Sentinel-2 Images Discover How Extraordinary Water Inputs Allow the Ephemeral Resurgence of Najas marina in a Shallow Hypertrophic Lagoon (Albufera of Valencia, Spain)

Anthropogenic activities represent a significant challenge to macrophyte conservation worldwide. Eutrophication, resulting from excessive nutrient inputs to aquatic ecosystems, is one of the main man-induced disturbances affecting the health of wetlands. Albufera of Valencia has experienced a hypertrophic and turbid state since the 1970s, with the consequent disappearance of macrophyte meadows and the predominance of phytoplankton. However, unique episodes of water clarity occurred in 2018 and 2022, leading to the reappearance of Myriophyllum spicatum and Najas marina, respectively. In the present study, the Normalized Difference Vegetation Index (NDVI) is used to monitor the emergence, growth, and disappearance of N. marina in 2022, as was previously done for M. spicatum. In November 2022, we obtained the maximum cover with 48.42 ha and began declining until March 2023. This methodology supports the potential of remote sensing in assessing the cover, density, and health of aquatic vegetation, while allowing us to examine the influence of water quality and quantity on this prominent phenomenon. After removing the outlier data, all variables except for suspended solids presented normal distribution. The results suggest that, by improving the water quality in the Albufera and maintaining an adequate ecological flow, managed by the competent authorities, the recovery of the macrophyte meadows that characterised this ecosystem more than five decades ago could be feasible.