Cascading ecological impacts: from eutrophic coastal lagoon collapse to decline of Posidonia oceanica meadows in the Mediterranean.

Environmental degradation in coastal lagoons can propagate across ecosystem boundaries, triggering cascading impacts due to their strong connectivity with adjacent marine habitats. Understanding these cross-system dynamics is essential for effective coastal management. We investigate whether the ecological collapse of the Mar Menor coastal lagoon, following the 2015 eutrophication crisis, extended its impacts to the adjacent Mediterranean Sea, affecting an extensive Posidonia oceanica meadow. We characterized the physicochemical properties of Mediterranean waters influenced by lagoon outflow and analysed seagrass structural and physiological parameters (shoot density, meadow cover, shoot size, and nitrogen content and isotopic signatures) across 29 sites with historical data from 2004 onward. GIS tools were used to evaluate the spatial extent of lagoon water influence. Hypersaline, nutrient-rich waters extended up to 1.8 km offshore, affecting tens of hectares of seagrass habitat, as evidenced by δ¹⁵N values. Plants in this area showed signs of chronic stress, including nitrogen accumulation and reduced shoot size, likely resulting from hypersalinity, nutrient loading, and persistent light limitation. Structural degradation was also apparent, with shoot density and cover markedly lower than pre-crisis conditions. Long-term monitoring (2012-2020) confirmed that these changes followed the lagoon collapse and the extreme turbidity and reduced light availability associated with massive algal blooms. Our findings underscore the cascading impacts of anthropogenic pressures on interconnected coastal ecosystems. Given the widespread occurrence of eutrophication in Mediterranean lagoons, integrated management strategies that accounts for ecosystem connectivity are urgently needed to prevent irreversible impacts on valuable and slow recovering ecosystems such as P. oceanica meadows.