Natural and anthropogenic tipping points in a Mediterranean karstic lake: evidence from Lake Estanya (NE Spain)

Lakes are not merely static pools of water; they are dynamic systems that breathe, mix, and change in response to the sky above and the land around them. In many parts of the world, particularly in regions with hot, dry summers and wet winters, these water bodies are incredibly sensitive to shifts in climate and human activity. Scientists who study the history of lakes, known as paleolimnologists, look at the layers of mud at the bottom to read a record of the past. They also monitor the water in real time to see how it reacts to current weather. A key concept in this field is the "tipping point," a moment when a small change in conditions pushes a system into a completely new state that it cannot easily return to. For example, a lake might suddenly stop mixing its top and bottom layers, trapping the deep water in a permanent state of low oxygen. Understanding when and why these shifts happen is vital, because the health of a lake determines the quality of water for drinking, farming, and the survival of the creatures that live within it.

In the Pyrenees mountains of northeastern Spain, a small, shallow lake called Estanya has become a critical case study for these changes. This lake sits in a basin surrounded by gypsum and limestone, a geological setting that makes its water naturally rich in salts and minerals. For decades, scientists believed Estanya was a "warm monomictic" lake, a technical term meaning it would warm up in the summer, stratify into layers, and then completely mix from top to bottom during the cold winter months. This annual mixing is essential because it brings oxygen down to the deep water, keeping the ecosystem alive. However, a new study combining modern monitoring with ancient sediment records has revealed that this lake has crossed a major threshold. It has shifted from a lake that mixes every year to one that is permanently stratified, with its deep waters cut off from the surface and the oxygen they need.

The story of this change begins with a close look at the last few years. Between 2018 and 2023, researchers placed automated sensors in the lake to track the temperature of the water at different depths every hour. They also collected water samples to measure salt levels and chemical signatures. The data told a dramatic story. In 2018, after a period of severe drought, the lake's surface water evaporated heavily, becoming much saltier and denser. When autumn rains finally arrived, they dumped fresh, light water onto the surface. Instead of sinking and mixing, this fresh water floated on top of the dense, salty deep water. The two layers refused to blend. By the following winter, the surface water cooled down, but it never got cold enough to become heavy enough to sink through the salty layer below. The deep water, trapped at the bottom, remained isolated, warm, and without oxygen. This state, known as chemical meromixis, has persisted every year since 2018, creating a permanent barrier between the surface and the depths.

To understand how this happened, the team dug into the lake's past. They extracted a core of sediment, a cylinder of mud about 41 centimeters long, which contained a history of the lake stretching back roughly 300 years. By analyzing the chemistry of the mud layers, they could see how the lake had behaved over centuries. The record showed that the lake had not always been this way. Around the middle of the 19th century, as the world was emerging from a cooler period known as the Little Ice Age, the lake underwent its first major shift. The climate became warmer and drier, leading to more evaporation. This natural change pushed the lake toward a saltier, more restricted state, marking a natural tipping point where the lake became less able to mix completely.

However, the most recent and most alarming shift occurred after the year 2000. The sediment layers from this period told a different story, one driven by human hands. The mud showed a sharp increase in nitrogen, a nutrient essential for plant growth but dangerous in excess. The chemical signature of this nitrogen pointed directly to agricultural fertilizers and livestock waste from the farms surrounding the lake. At the same time, the carbon isotopes in the sediment indicated a massive boom in algae growth. The lake had become eutrophic, a condition where too many nutrients cause plants and algae to grow wildly. When these organisms die, they sink to the bottom and are eaten by bacteria, a process that consumes oxygen. Because the lake was already stratified and the deep water was cut off from the surface, this oxygen consumption created a permanent, oxygen-free zone at the bottom. The researchers found that this was not just a natural fluctuation; it was an unprecedented anthropogenic tipping point, where intensive farming combined with a changing climate to push the lake into a new, unstable state.

The study also looked at sulfur, another element found in the lake's gypsum bedrock. The chemical history of sulfur in the mud showed a clear transition. For centuries, the sulfur isotopes indicated a system that was mixing and cycling oxygen. But as the lake moved toward its current state, the sulfur record changed, reflecting a shift toward a more stagnant, evaporative environment. This chemical evidence, combined with the temperature data and the nitrogen spikes, confirmed that the lake had crossed a line. It was no longer just reacting to the weather; it had entered a new regime where the deep water was permanently isolated.

The implications of this discovery extend far beyond a single mountain lake. The researchers argue that Lake Estanya is a warning sign for other karstic lakes in the Mediterranean region. These are lakes formed in limestone or gypsum areas, often shallow and sensitive to evaporation. If a prolonged drought concentrates the salts in the deep water, and a subsequent rain creates a fresh layer on top, the lake can become trapped in this stratified state. Once this happens, the deep water becomes a "time capsule" of the previous drought, holding onto high salt concentrations and toxic chemicals. If the lake remains in this state, the lack of oxygen can lead to the release of toxic compounds from the sediment, potentially harming the entire ecosystem.

The team emphasizes that while the first shift in the 19th century was driven by natural climate changes, the current situation is different. The combination of extreme weather patterns and the heavy use of agricultural fertilizers has created a perfect storm. The lake is now in a state where it is highly productive at the surface but dead at the bottom. The researchers suggest that without urgent changes in how the surrounding land is managed, specifically by reducing the amount of fertilizer runoff, the lake may not be able to recover. The window to prevent this irreversible damage is closing. By understanding the precise moment when the lake crossed its tipping point, scientists hope to provide a blueprint for protecting these fragile ecosystems. The story of Lake Estanya is a clear reminder that when climate change and human activity work together, they can push natural systems into new and dangerous territories, changing the very nature of the water we rely on.