Connecting hydrology, biology, and geochemistry in a coastal wetland: feedbacks between ecosystem processes toward predictive understanding

Abstract

Coastal wetlands are globally important reservoirs of carbon and the critical point of interaction between land and oceans where carbon is cycled and storm surges dissipated. Despite their importance, our understanding of coastal wetlands is predominantly two-dimensional, neglecting consideration of groundwater dynamics on ecosystem processes, services, and response. This limits our capacity to predict feedbacks with climate change and sea-level rise, value marsh ecosystem services related to carbon dynamics, and manage human activities in these fragile landscapes. This dissertation focuses on unraveling hydrological-biogeochemical-human interactions in a temperate coastal wetland, advancing our three-dimensional knowledge of linked hydrological-biogeochemical factors that influence carbon fluxes, improving predictive capacity under changing hydrologic conditions, and enhancing management strategies for coastal agricultural land. ☐ Field studies in a Mid-Atlantic coastal wetland uncovered interactions between crab burrowing activity, groundwater flow and exchange, and carbon fluxes. Through biophysical and biogeochemical interactions, I showed that crab burrowing activity enhances carbon loss from coastal wetlands. These processes are important to consider as burrowing marsh crabs expand their habitat range due to a warming climate, decreasing the carbon sequestration capacity of coastal wetlands globally. ☐ Field results also provided a larger-scale picture of hydrological-biogeochemical interactions, showing that subsurface redox potential mirrors the groundwater table elevation and oscillation pattern, and thus impacts both vegetation and carbon accumulation. These links enabled me to develop and use physics-based models to predict coastal wetland hydrological response to sea-level rise and extrapolate to changes in carbon accumulation. Results showed a decrease in coastal wetland area and, in turn, carbon accumulation with sea-level rise. However, what was unique was that the magnitude of decline was related not only to relative sea-level rise but also inland groundwater levels, highlighting the importance of regional hydrogeology and geology in the fate of coastal wetlands. ☐ Bordering many coastal wetlands in the Mid-Atlantic are agricultural fields and the communities reliant on their productivity, yet many of these coastlines are also threatened by storm surge inundation. Two-dimensional, variable-density, coupled surface-subsurface hydrological models were used to explore how coastal wetland migration impacts the extent and volume of surface and subsurface salinization due to storm surges. Results showed a decrease in overland flood extent and subsurface salinized volume with greater marsh migration with implications for crop yield. This study demonstrated that facilitated marsh migration into agricultural fields may be an effective management strategy to maintain both crop yield and water quality under storm surge conditions. ☐ Through these projects, I have shown the importance of hydrology in present-day and future ecosystem function, services, and management. Work from this dissertation fills gaps in our understanding of the hydrologic impacts on marsh biogeochemistry and identifies specific factors controlling carbon dynamics and fluxes between marshes and the coastal ocean. This research helps managers to better assess the value of coastal ecosystem services now and in the future and to justify the need for conservation, restoration, and management activities in coastal wetlands.