Lakes aren’t just storing water—they’re changing the carbon cycle

Lakes may look like picturesque stopping points along a river or stream, but they can have a big influence on what happens to carbon as it moves through a landscape. New research suggests that the more lake-rich a network of streams is, the more carbon is released into the atmosphere rather than carried downstream.

The finding, published in Geophysical Research Letters, comes from researchers who studied 32 connected stream-and-lake networks in northern Sweden, measuring how much carbon was released into the atmosphere and how much continued downstream. They found that networks with 10 times more lake surface area had almost twice the carbon emissions.

The results highlight a part of the carbon cycle that may have been overlooked by scientists. Rather than treating lakes and streams as separate pieces of the landscape, Fredrik Alriksson, of Sweden’s Umeå University, and colleagues argue that they need to be considered together as connected networks.

The race downstream

Streams and rivers constantly pick up carbon from the land around them and carry it downstream toward larger rivers and eventually the ocean. However, some is transformed into gases, including carbon dioxide, and escapes from the water into the atmosphere.

How much carbon each route takes depends partly on how long the water stays within the aquatic network. A fast-flowing stream can carry carbon downstream relatively quickly, while a lake can slow the journey considerably, giving chemical and biological processes more time to act on the carbon.

The researchers call the length of time water remains within a stream-and-lake network its “network residence time.” In the Swedish networks they studied, this varied enormously, from just 42 minutes to as long as 29 years. Lakes accounted for almost all of this residence time.

More water, more emissions

To investigate what this means for the carbon cycle, the team repeatedly sampled the networks during four different periods: spring snowmelt, early summer, late-summer low flow and autumn high flow. They measured carbon dioxide emissions across more than 360 sections of streams and from 42 lakes, while also estimating how much dissolved carbon was being transported downstream.

The pattern was striking. 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.

One likely explanation is that keeping carbon in the water longer gives microbes more opportunity to break down dissolved organic carbon. This process can ultimately produce carbon dioxide, which can then escape from the water into the atmosphere.

The researchers caution, however, that they did not directly measure all of the processes responsible for the pattern. They could not, for example, determine exactly how much of the effect came from the breakdown of organic carbon compared with other differences between lake-rich and lake-poor networks.

Summer’s stronger effect

The balance between carbon emissions and downstream export also changed substantially with the seasons. Carbon emissions relative to downstream export were lowest in spring and autumn and highest during summer.

This was particularly important because the summer measurements coincided with warmer, relatively dry conditions and lower stream flow. When less water is moving through a network, carbon can spend longer in the system, while warmer temperatures can also speed up processes that transform dissolved organic carbon.

The team found that the effect of lake-rich networks remained across the different environmental conditions they measured, including an unusually dry summer. Together, this suggests that both the amount of water in a network and how that water is distributed between lakes and streams can influence what happens to carbon.

Rethinking inland waters

The findings do not mean that lakes are simply replacing streams as the main source of carbon emissions. In fact, the study found that streams produced more carbon emissions overall. But in lake-rich networks, a greater proportion of the carbon was released into the atmosphere rather than exported downstream.

Lakes can also store carbon in their sediments, adding another possible destination for carbon that enters an aquatic network. This makes the overall picture more complicated than simply measuring how much carbon a river carries or how much carbon dioxide a lake releases.

The researchers say this is why studies of the global carbon cycle need to consider the different parts of inland water systems together. Lakes, streams, wetlands, reservoirs and floodplains can all alter the movement of carbon through a landscape, affecting how much is transported onward, stored or released into the atmosphere.

As the Arctic warms and hydrological conditions change, understanding these connections could become increasingly vital. This new study suggests that the shape and structure of a water network may be just as important as the amount of water flowing through it when determining what happens to carbon along the way.

Who’s behind this story?

Hannah Bird

PhD Earth Sciences in oceanography and paleontology. 10+ years translating complex science for mainstream media audiences.

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Sadie Harley

BSc Life Sciences & Ecology. Microbiology lab background with pharmaceutical news experience in oil, gas, and renewable industries.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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