For decades, the scientific understanding of riverine contributions to the global carbon cycle has been predominantly one-sided: rivers were believed to be net emitters of carbon dioxide (CO2), releasing more greenhouse gases into the atmosphere than they absorbed. This long-held assumption stemmed largely from extensive studies focused on temperate, forested regions, which constituted the bulk of empirical data available. However, groundbreaking new research challenges this traditional perspective by incorporating hitherto underrepresented arid and semi-arid river systems across the contiguous United States. This comprehensive analysis reveals that many rivers, particularly in Western arid landscapes, may act as significant carbon sinks, absorbing more CO2 than they release. This paradigm-shifting insight not only refines our global carbon budget calculations but also opens new pathways for understanding ecosystem dynamics in diverse climatic zones.
The research, spearheaded by aquatic biogeochemist Dr. Taylor Maavara at the Cary Institute of Ecosystem Studies, employs advanced machine learning techniques to upscale extensive river metabolism datasets, encompassing every stream and river network across the contiguous U.S. The study represents the largest synthesis of river metabolic processes to date, quantifying monthly and annual rates of photosynthesis and respiration across thousands of river reaches. By leveraging data from hundreds of monitoring sites, including those historically neglected due to their remote and arid locations, the researchers developed models that predict photosynthetic and respiratory dynamics with unprecedented geographical and temporal granularity.
Rivers play a dual ecological role by both emitting and absorbing carbon through complex biological and physical processes. Photosynthesis by aquatic plants and microorganisms facilitates the uptake of atmospheric CO2, transforming it into organic matter. Conversely, respiration by aquatic organisms – encompassing microbes, plants, and animals – releases CO2 back into the atmosphere as organic carbon is metabolized. The net balance between these antagonistic processes, termed river metabolism, determines whether a given water body is a net source or sink of carbon dioxide. Historically, scientific inquiry into river metabolism has favored temperate forest streams, where nutrient-rich waters and abundant organic substrates fuel higher respiration rates, culminating in net CO2 emissions.
This geographic bias overlooked the distinct metabolic regimes present in arid and semi-arid watersheds, where reduced canopy cover allows for increased sunlight penetration, and lower inputs of terrestrial organic carbon limit heterotrophic respiration. Maavara and colleagues challenged this assumption by explicitly incorporating data from these understudied regions, revealing starkly different metabolic patterns. In deserts and shrublands of the Western U.S., elevated photosynthetic activity coupled with diminished respiration results in river reaches that annually absorb more carbon than they emit, effectively functioning as carbon sinks. Approximately one-quarter of western river reaches fall into this category, in contrast to roughly 11% in the more heavily forested eastern regions.
The analysis was made possible by the integration of US Geological Survey data with machine learning algorithms trained to identify key environmental drivers of photosynthesis and respiration in rivers. These drivers include but are not limited to solar irradiance, water temperature, nutrient availability, organic matter concentrations, and hydrological flow regimes. The model’s predictive power allows for the estimation of metabolic rates in river segments lacking direct measurements, enabling a holistic assessment spanning diverse climatic and ecological contexts. Such modeling approaches represent a breakthrough in addressing the spatial heterogeneity and complexity of riverine carbon cycling at continental scales.
An intriguing aspect of this study is its implications amidst the context of climate change. In the Western United States, rising temperatures and altered precipitation patterns are influencing streamflow dynamics, often resulting in slower, lower-volume rivers. These changes increase light availability within the water column, enhancing photosynthetic rates and consequently carbon uptake. Nonetheless, this potential benefit is precarious; if drought conditions intensify to the point of riverbed desiccation, the absorption capacity is lost, and these aquatic systems could switch from carbon sinks to sources, releasing stored carbon back into the atmosphere and exacerbating greenhouse gas concentrations.
While the findings underscore the nuanced roles of rivers within regional and global carbon budgets, uncertainties persist. River metabolism is highly sensitive to climatic variability, land use changes, and anthropogenic impacts such as water diversion and pollution. The study’s robust methodology narrows major knowledge gaps but also highlights the need for expanded monitoring networks that capture comprehensive data across climatic gradients and seasonal cycles. Understanding these dynamic interactions is essential for predicting future feedbacks between freshwater ecosystems and climate systems.
The implications extend beyond academic curiosity, offering critical insights for environmental policy and management. Recognition of rivers as potential carbon sinks, particularly in arid landscapes, can inform conservation strategies aimed at preserving or enhancing these ecosystem services. For instance, maintaining or restoring riparian vegetation and minimizing watershed disturbances could bolster photosynthetic capacities and stabilize riverine carbon sequestration. Moreover, incorporating river metabolism into global carbon models may improve the precision of climate projections and carbon budget estimations, essential for meeting international emission reduction commitments.
Dr. Maavara emphasized the transformative nature of this research: “Our work reveals that rivers in arid regions, often overlooked in global carbon assessments, can play a substantial role in mitigating atmospheric CO2. It challenges traditional thinking and illustrates the importance of expanding scientific inquiry beyond well-studied temperate forests.” Co-author Pete Raymond from Yale University adds that “Quantifying stream metabolism at the continental scale has been a longstanding challenge, and this study offers a novel framework that can be applied worldwide to better understand freshwater carbon dynamics.”
The global significance of these findings cannot be overstated. Approximately 65% of the Earth’s terrestrial land surface is classified as arid or semi-arid, suggesting that the carbon sink potential observed in U.S. rivers may be echoed on other continents. This insight invites further investigations into river systems in similar climatic zones across Africa, Australia, and Asia, with the potential to revise global carbon budget models fundamentally. It also highlights the interconnectedness of hydrological and atmospheric processes in regulating Earth’s climate.
Funded by multiple prestigious agencies, including the United Kingdom’s Natural Environment Research Council, the United States Department of Energy, and the National Science Foundation, this research exemplifies the power of interdisciplinary collaboration and cutting-edge technologies in advancing ecological science. The integration of field observations, remote sensing, and machine learning creates a powerful toolkit capable of addressing complex biogeochemical questions across expansive spatial scales.
As the scientific community continues to unravel the intricacies of the global carbon cycle, this study stands as a landmark contribution, demonstrating that rivers are not merely passive conduits of carbon but dynamic, spatially variable players in Earth’s climate system. Recognizing the variable metabolism of river networks, especially in arid regions, opens new avenues for climate mitigation and ecosystem management, underscoring the critical need for nuanced, spatially informed environmental policies in the Anthropocene.
Subject of Research: River metabolism and its role in the carbon cycle across diverse climatic regions of the contiguous United States.
Article Title: River metabolism in the contiguous United States: A West of extremes
News Publication Date: 6-Nov-2025
Web References:
http://dx.doi.org/10.1126/science.adu9843
References:
Maavara, T., Raymond, P., et al. (2025). River metabolism in the contiguous United States: A West of extremes. Science, DOI: 10.1126/science.adu9843.
Image Credits: Taylor Maavara
Keywords: Rivers, Carbon sinks, Carbon cycle, Aquatic ecosystems
