Ecological Decline over a Decade in the Albufera of Valencia Coastal Lagoon (Spain): A Growing Environmental Hazard in a Hypertrophic System

The Albufera of Valencia is a shallow, hypertrophic Mediterranean coastal lagoon. Since the 1970s, the lagoon has undergone substantial ecological deterioration, marked by the decline of macrophyte beds and the predominance of phytoplankton. The objective of this study was to monitor key water quality variables over a 10-year period (2015–2025) to assess the persistence of eutrophication and the current ecological status of the lagoon. For this purpose, a remote sensing approach was applied using the Sentinel-2 constellation, complemented by newly developed algorithms specifically calibrated with ten years of in situ field data (2016–2025). This approach was employed to estimate variables such as the chlorophyll-a (Chl-a) concentration as an indicator of phytoplankton biomass, suspended solids (S.S.), and Secchi disk depth (ZSD). An analysis of temporal trends from 2017 to 2025 revealed a progressive system deterioration. The concentrations of both chlorophyll-a and suspended solids exhibited a statistically significant increasing trend (p < 0.01). Moreover, in line with these findings, water transparency (ZSD) decreased significantly (p < 0.001). Thus, there has been a progressive deterioration in the trophic status and ecological quality of the lagoon over the last decade, despite prior management interventions. The results from this research highlight the need to implement more effective conservation strategies, such as regulating nutrient inputs and increasing the water renewal time in the lagoon.