Process-based Blue Carbon Mapping in the Northeastern US Tidal Marshes Using Optical Remote Sensing

Tidal wetlands store significant amounts of soil organic carbon (SOC). However, variability in SOC within and between sites leads to large uncertainty of global carbon stocks and sequestration rates. SOC is affected by tidal inundation, vegetation productivity, and sediment supply. This mechanism allows us to use these variables that can potentially be derived from optical satellites to map SOC. We measured sediment carbon content at 410 tidal marsh sampling locations and related derived values to remotely sensed optical properties. We used 1 m resolution multispectral WorldView-3 imagery to test the optimal spatial resolution for mapping SOC and found 10~30m is optimal. Thereafter, we assemble ~20,000 images from Landsat 5-8 and Sentinel 2 using Google Earth Engine to examine the influence of vegetation phenology and tidal conditions (tidal hour, spring/neap tide, flood/ebb tide). Site-specific sediment supply, sea level rise (SLR), tidal range, temperature, and marsh vulnerability indices (UVVR) were also explored to understand the major control of SOC. Finally, 10m resolution SOC map and the total carbon stocks in top 0-100cm soils were reported.

The study found 1) either water index or vegetation index is able to capture inundation and vegetation characteristics. The use of the indices should consider the seasonal variability of the indices controlled by vegetation phenology and the hourly or daily variability driven by tidal conditions. 2) the importance of vegetation and inundation on predicting SOC varies across geomorphic settings. Both inundation and vegetation are significant factors on carbon stocks for barrier marshes, but inundation is the dominant factor for fluvial marshes. Tidal range is the first order control of SOC in barrier marshes, while sediment supply is the major control in fluvial marshes. 3) site-specific LOI/BD ratio is a potential indicator of marsh vulnerability to SLR. The lower value indicates that the marsh growth is limited by vegetation productivity; higher value indicates that the marsh growth is limited by lack of sediment supply; and median value means the vegetation productivity and sedimentation reach equilibrium. The spatial distribution of SOC, regional carbon stocks, and restoration implication will be further discussed during the meeting.