Assessing salt marsh extent and condition changes with 35 years of Landsat imagery: Tagus Estuary case study

Abstract Reliable information on salt marsh extent and condition is crucial to promote effective management strategies towards their maintenance and recovery. Most of previous studies on salt marsh extent assessment used Landsat-derived Normalized Difference Vegetation Index (NDVI), being limited the current knowledge about the performance of other Vegetation Indices (VI). Based on Landsat imagery this study proposes a new methodology to map salt marsh extent in estuarine systems by combining Normalized Difference Water Index (NDWI) and VI, exploring their performance when using different VI. Moreover, it aims to assess the extent and condition changes between 1984 and 2018 in two salt marshes (Pancas and Corroios) located within Tagus Estuary. The VI best-performing salt marsh extent was determined and the methodology applied to assess salt marsh extent changes. Condition change was investigated by statistically analyzing spatially averaged VI over salt marsh extent change regions. Results demonstrated that NDWI and VI combined can be used to efficiently map the marsh extent and NDVI was the VI with the best performance. Corroios revealed mostly stable without noticeable changes in its extent and condition. Oppositely, Pancas registered a continuous seaward progression at a mean rate of 3 ha/year since 1984, while restricted upper regions dieback after 2004–2005, likely due to high soil salinity conditions. In general, NDVI performs better salt marsh extent, but others VI corrected to minimize soil effects perform better when tidal flats are entirely, or almost entirely, exposed, opening perspectives to the development of new methods combining the use of different VI to optimize salt marsh extent detection.