PNNL

Abstract

The hydrological environment of vegetated coastal ecosystems is directly influenced by rainfall and seawater flooding, which mediates biogeochemical processes within these areas. However, the specific effects of dynamic rainfall and flooding on oxidation-reduction conditions in these complex terrestrial-aquatic interfaces (TAIs) are not well understood, particularly when considering the ecological processes of above-ground plants. To address this gap, this study used integrated physics-based models, the Advanced Terrestrial Simulator and PFLOTRAN, to examine the effects of hydrological and ecological processes on biogeochemical reactions and exchange flux across a vertical transect spanning a coastal upland forest, saltmarsh, and open water boundary condition. Our numerical experiments showed that spatio-temporally dynamic surface-subsurface exchange fluxes and distribution of oxic subsurface zones within the TAIs are significantly influenced by hydrological processes. The concentration of oxygen along a flow path can be either diluted or intensified by rainwater infiltration and seawater inundation. The effect of vegetation on the hydro-biogeochemical process was also assessed using integrated modeling. Evapotranspiration accelerated the fluxes changed between the surface and subsurface, which in turn resulted in higher dissolved oxygen concentration. The transportation of decomposed organic carbon to the TAIs from the ground surface increased the concentration of dissolved organic carbon and disrupted the aerobic respiration and denitrification processes. The sensitivity of aerobic respiration rate was analyzed, showing the dominant role of rate constant in controlling the response of the reaction environment to hydrodynamic events. This work investigated multi-physics effects on reactions and fluxes by linking numerical simulations and site information. Local reaction properties and site-scale properties and their interactions were considered in simulations, which contribute to an enhanced predictive understanding of hydro-biogeochemical processes along the TAIs in three dimensions. Ultimately, the models used here are capable of coupling with larger-scale Earth Systems Models, an important step towards representing coastal ecosystems in global models.