Understanding Global Salt Marsh Change with Earth Observation

Abstract:

Salt marshes are highly dynamic ecosystems that are tolerant of environmental extremes, adaptable to sea-level rise (SLR), an extensive blue carbon stock, and providers of coastal protection and water quality benefits. Regionally, a variety of salt marsh loss drivers have been identified, including eutrophication, herbivory, sea-level rise, storm events, mangrove intrusion, and reclamation. However, globally the magnitude and location of salt marsh change are unknown. Utilizing an existing global map of salt marsh extents, we used Google Earth Engine and the Landsat archive to identify change anomalies within the salt marsh and surrounding area. Salt marsh change was quantified by country and watershed for a baseline of 1984-1989 and a change detection period from 1990-2019. Additionally, a time series of salt marsh change with a reference period of 1984-1999 for four five-year periods 2000-2004, 2005-2009, 2010-2014, 2015-2019, was calculated. Recovery from disturbance is typical within salt marsh environments, as such, loss pixels were examined across time to determine recovery rates. In the conterminous United States (CONUS), 17% of the salt marsh area, which experienced a loss between 2000 and 2004, recovered to pre-disturbance levels by 2019. Water and terrestrial pixels were identified with the National Land Cover Database (NLCD) and analyzed to detect salt marsh migration. The country with the largest loss, from 1990-2019, was the USA, which lost 44,194 ha of salt marsh. Russian salt marshes experienced the second-largest loss of 31,494 ha. The uncertainty surrounding global salt marsh change both currently and into the future necessitates additional global monitoring and mapping of the ecosystem.