Quantifying the effect of changes in land use and land cover on the carbon balance of coastal wetlands in the Mississippi River Delta

Background/Question/Methods

Coastal wetlands, such as marshes, mangroves and seagrasses, store significant amounts of “blue carbon.” As such, they represent important opportunities for natural climate solutions (NCS): conservation, restoration and other land management actions that can serve to increase or protect ecosystem carbon storage and thus help mitigate global greenhouse gas emissions. The carbon dynamics of coastal wetlands, however, are generally poorly understood, making it difficult to assess their current and future potential for NCS. Furthermore, many coastal wetlands, and their associated below-ground carbon stores, are also extremely vulnerable to changes in climate and land use/land cover (LULC). In this study, we examined how historical changes in LULC, including both natural and anthropogenic disturbances, have impacted the carbon balance of coastal wetlands in the Mississippi River Delta. This region represents 37% of the tidal marshes (17,000 km2) in the conterminous U.S. and has seen some of the highest rates of coastal wetland loss in the nation over the past several decades. These changes are due to both climate-related disturbances, such as hurricanes and flooding, and LULC changes, such as agricultural development and urbanization.

Results/Conclusions

Combining remotely sensed LULC data for the past 20 years with field measurements of carbon stocks and fluxes, we developed an integrated, spatially explicit simulation model of LULC change and stock-flow carbon dynamics for this coastal wetland ecosystem. We then used this model to estimate the historical net ecosystem carbon balance over the past two decades, including: (1) how much carbon has transferred between wetlands, the atmosphere, and the estuary; and (2) which LULC transitions have accounted for the largest fluxes of carbon. Our results indicate that there has been a loss of wetlands with a corresponding decrease in the rate of accumulation of belowground carbon over time, due primarily to transitions from coastal wetlands to open water. However, estimates of belowground carbon are extremely sensitive to assumptions about the release of soil carbon when a coastal wetland converts to open water. This study demonstrates how remotely sensed LULC datasets can be combined with targeted field measurements of carbon stocks and fluxes to quantify the present and future NCS potential of coastal wetlands. Next steps will be to apply this approach to assess the NCS potential for all coastal wetlands in the conterminous U.S.