Massive increase of intertidal seagrass coverage in a large estuarine system revealed by four decades of Landsat imagery

The ecosystem services and functions of seagrass meadows are indisputable, and knowledge about their coverage is critical for coastal managers worldwide. In this study, the surface area coverage of the foundation species Zostera marina L. (eelgrass) was investigated in four contrasting subregions of the Estuary and Gulf of Saint Lawrence (EGSL), eastern Canada. The meadows in all subregions mainly occupy intertidal zones. Our analysis covered broad spatial (meters to kilometers) and temporal (annual to decadal) scales and revealed unprecedented insights at a local and regional context. We processed surface reflectance products of the Landsat archive through the Google Earth Engine cloud computing platform. The processing scheme only considered emerged areas of intertidal zones from imagery acquired at the lowest tide levels because of inherent limitations imposed by water clarity and the poor radiometric quality for water applications of the early Landsat sensors. The polygons classified as eelgrass encompassed at least 25% coverage of eelgrass for each patch, and the classification scheme showed a very good agreement with coastal ecosystem habitats maps generated by photointerpretation and field validation for the period between 2015 and 2019, with an overall accuracy of approximately 94%. From the 40-year period analyzed (1984–2023), the meadows’ surface area dramatically increased 10- (from approx. 0.3 to 2.5 km 2 ) to 21-fold (from approx. 0.8 to 16.7 km 2 ). The percentage of the intertidal area occupied by eelgrass meadows varied by subregion, ranging between 17% and 82%. In some subregions, meadows expanded landward. Some meadows experienced relatively short-term losses (interannual scale) in three subregions, although these losses differed in their timing. We propose several hypotheses involving hydrodynamic, sedimentological, drift ice and climatic processes that could explain long- and short-term variability of the meadow coverage. However, this complex relationship remains to be investigated. Overall, while showing suitable habitats for eelgrass colonization, this study also revealed the EGSL tidal flats as potentially important areas of biodiversity, carbon storage, and coastal protection against erosion. • The seagrass meadow coverage of a large estuarine system was investigated • A substantial increase in area coverage was observed over 40 years of analysis • A landward growing pattern of the meadows was noticeable in some subregions • Short-term (interannual) losses of some meadows occurred differently between subregions