Lake-Rich Stream Networks Release Far More Carbon to the Atmosphere, Study Finds

New research shows that the more lake-rich a network of streams is, the more of its carbon is released into the atmosphere rather than carried downstream. The study, led by Fredrik Alriksson of Sweden's Umeå University and published in Geophysical Research Letters, examined 32 connected stream-and-lake networks in northern Sweden.

Networks with ten times more lake surface area had roughly 35 times longer "network residence time" — how long water stays within the system — and produced about 1.8 times more carbon emissions relative to the carbon exported downstream. Residence time across the networks studied ranged from just 42 minutes to as long as 29 years, with lakes accounting for almost all of it.

Researchers repeatedly sampled the networks during four seasons — spring snowmelt, early summer, late-summer low flow and autumn high flow — measuring carbon dioxide emissions across more than 360 stream sections and 42 lakes, and estimating how much dissolved carbon continued downstream.

Keeping carbon in the water longer appears to give microbes more time to break down dissolved organic carbon into carbon dioxide, which then escapes into the air; the researchers note they did not directly measure every process responsible for the pattern. The effect relative to downstream export was weakest in spring and autumn and strongest in summer, when warmer temperatures and lower flow let carbon linger longer in the system.

Streams overall released more carbon than lakes in absolute terms, the study found, but in lake-rich networks a greater share of that carbon escaped into the atmosphere rather than being carried downstream.