity distribution, a salinity-based index for assessing VHC is developed. This VHC index effectively indicates elevation-dependent ecogeomorphological zonation, highlighting the role of VHC in regulating the interactions between hydrological and ecological processes. Topography and eco-geomorphic feedbacks greatly affect the spatial differentiations of VHC in the intertidal wetland system. This study provides the first evidence that soil water salinity serves as a cost-effective indicator for quantifying VHC. Optimizing VHC through targeted geomorphic engineering and vegetation restoration could enhance the resilience of degraded coastal wetlands.