How Climate Change Is Stripping Lakes of Their Self-Cleaning Power

For decades, lakes have quietly helped filter excess nutrients from the water they hold. But as temperatures rise, scientists say this natural cleaning process may be starting to weaken. A study has found that rising temperatures are disrupting seasonal processes that help lakes remove excess nitrogen from the water, potentially allowing more pollution to flow downstream into rivers, estuaries and coastal ecosystems. Researchers warn that as lakes warm and winter seasons shorten, their natural capacity to filter pollutants could weaken, increasing the risk of harmful algal blooms, oxygen-deprived “dead zones” and deteriorating water quality.

Winter is when lakes do most of their cleaning

For years, scientists knew that lakes play an important role in removing nitrogen, a nutrient that can trigger excessive algal growth when present in large quantities. What remained unclear was when most of this cleanup occurred. The new research found that the bulk of nitrogen removal happens during winter, when lakes undergo seasonal mixing. During this period, colder surface waters sink and mix with deeper layers, creating conditions that help microbes convert nitrogen into gas that escapes harmlessly into the atmosphere.

Researchers studying a Swiss lake found that nitrogen removal during the winter mixing season was nearly 50% higher than during the calmer summer months. The process appears to depend on a complex interaction between different groups of microbes living in lake sediments. One group breaks down organic material such as dead algae and tiny aquatic organisms, while another carries out denitrification, the final step that removes nitrogen from the water.

Scientists say this seasonal rhythm has largely been overlooked in previous assessments of lake health. The findings suggest that winter is doing much of the ecological “heavy lifting” that keeps lake ecosystems functioning and prevents excess nutrients from accumulating.

Warming temperatures could shorten lakes’ cleanup season

Using computer simulations, researchers examined how lakes might respond under a high-emissions future. The results showed that warming temperatures could shorten the winter mixing season by about 27 days, effectively removing nearly a month of the period when lakes are most efficient at cleaning themselves. As a result, overall nitrogen removal declined by roughly 10%.

While a 10% reduction may appear modest, scientists warn that the impact could be substantial when multiplied across thousands of lakes worldwide. Lakes in many regions are already warming and experiencing weaker seasonal mixing as global temperatures continue to rise. According to IPCC’s Sixth Assessment report, Earth’s average temperature has increased by about 1.1°C since the late 19th century, contributing to widespread changes in freshwater ecosystems.

Researchers say the findings provide another example of how climate change is altering natural processes that people often take for granted. Unlike visible impacts such as floods or heatwaves, the weakening of a lake’s self-cleaning ability occurs gradually and may go unnoticed until water quality begins to deteriorate.

The consequences could extend far beyond lakes

The study warns that nitrogen not removed by lakes does not simply disappear. Instead, it continues downstream through rivers and eventually reaches coastal waters. There, excess nutrients can fuel large algal blooms that consume oxygen as they decompose, creating low-oxygen zones where fish and other aquatic life struggle to survive. These so-called dead zones already affect many coastal regions around the world. Scientists say a decline in the natural filtering capacity of lakes could add further pressure to ecosystems that are already dealing with pollution, biodiversity loss and climate change.

The findings also have implications for water management. Researchers suggest that the length of the winter mixing season could become an important indicator for monitoring lake health in a warming climate. Understanding how seasonal changes affect nitrogen removal may help governments and water managers better anticipate future pollution risks and develop strategies to protect freshwater resources. 

As global temperatures continue to rise, scientists say the research highlights a broader reality: climate change is not only altering weather patterns and ecosystems, but also weakening some of the natural processes that quietly help keep water bodies clean. What happens during a few weeks of winter in a lake may ultimately influence water quality and ecosystem health far beyond its shores.

References:

https://www.nature.com/articles/s41564-026-02349-9

https://www.earth.com/news/lakes-are-losing-their-natural-self-cleaning-power-as-the-climate-heats-up

https://science.nasa.gov/climate-change/effects

Banner image: Photo by Tom Gainor on Unsplash 

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