Accelerated sea level rise and Phragmites australis expansion alter tidal marsh water table dynamics

Abstract Groundwater hydrology plays an important role in coastal marsh biogeochemical function, in part because groundwater dynamics drive the zonation of macrophyte community distribution. Changes that occur over time, such as sea level rise and shifts in habitat structure are likely altering groundwater dynamics and eco-hydrological zonation. We examined tidal flooding and marsh water table dynamics in 1999 and 2019 and mapped shifts in plant distributions over time, at Piermont Marsh, a brackish tidal marsh located along the Hudson River Estuary near New York City. We found evidence that the marsh surface was flooded more frequently in 2019 than 1999, and that tides were propagating further into the marsh in 2019, although marsh surface elevation gains were largely matching that of sea level rise. The changes in groundwater hydrology that we observed are likely due to the high tide raising at a rate that is greater than that of mean sea level, as well as potential increases in soil porosity, hydraulic conductivity, and evapotranspiration from Phragmites australis expansion. These results suggest that the interactive effects of climate change and shifts in plant community composition are impacting the tidal marsh water table at Piermont Marsh. These effects may exacerbate or mitigate impacts of climate change on tidal marsh vegetation. Considering these results, we recommend that habitat models consider tidal range expansion and groundwater hydrology as metrics when predicting the impact of sea level rise on marsh resilience.