A collaborative study led by the University of Basel and Eawag, published in Nature Microbiology, finds that climate warming could weaken lakes’ natural nitrogen filtration by altering seasonal mixing and reducing denitrification.

Researchers used two complementary methods—15N-labelled sediment experiments and an integrated lake balance model—to quantify denitrification and confirm that winter denitrification is particularly important for nitrogen removal.

In Lake Baldegg in Switzerland, denitrification peaks during winter turnover when surface, transition, and deep waters mix, whereas summer stratification cuts activity by about half.

The study notes that winter mixing creates a denitrification hotspot, with rates about 50% higher in winter than in summer due to the full mixing of water layers.

Lakes act as natural nitrogen sinks through denitrification, converting reactive nitrogen to nitrogen gas and thereby protecting downstream ecosystems from nutrient pollution, supporting roughly one-fifth of inland nitrogen removal.

Experts warn that even modest changes in seasonal mixing can cascade through lake biogeochemistry and the global nitrogen budget, underscoring the need for integrated freshwater management under climate change.

Climate warming could shorten the winter mixing window by about 27 days, reducing denitrification and potentially increasing nitrogen export to rivers and oceans with downstream risks like algal blooms and dead zones.

Reductions in winter mixing driven by climate change may weaken nitrogen removal efficiency, threatening the lakes’ purification role.

The study identifies a microbial mechanism: chitin degradation in sediments fuels a denitrifying community, linking microbial activity to winter nitrogen removal under low-oxygen conditions.

Future work will examine how denitrification shifts affect nitrous oxide production, a climate-forcing gas, in addition to nitrogen removal.

The findings reflect inputs from multiple agencies and highlight broader implications for freshwater ecosystems facing climate-driven changes.

A comprehensive nitrogen budget for Lake Baldegg was developed using ^15N tracing and an integrated model that combines physical mixing, biogeochemistry, and microbial ecology to validate the results.