A worldwide study led by ICRCA, UB and UFZ reveals that rivers, lakes and reservoirs emit large amounts of carbon dioxide into the atmosphere when they dry up

Decades ago, science began to understand the capital role of ecosystems in regulating the concentration of carbon dioxide in the atmosphere. The pieces of this gigantic puzzle still don't quite fit together, and every now and then new pieces are found to fit. An international research project led by scientists from the Catalan Institute for Water Research (ICRA), the University of Barcelona (UB), and the Helmholtz Center for Environmental Research in Germany (UFZ) has just announced a surprising finding that adds a piece to the puzzle: rivers, lakes and reservoirs emit large amounts of carbon dioxide into the atmosphere when they dry up. The finding has been published today in the journal Nature Communications.

"It all started in 2012, during a measurement campaign on the Fluvià river," says Biel Obrador, from the UB's Department of Evolutionary Biology, Ecology and Environmental Sciences. Together with Rafa Marcé (ICRA) and Daniel von Schiller (UB), they studied the release of greenhouse gases in small dams on this river. "It was summer and parts of the riverbed were dry. Out of curiosity, we decided to take some measures also in those areas," explains Rafa Marcé. "The results surprised us; we expected limited biological activity due to the lack of water in these areas, but they released huge amounts of carbon dioxide. We wondered if this could be the case in other ecosystems in the world and if we were missing an important piece to understand the regulation of atmospheric carbon dioxide by ecosystems" says Matthias Koschorreck of the UFZ.

To answer this question, they decided to communicate the finding to researchers around the world to organize a program for measuring emissions in dry freshwaters on a global scale. "Often we researchers are reluctant to share ideas about future projects, because competition in the world of research is tough and that pushes you to secrecy," explains Rafa Marcé, "but in this case we saw it clearly: no one had data on carbon dioxide emissions from dry sediments, so if we wanted to know if they were widespread, we had to share the idea with everyone who wanted to collaborate. " This is how the DryFlux network was born. "We used our international contacts to convince research teams to measure carbon dioxide emissions in dry riverbeds, lakes, and reservoirs using a common protocol with just the right doses of complexity so that everyone could replicate it," adds Núria Catalan, now in the United States Geological Survey and who led the writing of the protocol. The response exceeded expectations. A total of twenty-four research teams from around the world decided to participate, contributing ecosystem measures on all continents except Antarctica. Daniel von Schiller explains that "we were surprised by this very positive response, because although the methodology and equipment were relatively affordable for a research team, participating in field work has substantial costs, and each team had to take care of its own, we did not have the resources to cover all those expenses. "

The data obtained was analyzed by Philipp Keller, doctoral researcher at the UFZ Lakes Research Department and first author of the study. "We have found significant carbon dioxide emissions from dry areas of freshwater ecosystems in all climate zones," says Keller. "So it really is a global phenomenon. If you take these emissions into account in today's global estimates for freshwater, your emissions are up six percent." But what mechanisms are responsible for the release of carbon dioxide in dry sediments? "Microorganism respiration processes," says Phillip Keller. "We still don't fully understand the biological mechanisms behind it, but it is clear that the more humidity, temperature and organic matter, the more carbon dioxide is released." From the study results, the researchers concluded that the factors responsible for the release of carbon dioxide are essentially the same worldwide. "It is surprising that the dry sediments of a mountain river or a salty lagoon in the middle of a desert plain respond in the same way to environmental factors such as humidity or temperature, suggesting the presence of microorganisms capable of adapting to extraordinarily diverse conditions and in some cases very hard", explains Nuria Catalán.

What, then, do the study results mean for the future assessment of the role of ecosystems in regulating atmospheric carbon dioxide? "Our study shows that we still lack many pieces to fully understand the carbon dioxide cycle on a planetary scale, because there are many small gears that must be understood if we want to predict how ecosystems will respond to the current increase in carbon dioxide in the atmosphere" Biel Obrador says, “We hope that our work helps to ensure that dry areas of freshwater ecosystems are included in future calculations. With the progression of climate change and human impacts, freshwaters will dry up more and more frequently in large regions of the planet, and some of them forever, such as the Aral Sea in Central Asia. Therefore, emissions from dry freshwaters will only increase". For Rafa Marcé "this study includes another message: competing is not the only, nor the best, way of doing research. Collaborating openly allows you to answer more ambitious questions. In the times that we are living, it is a lesson that we should take into account".

S. Keller, N. Catalán, D. von Schiller, H.-P. Grossart, M. Koschorreck, B. Obrador, M. A. Frassl, N. Karakaya, N. Barros, J. A. Howitt, C. Mendoza-Lera, A. Pastor, G. Flaim, R. Aben, T. Riis, M. I. Arce, G. Onandia, J. R. Paranaíba, A. Linkhorst, R. del Campo, A. M. Amado, S. Cauvy-Fraunié, S. Brothers, J. Condon, R. F. Mendonça, F. Reverey, E.-I. Rõõm, T. Datry, F. Roland, A. Laas, U. Obertegger, J.-H. Park, H. Wang, S. Kosten, R. Goméz, C. Feijoó, A. Elosegi, M. M. Sánchez-Montoya, C. M. Finlayson, M. Melita, E. S. Oliveira Junior, C. C. Muniz, L. Gómez-Gener, C. Leigh, Q. Zhang & R. Marcé (2020): Global CO2 emissions from dry inland waters share common drivers across ecosystems. Nature Communications https://doi.org/10.1038/s41467-020-15929-y

Additional information: Dryflux project: https://www.ufz.de/dryflux/

Additional scientific publication: Marcé, R., Obrador, B., Gómez-Gener, L., Catalán, N., Koschorreck, M., Arce, M. I., Singer, G., von Schiller, D. (2019): Emissions from dry inland waters are a blind spot in the global carbon cycle. Earth-Sci. Rev. 188, 240 - 248

Gallocanta lagoon (Saragossa) June 2017 ©Rafael Marcé