Fauna of Spain's Mar Menor is fleeing due to the lack of oxygen in its waters

Wildlife in Spain’s Mar Menor is abandoning the southern basin as oxygen at the bottom collapses. Near zero oxygen leaves fish and invertebrates with one option, move up or die.

The trigger was a post-storm split in the water column after heavy rain from Storm Alice. Layering locked the deepest water away from fresh oxygen long enough to set off a chain reaction.

What an oxygen crash does

When anoxia, absence of dissolved oxygen, settles over bottom waters, animals that live close to the seabed soon struggle. Many flee upward where oxygen still lingers and light reaches.

Field teams recorded the first signs on October 23, 2025, when animals were seen rising from the bottom to escape the dead zone, a finding summarized in the project ‘s emergency report. 

The work was led by Dr. Juan Manuel Ruiz at the IEO-CSIC’s Murcia Oceanographic Center. His research focuses on seagrass ecology and coastal-lagoon restoration.

By October 19, oxygen in the south had already fallen below 2 mg/L, a level linked with severe stress in coastal species. Marine managers often use 2 mg/L as a critical hypoxia threshold, according to federal criteria.

How a storm set the stage

A fresh lens of rainwater formed at the surface and sat on top of saltier, heavier water below. This setup is known as thermohaline stratification, layered by temperature and salt that limits mixing.

As bottom water stopped exchanging with the surface, oxygen was consumed faster than it could be replaced.

The report noted that clearly anoxic conditions had developed in the deep layer linked to thermohaline stratification, marking the point when the lowest waters entered a full oxygen collapse.

The hardest hit area now sits between Varón Island and Ciervo Island in the south. The anoxic layer thinned in recent days from roughly 3.3 feet to about 1.6 feet, a small but notable improvement if it continues.

Scientists also flagged the long-standing “white patch” along the central west coast. Earlier work showed this pale, turbid zone is made of suspended calcite crystals, a classic whiting event. 

Signs in the Mar Menor water

A surge in chlorophyll a, which is a pigment used to track algae growth, peaked on October 16 in the central and southern lagoon. Values then declined but stayed high enough to keep the water green and murky.

Bottom-dwelling life, known as benthic, living on or in the seabed, suffers when light cannot reach the plants that stabilize the floor. The team warned that light at the bottom was close to limiting levels for bottom vegetation in several sites.

Underwater irradiance, light energy reaching a surface, matters because seagrasses need a share of the surface light to survive and grow. Many seagrasses struggle when light at the bottom drops below roughly 5 to 11 percent of the surface.

Turbidity across the lagoon showed a general downward trend after the storm. Even so, southern waters still looked cloudier than the rest, which further cuts the light that reaches the bottom.

What history tells us

Years of eutrophication, nutrient overload that fuels algae blooms, have primed Mar Menor for fast blooms and sudden oxygen loss. Modeling and monitoring studies describe how sustained runoff makes the ecosystem brittle when storms hit.

Storm Alice did not invent the problem. Major collapses in 2016, 2019, and 2021 left scars in seagrass meadows and shellfish beds that have not fully healed.

Globally, low-oxygen events in coastal waters are becoming more frequent in stressed lagoons and estuaries. Large reviews point to nutrient inputs and warming as drivers that stack the odds toward hypoxia, a trend outlined in a broad synthesis.

When oxygen drops, mobile animals try to escape while plants and slow movers lag behind. The pattern seen this week, with animals moving up from the bottom, fits that playbook.

Why the lagoon keeps collapsing

For decades, the Mar Menor has acted as a catch basin for farm runoff, stormwater, and urban waste. The flat terrain around the lagoon channels nutrients and sediments into its shallow waters after each major rain.

These inflows feed algae blooms that consume oxygen when they decay, making each storm a potential trigger for collapse.

Without long-term limits on fertilizer use and drainage controls, the lagoon remains vulnerable even in calm years.

Scientists emphasize that recovery requires restoring wetlands around its perimeter, improving wastewater systems, and reducing direct runoff from nearby fields to stop the cycle of stress and collapse.

Lessons from Mar Menor

The monitoring network suggests the layered structure in the south is losing strength. If winds and exchange with the Mediterranean increase, mixing could restore oxygen at the bottom in short order.

Prevention still hinges on cutting nutrients that feed blooms before the next storm loads the deck.

Managers will also watch the white patch, because its turbidity further dims the light that bottom plants need.

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