Assessing Greenhouse Gas Structural and Functional Resilience of Freshwater Coastal Wetlands Subject to Persistent Saltwater Intrusion Events

Authors

Jorge Villa* (jorge.villa@louisiana.edu), Diana Taj

Institutions

University of Louisiana–Lafayette, LA

Abstract

Greenhouse gas production and fluxes in coastal wetlands are controlled by complex interacting hydrological and ecological processes whose dynamics are, in turn, modified by saltwater intrusion (SWI) events. In suburban and urban transition zones of many coastal areas, the floodwalls and levee systems built to protect infrastructure and urban development intensify the SWI events resulting from floods during storm surges. When the flood waves travel inland, they meet these barriers, and saltwater can remain stagnant for prolonged periods, increasing the exposure of adjacent coastal wetlands to salt stress (i.e., persistent SWI events). These stress conditions induced by SWI are rarely represented in biogeochemical models, limiting the capacity to predict and assess the long-term effects of SWI on coastal greenhouse gas production and fluxes. Moreover, significant knowledge gaps exist in how the ecosystem reorganizes after SWI disturbance.

Specifically, what trajectories do biological components (ecosystem structure), greenhouse gas production, and emission (ecosystem function) follow after SWI? Also, it is not clear what the relevant scales are for evaluating these trajectories. Using an ecohydrological patch approach defined by plant functional types and water levels, this early career proposal aims to better understand how ecosystem structure and function linked to greenhouse gas fluxes in freshwater coastal wetlands affected by the built environment are influenced by SWI in the short- (days to weeks) and long- (years to decades) term. This project evaluates carbon dioxide (CO₂) and methane (CH₄) pools and fluxes before, during, and after simulated SWI events covering a wide range of salt exposure (salinity concentration, duration, and frequency).

The measurements will be conducted with a multisetting sampling approach composed of controlled mesocosm in greenhouses and experimental wetland ecosystem units. It will be complemented with time-for-space replacements in coastal wetlands with a history of persistent SWI. Data collected will be used to produce and incorporate an explicit salinity-dependent function into the CH₄ biogeochemistry module integrated into the E3SM Land Model (ELM v1). Early testing on experimental wetland ecosystem units with simulated SWI events of ~2 psu salinity concentrations and durations of one, three, and five days indicate that CO2 and CH4 fluxes were affected after five days in cattail and maidencaine patches. CH4 fluxes in maidencaine patches were also affected by SWI events of three days.