The very water of the Mar Menor is poisoning itself. A new study from the Institute of Environmental Science and Technology at the Universitat Autònoma de Barcelona (ICTA-UAB) in Spain has found that more than 90% of the key nutrients destroying this coastal lagoon, including ammonium, phosphorus, and silica, do not arrive from rivers or groundwater. Instead, the lagoon's own water seeps into the sediments, picks up decades of accumulated nutrients, and then re-emerges loaded with contaminants.
The hidden loop that keeps the lagoon sick
Researchers discovered a slow but relentless cycle. Water from the Mar Menor filters down through the seabed. Inside the sediment, it dissolves nutrients that have built up over years from agricultural runoff and other sources. That nutrient-rich water then rises back into the lagoon. This internal recycling mechanism, previously overlooked, now appears to be the dominant source of the pollution that has triggered massive fish kills and algae blooms in recent years.
The study directly challenges the current restoration plans for the Mar Menor. Those strategies focus on cutting off external sources of pollution, such as fertilizers washing in from nearby farms. But the new findings suggest that even if all outside contamination stopped tomorrow, the lagoon would continue to degrade itself from within. The sediments act like a slow-release battery of nutrients, constantly feeding the water with the very compounds that cause its ecological collapse.
Why this matters for the people of the region
The Mar Menor is Europe's largest saltwater lagoon, located on the coast of Murcia in southeastern Spain. It is a vital economic and ecological resource. Local fishing communities, tourism operators, and residents have watched the lagoon turn green with algae and suffer repeated die-offs of marine life. The regional government has invested heavily in measures to reduce nutrient inflows from agriculture and urban runoff. This new research suggests those efforts, while necessary, may not be enough to reverse the damage.
Lead researcher from ICTA-UAB explained that the internal nutrient loading is so significant that restoration plans must be redesigned to address the sediment problem directly. Without intervention, the lagoon could remain trapped in a state of chronic eutrophication for years or decades.
A quiet crisis beneath the surface
The study's conclusion is stark. The mechanism of water infiltrating and re-emerging from sediments has been largely ignored in environmental management. The Mar Menor is not just a victim of what flows into it. It has become its own worst polluter. The findings force scientists and policymakers to look below the surface, literally, to understand why the lagoon is not healing despite efforts to clean up its shores.
