A new study supported by the Northeast CASC provides the most precise estimate to date of carbon storage in salt marshes along the U.S. Northeast coastline. It also provides the first 10-meter resolution map of “blue carbon” (carbon stored in marine and coastal ecosystems) in the region.  

Salt marshes act as long-term carbon sinks, continuously accumulating carbon as tides and storms deposit sediment into dense marsh grasses. Unlike forests, which have storage limits, salt marshes can expand vertically, increasing their ability to trap carbon over time. In other words, they have no upper storage limit. However, the natural spatial variability within marshes makes it difficult to estimate how much total carbon is actually stored or ongoing sequestration rates. Traditional soil sampling methods are highly accurate but costly and time-consuming, while satellite methods cover more ground but measure indicators, like water depth and vegetation density, rather than carbon directly. Adding to this complexity, within a marsh, vegetation density and water depth also vary seasonally and with tidal cycles. 

The researchers tackled these challenges by combining both approaches. They used soil samples from 15 sites across five states, spanning Long Island Sound to the Gulf of Maine, and the “Normalized Difference Water Index” (NDWI) from satellite images taken across seasons and tidal levels. After comparing soil carbon measurements to satellite estimates, they found that satellite images most accurately reflect soil properties when taken at high tide. This insight allowed them to select the most reliable satellite images to refine carbon estimates across larger coastlines – an important advance when extensive soil sampling is not feasible. The study also provides insights for soil formation and carbon storage in different types of marshes. 

This study offers a scalable method for estimating blue carbon worldwide but also warns that environmental degradation of salt marshes could release massive amounts of stored carbon back into the atmosphere.  

This research was supported by the Northeast CASC Project “Effects of Urban Coastal Armoring on Salt Marsh Sediment Supplies and Resilience to Climate Change.”