Quantifying lateral carbon export from coastal wetlands using space-borne sensors and hydrodynamic modeling

Coastal wetlands store 20–30% of the Earth’s soil carbon yet occupy <8% of its surface. Despite their outsize role in the global carbon cycle, the magnitude of lateral carbon export from wetlands, such as mangroves, to the ocean, remains unknown. To fill this source-to-sink knowledge gap for coastal ecosystems, we created a coupled hydrodynamic–biogeochemical model (Telemac-AED2) for the Guayas Estuary in Ecuador. Initial states and boundary conditions were derived from remote sensing products—above ground biomass was estimated from TanDEM-X (2010), land use / ocean extent from Sentinel-2 (2015), and tidal harmonics from OSU-TPXO model inputs TOPEX/Poseidon (1992), Jason 1-3 (2001, 2008, 2016), and Sentinel-6 (2020). Modeled hydrodynamic conditions were validated against local tide gauges. However, tide gauge data is often sparse in remote areas. To overcome this limitation, we present one of the first ever test cases for hydrodynamic model validation using SWOT (2023). Modeled surface water elevations were validated at unprecedented vertical and spatial resolutions of up to 3 cm and 50 m, respectively, every 21 days or less. With below ground biomass, inundation extent, and the tidal pumping regime constrained, we were able to estimate dissolved inorganic carbon (DIC) fluxes using AED2. Subsequent carbon speciation and transport was modeled. These initial results were then compared and calibrated against data collected during previous carbon field campaigns (2018). Our high-resolution coupled numerical model, validated using novel remote sensing products, sheds light on the critical dynamics responsible for the ultimate fate of terrestrially sourced carbon into the ocean.