Lakes Can Increase Carbon Emissions by Slowing the Movement of Water, Study Finds 

Lakes are not simply passive stops along rivers and streams. New research from Sweden shows that their presence can substantially change what happens to carbon as it moves through inland water networks, increasing the share released into the atmosphere rather than carried downstream. Researchers studied 32 interconnected Arctic stream-lake networks in northern Sweden. They found that networks containing more lakes had longer water residence times and higher carbon emissions relative to downstream carbon export. The effect was strongest during warmer, low-flow conditions, when water moved more slowly through the landscape. The findings suggest that lakes and streams need to be considered together when assessing the role of inland waters in the global carbon cycle.

Lakes Give Microbes More Time to Break Down Carbon

The study, led by Fredrik Alriksson of Umeå University, analysed 362 stream segments and 42 lakes across the 32 Arctic networks. Researchers repeated their measurements across four seasons with contrasting environmental conditions, allowing them to compare how the balance between atmospheric carbon emissions and downstream carbon transport changed over the year.

The researchers used the term “network residence time” to describe how long water remains within a connected stream-and-lake system. In the Swedish networks, this ranged from just 42 minutes to 29 years, with lakes accounting for almost all of the longest residence times. Networks with 10 times more aquatic surface area had approximately 35 times longer residence times and 1.8 times higher carbon emissions relative to downstream carbon export, according to the study.

The longer water remains in a lake-rich network, the more opportunity microbes have to process dissolved organic carbon. One important pathway is the mineralisation of dissolved organic carbon, which can produce carbon dioxide that subsequently escapes from the water into the atmosphere. The researchers, however, caution that their measurements could not identify the precise contribution of every process behind the observed relationship.

Warm, Low-Flow Summers Shift the Carbon Balance

The balance between carbon released to the atmosphere and carbon transported downstream also changed markedly with the seasons. Carbon emissions relative to downstream export were lowest during spring and autumn but highest in summer. The summer measurements coincided with warmer, relatively dry conditions and reduced stream flow, creating conditions in which carbon could remain within the aquatic network for longer.

The study found that lake-rich networks maintained higher carbon emission-to-export ratios across different environmental conditions. That is important because inland waters can send carbon along several different pathways. Some carbon is transported downstream, some is released as gases such as carbon dioxide, and some can be retained within aquatic ecosystems. Focusing on only one of these pathways can therefore give an incomplete picture of how inland waters influence the carbon cycle.

The researchers argue that lakes should not be treated as isolated features when scientists assess carbon movement through rivers and streams. Instead, lakes, streams and other components of inland water networks need to be evaluated together. Their findings indicate that the physical structure of a water network, particularly the presence and extent of lakes, can influence how long carbon remains in the system and whether it is ultimately released to the atmosphere or transported farther downstream.

As Arctic temperatures and hydrological conditions change, the relationship could become increasingly important for understanding regional carbon cycling. The study does not establish that warming will automatically make every lake a larger carbon source. Rather, it shows that changes in water residence time and network structure can alter the balance between carbon emissions and downstream export, highlighting a part of the inland carbon cycle that has often been considered separately.

References:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2025GL120340

https://phys.org/news/2026-08-lakes-theyre-carbon.html

Banner image: Photo by Pietro De Grandi on Unsplash 

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