Lakes like Lake Baldegg act as natural nitrogen sinks, helping protect downstream ecosystems from nutrient pollution. Climate warming is weakening this important filtering function. Image credit: Pro Natura Lucerne.

Researchers have shown that climate change could weaken the vital filtering role of lakes in removing excess nitrogen from the ecosystem.

Weakening this natural purification process would have consequences extending all the way to coastal marine ecosystems.

When we think of lakes, fish and frogs often come to mind, along with birds on the shore or places to swim.

But lakes also play a central role in the global nitrogen cycle; micro-organisms convert nitrogen compounds such as nitrate or ammonia into dinitrogen gas (N₂), which is released to the atmosphere and effectively removed from the biosphere.

This nitrogen removal process is called ‘denitrification’.

About 20% of natural nitrogen removal in inland waters is attributable to such processes.

A new study led by the University of Basel and Eawag shows that this purification function is highly sensitive to warming caused by climate change.

A key tool is the gravity corer, used to collect sediment cores. These cores are then used to initiate 15N tracer addition experiments, enabling quantification of denitrification activity in the lake. Image credit: Cameron Callbeck/ University of Basel.

Particularly active in winter

For their research, the scientists collected samples from Lake Baldegg in the Lucerne Lake District.

The 5.3² km lake is typical of many lakes in our latitudes, where the water completely mixes once a year.

The researchers showed that denitrification activity is closely linked to this seasonal mixing.

In winter, the three water layers in Lake Baldegg mix completely: the warm, oxygen-rich surface water, the transition zone, and the cold, oxygen-poor deep water.

During this phase, denitrification is nearly 50% more active than during summer stratification. This is precisely where a potential danger of climate change lies.

Lead author, environmental scientist Cameron Callbeck, said: “The ability of lakes to remove nitrogen from the water is highly dependent on the season. And this is being altered by climate change.”

In a severe warming scenario, the winter mixing phase could shorten by about 27 days. Nitrogen removal in the lake would decrease accordingly.

Callbeck added: “However, we don’t yet know why denitrification occurs particularly in winter.”

Lakes play a crucial filtering role in the global nitrogen cycle.

If they do not retain and break down nitrogen, it flows into the ocean via rivers.

The consequences can be severe: algal blooms in coastal regions, so-called ‘dead zones’ with oxygen depletion, and stress on sensitive ecosystems.

Professor Dr Moritz Lehmann, the study’s last author, said: “The study shows that even relatively small shifts in the seasonal mixing rhythm of lakes can measurably affect nitrogen cycling at the lake scale and, in aggregate, the global nitrogen cycle.”

A combination of sediment samples and lake balance

To measure denitrification in the lake, the researchers used two methods.

First, they spiked sediment samples with nitrogen molecules containing the rare isotope ¹⁵N.

This isotope allowed them to use special measurement techniques to determine how much of the labelled nitrogen was converted into nitrogen gas, a measure of denitrification activity.

Second, they created a model for the entire Lake Baldegg to estimate how much nitrogen the lake breaks down in total.

Callbeck said: “The isotope measurements and the calculated total balance matched between the observations and the model.

“This allowed us to make reliable predictions of lake nitrogen removal, and revealed that the winter period is indeed a hotspot of denitrification.”

Microbial partnership

The researchers also discovered a kind of microbial teamwork in the sediment.

Certain bacteria break down chitin there, a robust molecule derived from the shells of zooplankton or dead algae, among other sources, which accumulates on the lake bottom.

This breakdown produces compounds that serve as an energy source for other micro-organisms.

These, in turn, drive denitrification, converting nitrate into dinitrogen gas.

Chitin degradation thus provides, in a sense, the fuel for the process through which the lake removes excess nitrogen from the water.

In the next step, the researchers plan to investigate whether the observed processes also affect the production of climate-damaging nitrous oxide in lakes.

This is linked to denitrification and other key nitrogen transformation processes in lakes.

The results are published in Nature Microbiology.