Surface Water and Groundwater Interactions in Salt Marshes and Their Impact on Plant Ecology and Coastal Biogeochemistry

Abstract

Salt marshes are highly productive intertidal wetlands providing important ecological services for maintaining coastal biodiversity, buffering against oceanic storms, and acting as efficient carbon sinks. However, about half of these wetlands have been lost globally due to human activities and climate change. Inundated periodically by tidal water, salt marshes are subjected to strong surface water and groundwater interactions, which affect marsh plant growth and biogeochemical exchange with coastal water. This paper reviews the state of knowledge and current approaches to quantifying marsh surface water and groundwater interactions with a focus on porewater flow and associated soil conditions in connection with plant zonation as well as carbon, nutrients, and greenhouse gas fluxes. Porewater flow and solute transport in salt marshes are primarily driven by tides with moderate regulation by rainfall, evapotranspiration and sea level rise. Tidal fluctuations play a key role in plant zonation through alteration of soil aeration and salt transport, and drive the export of significant fluxes of carbon and nutrients to coastal water. Despite recent progress, major knowledge gaps remain. Previous studies focused on flows in creek‐perpendicular marsh sections and overlooked multi‐scale 3D behaviors. Understanding of marsh ecological‐hydrological links under combined influences of different forcing factors and boundary disturbances is lacking. Variations of surface water and groundwater temperatures affect porewater flow, soil conditions and biogeochemical exchanges, but the extent and underlying mechanisms remain unknown. We need to fill these knowledge gaps to advance understanding of salt marshes and thus enhance our ability to protect and restore them.

Impact of Coastal Marsh Eco‐Geomorphologic Change on Saltwater Intrusion Under Future Sea Level Rise

“…Precipitation and evaporation may also affect surface and subsurface saltwater transport and distribution (Geng et al., 2016; Morris, 1995; Payne, 2010; Wang et al., 2007; Werner & Simmons, 2009; Xin et al., 2022). We acknowledge that precipitation and evaporation would play an important role in changing water salinity on the land surface and in the upper subsurface zone.…”

A Review on Bank Retreat: Mechanisms, Observations, and Modeling

“…These studies are as yet limited to fluvial erosion. Other erosion processes typical of tidal environment, such as recursive water level oscillations, subsurface flow and wind waves, have been shown to affect soil pore‐water pressure (Cao et al., 2012; Fox et al., 2006; Francalanci et al., 2013; Xin et al., 2022) and therefore call for their implementation in numerical models.…”

Salt Transport Under Tide and Evaporation in a Subtropical Wetland: Field Monitoring and Numerical Simulation

“…Similarly, the hydrology and ecology in tidal wetlands are strongly affected by the tide‐influenced porewater flow and salt transport in the wetland subsurface (Hughes, 2016; Moffett et al., 2012; Ursino et al., 2004). However, subsurface hydrology in tidal wetlands, particularly the salt transport patterns, is understudied compared to beach environments (Guimond & Tamborski, 2021; Xin et al., 2022). The sand‐dominated beach is characterized by receiving fresh groundwater from inland and discharging it through a submarine zone above the seawater wedge (Figure 1b) (e.g., Burnett et al., 2006; Li et al., 1999; Robinson et al., 2007; Taniguchi et al., 2002).…”

“…Despite the previous numerical characterizations on intertidal wetland salinity, only tidal force was considered as the governing force to the porewater flow and salt transport in their systems. Salt transport and distribution could be affected by evaporation, which was however ignored in their models (Xin et al., 2022). Surface evaporation results in salt accumulation, particularly near the soil surface (Wang et al., 2007; Zhang et al., 2014).…”