Evaluation of long-term estuarine vegetation changes through Landsat imagery

Salt marshes support estuarine biodiversity and provide ecosystem services; however, their general decay is being observed worldwide, in large part due to land reclamation. Accordingly, there is a growing concern about salt marsh preservation status having in mind the promotion of effective management decisions towards their conservation and restoration. Satellite imagery offers the opportunity to monitor land surface dynamics, constituting a fundamental information source for wetland monitoring. This study analyses spatial and temporal vegetation changes within Ria de Aveiro coastal lagoon between 1984 and 2017, by processing and analyzing TM and ETM+ Landsat imagery. A database consisting of 264 cloud-free images was collected and analyzed. The Normalized Difference Water Index was computed using the remote surface reflectance and was then used to distinguish land from water and to estimate the flooded lagoon area. Moreover, the tidal state was determined for each image from a tidal elevation record monitored at the lagoon entrance. Subsequently, four vegetation indices (VI) were computed and their spatial variability in the lagoon area uncovered by water was assessed. Spatially averaged spectral indices were also statistically analyzed and seasonal variations and interannual trends evaluated. Results show that the intertidal area increased, and VI values decreased indicating a possible reduction in the Chlorophyll content and suggesting that the new intertidal regions are mostly covered by mud. The spatially averaged VI values show seasonal patterns, with peaks in spring and summer, coinciding with high biomass productivity periods. The largest flooded area and VI modifications occurred after 1999, suggesting that changes are associated with dredging activities performed in the main lagoon channels. This study reinforced the potential of Landsat archives to monitor coastal wetlands, highlighting their importance for coastal managers of threatened systems, and therefore helping to define management strategies about the ecological conservation of estuarine systems.