Climate determines the role of rivers in carbon processing

We are glad to present the latest work of the DOMIPEX project, where they show that climate determines the role of rivers in the carbon cycle. The article, recently published in the journal Global Biogeochemical Cycles, in the study, the authors have found that climate conditions the ability of rivers to degrade organic matter and recycle the nutrients of water, one of the main ecosystem services of rivers.

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Rivers play a fundamental role in carbon cycling
Rivers and streams are fascinating elements of the landscape. They transport water and materials connecting the entire watershed, from the mountains to the sea. But rivers are not simple passive pipes. During the journey downstream, rivers transform the materials they transport, which can accumulate in aquatic organisms and sediments or become gases, such as carbon dioxide (CO2) and methane (CH4). One of the main materials that come from terrestrial ecosystems and reach rivers and streams is organic matter (for example, leaves and branches from the surrounding vegetation), rich in carbon, which serves as indispensable food for fluvial organisms, such as bacteria and fungi. Thus, the rivers, and especially the smaller streams, are very active systems where a multitude of biogeochemical reactions take place and with a great capacity for processing the organic matter that reaches them, which is why they play a key role in the cycle of carbon.

DOMIPEX: a collaborative project to study the role of headwater streams in the carbon cycle
In this study, researchers explore how rivers process dissolved organic matter along a broad biogeographical gradient, from the humid forests of southern Germany and Switzerland to the semi-arid areas of Murcia. To carry out a study of this kind, which covers such a wide geographical area simultaneously, we were fortunate to have the support of the first call for collaborative studies of the Iberian Limnology Association for young researchers.
Initially, the young researchers of the association who decided to get involved sampled 11 different rivers during two periods (summer and autumn 2014) using a common protocol. The experiment consisted of the addition of acetate, a compound similar to sugar, and nitrate to determine the rates of consumption of organic matter of the river by the microbial aquatic community. To measure the capacity of aquatic microbes to consume these compounds, they measured how the concentration of acetate and nitrate decreased after their experimental addition in the river. Thus, when the concentration of added acetate decreases rapidly, it indicates that microbes have a high capacity to degrade organic matter. Additionally, daily cycles of dissolved oxygen measurements were performed, which allowed to calculate the capacity of these rivers to produce (primary production) and consume organic matter (respiration), which together give us information on the ecosystem metabolism. Finally, the groups that participated in the project took water samples that were analyzed to identify the type of organic compounds dissolved in streams water, to see if there were differences between rivers in humid and arid zones.

The results of this work indicate that the differences in the type of organic matter and its processing in rivers depend on the biogeographic area where the river is located and its climate. In general, in rivers located in more rainy areas, the dissolved organic matter was composed of molecules of humic type, that is, long aromatic and colored molecules, such as lignin or cellulose, from plant material from the forests of the catchment.
The microbial organisms of rivers located in rainy areas seem to depend on this type of humic organic matter, as indicated by fluvial metabolism values. On the other hand, in the most southerly and arid rivers, with scarce rainfall, organic matter was mostly composed of smaller, protein-like molecules that have probably been produced upstream within the river itself by microbes, algae or macrophytes. This coincides with a higher primary production in these warmer rivers. At the same time, rivers process organic matter (acetate) very differently depending on the climate. Thus, the rivers of arid zones are able to consume much more acetate than the rivers of northern Europe, because in natural conditions, the rivers of more arid areas receive very little contributions of organic matter of terrestrial origin due to the lower presence of trees and organic matter in the soil.

This study has been a pioneer in determining the relationship between the type of dissolved organic matter, its process and the ecosystem metabolism in streams of different regions. In this way, we can better understand how rivers process the organic matter they receive from terrestrial ecosystems. This information is essential to highlight the key role that aquatic ecosystems have in the carbon cycle, and to predict the effects of global change on the functioning of ecosystems.

This work has been possible thanks to the illusion and the coordinated effort of a group of young researchers, who believe in the value of collaboration to face the challenges of global change, to which ecology, environmental sciences and society in general are currently facing.

Complete paper:

N. Catalán, J. P. Casas-Ruiz, M. I. Arce, M. Abril, A. G. Bravo , R. del Campo, E. Estévez, A. Freixa, P. Giménez-Grau, A. M. González-Ferreras , Ll. Gómez-Gener, A. Lupon, A. Martínez, C. Palacin-Lizarbe , S. Poblador, R. Rasines-Ladero, M. Reyes , T. Rodríguez-Castillo, P. Rodríguez-Lozano, I. Sanpera-Calbet, I. Tornero, and A. Pastor. 2018. Behind the Scenes: Mechanisms Regulating Climatic Patterns of Dissolved Organic Carbon Uptake in Headwater Streams. Global Biogeochemical Cycles. https://doi.org/10.1029/2018GB005919