Streams and rivers are more important for the global carbon cycling than previously thought

Streams and rivers transport nutrients and carbon compounds from terrestrial ecosystems to lakes and finally to oceans and are thus of great importance for regional as well as global carbon cycling and greenhouse gas emissions. Carbon dioxide emissions from running waters are estimated to be four times higher than the total carbon flux to oceans. The estimates of emissions are based on concentrations of carbon dioxide in water and these determinations are usually from manual, discrete samples taken during day light. Thus, there is an assumption that carbon dioxide concentrations in streams and rivers do not vary diurnally. Nowadays scientists have access to continuously operating sensor technology, which makes it possible to assess carbon dioxide concentrations in running waters at all hours of the day and to reveal the true temporal variability in concentrations and emissions.

An international group of scientists took advantage of the sensor technology and compiled all the available continuous data on carbon dioxide concentrations in running waters. The data set consisted of 66 streams and rivers, located from tropics to Arctic tundra and included also the Boreal zone and a Finnish site. All together the data set covered 57 years. The results showed that streams and rivers emitted ca. 30 % more carbon dioxide at night than on daytime, i.e. there is a clear diurnal pattern in carbon dioxide dynamics. When this dynamic was considered, the global estimate of annual carbon dioxide fluxes from streams and rivers increased from the present value of 0.65 -1.8 Pg C a

by 0.20-0.55 Pg C a

.

The diurnal pattern in carbon dioxide concentrations and emissions is a result of light dependent photosynthetic activity of aquatic plants and algae residing in streams and rivers. In running waters located in open landscapes and thus without shadowing trees in the riparian zone, the diurnal pattern was very clear whereas in streams and rivers running e.g. through tropical and boreal forests, temporal variation was much smaller. In these ecosystems, light climate of aquatic primary producers was further deteriorated because of higher concentrations of dissolved organic carbon of terrestrial origin. This means that e.g. in Finland estimates of carbon dioxide emission from streams and rivers in forested areas, when based on traditional daytime measurements only, are less biased than in general globally.

“This study highlights the importance of nocturnal emissions in dynamics of carbon dioxide in streams and rivers and thus, improves our understanding of global carbon cycling”, says Luke's Senior Scientist, Dr Anne Ojala who was involved in the international group of scientists behind the study.