Application of remote sensing methods for monitoring extent, condition and blue carbon storage in salt marshes

Salt marshes are fragile coastal ecosystems that provide a wide range of ecosystem services, including carbon sequestration and storage. Despite the importance of salt marshes for blue carbon sequestration, few studies have been done on blue carbon monitoring using satellite remote sensing techniques. This study arose to fill this knowledge gap, and aimed to determine the amount of blue carbon sequestered in salt marshes by analyzing the particular case of the Pancas salt marsh (Tagus Estuary, Portugal). To achieve this, the extent and condition of the Pancas salt marsh were assessed using remote sensing methods, by establishing a methodology that effectively identified and validated its extent based on Sentinel-2 imagery from 2016 to 2022. This method involves generating a reference salt marsh extent map followed by mapping salt marsh extent through four Vegetation Indices (VI) from Sentinel-2 data to compare and thus validate the salt marsh extent based on the information provided by the satellite. Considering the extent of the salt marsh and the changes it has undergone over the 7 years studied, together with reference values for sequestered carbon in the salt marsh sediments, approximate values for sequestered blue carbon were determined. The results highlighted the validation of Normalised Difference Vegetation Index (NDVI) as the best-performing VI for this study and as a proxy for the condition of the salt marsh in terms of the spatial and temporal distribution of its aboveground biomass. It was also verified that the salt marsh expanded its frontal region by 0.22 km2 between 2016 and 2022, while certain areas of high marsh have suffered dieback. Concerning sequestered carbon, an increase of 508.2 tonnes since 2016 was quantified, corresponding to 66500 tonnes of blue carbon stored for 2022. Our study revealed the intrinsic relationship between the extent of a salt marsh and its carbon sequestration capacity highlighting the importance of future application of more remote sensing techniques, to emphasize the significant role of these ecosystems in carbon sequestration and climate change mitigation.