Over 90% of Nutrients Degrading Mar Menor Lagoon Come from Underground Flows

On July 8, 2026, a study by the Institute of Environmental Science and Technology at the Universitat Autònoma de Barcelona revealed that over 90% of key nutrients degrading the Mar Menor lagoon originate from recirculated underground flows. This finding challenges current restoration strategies that overlook this significant contamination pathway.

Understanding Nutrient Sources in Mar Menor

The Mar Menor, located in southeastern Spain, has faced severe ecological degradation, notably since 2016. The lagoon, once a popular tourist spot, has increasingly become known for episodes of eutrophication, which depletes oxygen in the water and leads to massive species mortality, such as the fish die-off in 2019.

Traditionally, pollution was attributed to agricultural fertilizers rich in phosphates and nitrates entering through the Albujón stream, the lagoon's only surface watercourse. However, the recent study utilized radium isotopes to trace groundwater types entering the lagoon and identified three main groundwater discharge processes:

• Freshwater groundwater discharge from irrigation.
• Continuous recirculation of saline lagoon water through the aquifer.
• Porewater exchange at the lagoon's sediment bottom.

The Role of Recirculated Flows

The study highlights that the latter two processes are the primary sources of ammonium, dissolved organic nitrogen, dissolved inorganic phosphorus, and dissolved silica in the Mar Menor. These processes operate on two temporal scales:

1. Large scale
2. Small scale: Water circulates over hours or days, mainly due to wave pumping and sediment-disturbing organisms.

In both scenarios, the water acts as a vector, transporting nutrients accumulated in sediments from decades of agricultural and mining activities back into the lagoon.

Implications for Environmental Management

The implications of these findings are critical for the management of the Mar Menor lagoon. The current Priority Actions Framework for its restoration focuses on recovering surrounding ecosystems and restoring natural stream channels but fails to address nutrient inputs from groundwater recirculation.

Researchers emphasize that incorporating this overlooked pathway into restoration strategies is essential for improving their effectiveness. During summer, small-scale recirculation particularly heightens phosphorus inputs, increasing the risk of algal blooms and hypoxia events, symptoms of the lagoon's ecological decline.

“Incorporating this pathway into restoration strategies will be essential to improve their effectiveness,” the researchers concluded.