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

Authors

Margaret Zimmer¹* (margaret.zimmer@ucsc.edu), 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

Institutions

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

URLs

• http://mzimmer.weebly.com/nitrates.html

Abstract

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.