Research papersTidal groundwater flow and its ecological effects in a brackish marsh at the mouth of a large sub-tropical river

•Numerical quantitative studies linking saturation and plant zonation were conducted in real marsh systems.•The subsurface saturated/unsaturated conditions could explain the observed plant zonation.•Seepage face is responsible for the boundary of riverward vegetated zone.

Soil saturation is thought to be an important control on plant zonation in intertidal wetlands, but quantitative studies linking saturation and plant zonation in real marsh systems are few. We conducted a combined field and modeling study to examine the potential links between groundwater flow and ecological zonation in a brackish marsh in the Yangtze River estuary. The intertidal marsh transect contained two plant zones (Phragmites australis and Spartina alterniflora) and an unvegetated mud flat adjacent to the estuary. Numerical simulations were conducted to quantify soil saturation index (SSI, ratio of saturated time to the whole observation period) for analyzing plant zonation. Models considered multiple factors, including aquifer stratification, anisotropy, evapotranspiration, surface topography (particularly slope breaks), seepage face formation and tidal loading. Simulations revealed that the average SSI over the rhizosphere depth from 0.0 m to 0.3 m increased abruptly by 11.4% at the interface of the two plant zones, and by another 10.5% at the riverward boundary of the vegetated zone. The significant increase of SSI was not caused by slope breaks but seepage face was responsible for the riverward increase of SSI. Given known differences between Phragmites australis and Spartina alterniflora in their tolerances to anoxic conditions, the subsurface saturated/unsaturated conditions quantified by SSI are most probably responsible for the observed plant zonation.