The Push and Pull at the Land–Sea Interface: How Bidirectional Freshwater and Marine Inputs Drive Hydro-Biogeochemical Function in Salt Marsh Sediments


Margaret Zimmer1* (, Jasper Romero1, Adam Haynes1, Maya Montalvo1, Emilio Grande1, Corianne Tatariw2, Erin Seybold3, Anna Braswell4, Ate Visser5, Bhavna Arora6, Dipankar Dwivedi6, Hans Carlson6, Alice Kleinhuizen1, Michael Wilshire1


1University of California–Santa Cruz, CA; 2University of Alabama, Tuscaloosa, AL; 3Kansas Geological Survey,University of Kansas–Lawrence, KS; 4University of Florida, Gainesville, FL; 5Lawrence Livermore National Laboratory, Livermore, CA; 6Lawrence Berkeley National Laboratory, Berkeley, CA



Salt marshes exist at the terrestrial–aquatic interface between watersheds and adjacent marine environments. These systems receive hydrologic inputs at a range of time scales, from daily tidal cycles to seasonally elevated groundwater. These bidirectional water sources can cause rapid as well as seasonal changes in a range of environmental conditions in marsh sediments, including saturation, salinity, and redox conditions. However, there is limited understanding of how these time-variable environmental conditions drive the hot moments, or temporal dynamics of nutrient processing, within marshes. To address this knowledge gap, researchers instrumented a 25 m transect along a representative salt marsh platform at the Elkhorn Slough National Estuarine Research Reserve in California. The team installed variable-depth redox probes, nested piezometers, and a field-deployable spectrophotometer at lower, mid, and upper marsh positions to allow for characterization of subsurface hydrologic cycling and biogeochemical behavior at a high frequency. Researchers also conducted seasonal sediment incubation experiments to quantify nitrogen processing rates as well as monthly vegetation surveys, monthly pore water sampling campaigns, and subsurface sediment characterization. The team found that biogeochemical behavior ranged as a function of time scale. Findings suggest that intra-annual changes in source water contributions across the marsh result in functional zonation, where lower marsh position functions may be regulated by tidal flushing and upper marsh position functions may be regulated by freshwater contributions. A reactive transport model is being developed to extend process understanding into additional scenarios that investigate how changing hydrologic conditions (e.g., sea level rise, precipitation extremes) may affect marsh sediment biogeochemistry.