FLATHEAD LAKE – Rivers are critical for Earth’s ability to sustain life. They serve as the connective transport system, much like the circulatory system in the human body, delivering water, energy and matter from the mountains to the oceans.
But our understanding of the role of rivers in the global carbon cycle remains limited, making it difficult for scientists to predict how global change may alter the future health of not only our rivers but the planet itself.
Robert Hall, a stream ecologist with the University of Montana’s Flathead Lake Biological Station, recently joined an international team of scientists to review the state of river ecosystem metabolism research and synthesize the best available estimates of river ecosystem metabolism. Their work was published in a new paper in the prestigious scientific journal Nature.
River ecosystem metabolism refers to the sum of carbon produced by photosynthetic organisms and carbon breathed by all organisms in a given river ecosystem. Scientists often calculate these fluxes by measuring the uptake and release of oxygen, which is produced by photosynthesis and consumed by respiration and is more easily measured than carbon.
Then scientists convert these rates from oxygen into carbon. Rivers tend to have much higher respiration of carbon than production because they receive subsidies of carbon from the terrestrial landscape.
Once considered to be the equivalent of pipes simply transporting organic carbon (carbon derived from living things) from land to the ocean, rivers now are recognized by scientists as biogeochemical reactors that interact with organic carbon in two significant ways.
First, rivers can transform organic carbon in transport, emitting the greenhouse gases carbon dioxide and methane along the way. They also can store organic carbon in the biological community or in the sediments of floodplains and river deltas.
For this study, researchers quantified the organic and inorganic carbon fluxes from land to rivers across the globe, revealing that carbon dioxide emissions shift the balance of carbon from organic to inorganic while traveling through river networks.
“Generally, the Earth’s land mass and terrestrial plants absorb more atmospheric carbon dioxide than it emits,” Hall said. “But a fraction of that carbon does leak into streams and rivers where it is either emitted as carbon dioxide, stored or exported to the ocean.”
Hall said this particular study detailed how much of that carbon dioxide came from the metabolism of organic carbon in the river itself versus the river acting as a simple chimney for carbon dioxide that travels to rivers via groundwater.
Additionally, the study explored how global change may affect river ecosystem metabolism and related carbon fluxes. It also identified research directions that can help better predict the effects of global change, such as changing land use or climate, on river ecosystem processes.
Over the past few decades, global change – a term that encapsulates changes in the global environment that have the potential to impact the Earth’s capacity to sustain life – has altered the planet’s carbon cycle, a natural process that controls the amount of carbon dioxide in the atmosphere. As the largest biogeochemical bridge between the planet’s continents, oceans and atmosphere, river networks play a prominent part in the global carbon cycle.
In the study, scientists address how global change affects river ecosystem metabolism and greenhouse gas emissions. In one example, the study shows how increased amounts of atmospheric carbon dioxide and nitrogen deposition are factors that – combined with increased growing seasons resulting from a warming climate – are increasing terrestrial plant growth, which results in more carbon inputs and recycling in rivers.
In another example, lower snowpack and earlier snowmelt during milder winters have shifted the metabolism of alpine river networks. As winters become milder and precipitation increasingly comes as rain instead of snow, there may be increases in carbon dioxide emissions from alpine rivers.
Additionally, the study highlights the impacts of changing river flow regimes, which are the typical fluctuations of river flows over time. Because the atmosphere’s ability to hold water is highly sensitive to temperature, precipitation extremes will become more frequent and intense as the climate warms. In some regions, river flows will be shaped by drought. In others, flows will be impacted by flash floods attributed to intense storm runoff. Such changes to flow will alter how rivers cycle carbon from increased photosynthesis during drought to higher scour and transport of carbon during floods.
These aren’t the only factors impacting our rivers. Researchers raise additional concerning trends, including effects of shrinking ice cover on river flow regimes, higher flow variability in seasonal rivers and large-scale deoxygenation in rivers that could imperil biodiversity.
“Here in the Flathead,” Hall noted, “we might expect earlier snowmelt peak flows leading to longer, low-flow summers that increase both photosynthesis and respiration of stored carbon.”
But focusing research on the scale of individual rivers does not show the patterns that develop over space and time in large-scale river networks. To best understand and predict the future of our planet’s river systems, researchers contend, there is an essential need to expand river carbon research from individual rivers to a global scale.
To address this need, the study’s authors call for developing a global River Observation System (RIOS), similar to those established for other ecosystem types like oceans. Such a monitoring system would help promote important and ambitious research and allow the proper accounting of regional and global carbon fluxes at the interface between land, river, atmosphere and the coastal ocean.
“We need an accurate accounting of how the globe cycles carbon to predict future carbon dioxide emissions and therefore climate,” Hall said. “We cannot ignore rivers, because they emit a large fraction of the terrestrial carbon dioxide sink, and riverine emissions will change, perhaps in unexpected ways.”
This study was led by Tom J. Battin of the École polytechnique fédérale de Lausanne, Switzerland. Additional contributors include Ronny Lauerwald (Université Paris-Saclay), Emily S. Bernhardt (Duke University), Enrico Bertuzzo (Università Ca’ Foscari Venezia), Lluís Gómez Gener (Universitat Autònoma de Barcelona), Erin R. Hotchkiss (Virginia Polytechnic Institute and State University), Taylor Maavara (University of Leeds), Tamlin M. Pavelsky (University of North Carolina), Lishan Ran (The University of Hong Kong), Peter Raymond & Judith A. Rosentreter (Yale University), and Pierre Regnier (Université Libre de Bruxelles).
For the complete study, visit the Nature website at https://www.nature.com/articles/s41586-022-05500-8.
