Human-driven impacts on carbon release from rivers

Researchers use field data and predictive modelling to show the human impacts on aquatic ecosystems and how they contribute to the global climate crisis.

A new study published in the journal Science estimates the rates of organic-matter decomposition in freshwater ecosystems, which is a source of carbon emissions.

The paper is titled “Human activities shape global patterns of decomposition rates in rivers.”

It looks at decomposition across the world, including areas like the tropics that have previously been underrepresented. 

Professor Lee Brown (School of Geography) is a co-author of the study. The lead authors are Professor Scott Tiegs (Oakland University), Dr Krista Capps (University of Georgia) and Dr David Costello (Kent State University). 

Highest decomposition rates in agricultural areas 

The researchers collected field data from 550 rivers around the world and used predictive modeling and machine-learning algorithms to complete their dataset. 

Professor Brown contributed to the study by collecting new datasets for four UK rivers. 

He said "The results show clear variations in leaf decomposition and carbon processing between different river types. Environmental management groups can use this new information to target interventions such as pollutant control or river restoration schemes." 

Professor Capps said “Through a large, collaborative effort using a standardized, experimental approach, we were able to generate the first estimates of decomposition rates in streams throughout the world, including understudied areas such as the tropics. 

“Integrating experimental data collected at such a large scale with modelling is a powerful approach to examine global patterns in important ecological processes like decomposition.” 

The study reported the highest decomposition rates among densely populated, agricultural areas, including parts of the United States, Europe and Southeast Asia. 

Agricultural and urban runoff is a major contributor to the increase in carbon emissions that are responsible for climate change. 

“When we think of greenhouse gas emissions, we tend to think of tailpipes and factories, but a lot of carbon dioxide and methane can come from aquatic ecosystems,” said Professor Tiegs.  

“This is a natural occurrence, but when humans add nutrient pollution (like fertilizer) to fresh waters and elevate water temperatures, we increase the decomposition rates and direct more CO2 into the atmosphere.” 

Human impacts on decomposition in rivers

One of the study’s key takeaways is that humans are impacting decomposition rates in rivers on a global scale – a finding illustrated by the maps presented in the study. 

The maps are freely accessible through a website built by the authors.  

“The online mapping tool lets anyone see how fast we think different kinds of leaves will decompose in their local rivers,” said Costello. 

“Even if no researcher has made measurements in a river, we have an estimate for how fast leaves should decompose.” 

Using predictive modelling, the researchers identified key environmental variables responsible for increased decomposition rates, notably temperature and nutrient concentrations. 

The primary variables driving decomposition rates are impacted by human activities, such as nutrients and temperature. 

The authors noted that reducing human impacts on decomposition will keep more carbon in rivers, preventing it from being released into the atmosphere as CO2.  

“We need to minimize human impacts on fresh waters to more effectively manage our global carbon cycle,” said Professor Tiegs.