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.
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
