Humanity's harm to waterways contributes to the climate crisis

Humans are accelerating the rate at which organic matter decomposes in rivers and streams on a global scale. These human impacts on waterways pose a threat to biodiversity and increase atmospheric carbon, potentially exacerbating climate change. 

The research was led by the University of Georgia (UGA), Oakland University, and Kent State University.

Humans fundamentally change waterways

The study is the first to combine a global experiment with predictive modeling to illustrate human impacts on waterways contributing to the global climate crisis.

“Everyone in the world needs water. When human activities change the fundamental ways rivers work, it’s concerning,” said study co-author Krista Capps, an associate professor in UGA’s Odum School of Ecology and Savannah River Ecology Laboratory. 

“Increases in decomposition rates may be problematic for the global carbon cycle and for animals, like insects and fish, that live in streams because the food resources they need to survive will disappear more quickly, lost to the atmosphere as carbon dioxide.”

Disruptions in the global carbon cycle

Rivers and streams play a key role in the global carbon cycle by storing and decomposing large amounts of leaves, branches, and other plant matter. 

Normally, the process would involve leaves falling into rivers, where bacteria and fungi colonize the leaves. Insects then eat the bacteria and fungi, using the carbon stored in the leaves to grow, and fish subsequently eat the insects. This process ensures carbon is passed through the food chain.

However, the study found that this process is being disrupted in human-impacted areas, such as those affected by urbanization and agriculture, where leaf litter decomposes more quickly. 

Faster decomposition means insects cannot absorb the carbon from the leaves, leading to more carbon being released into the atmosphere, contributing to greenhouse gas pollution and disrupting the food chain.

Emissions from aquatic ecosystems 

“When we think of greenhouse gas emissions, we tend to think of tailpipes and factories,” said co-author Scott Tiegs, a professor of biological sciences at Oakland. “But a lot of carbon dioxide and methane comes from aquatic ecosystems.” 

“This process is natural. 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.”

Decomposition rates in rivers and streams 

To investigate, the researchers collected field data from 550 rivers across the globe, collaborating with over 150 researchers in 40 countries. They generated one of the first estimates of decomposition rates in rivers and streams worldwide, including understudied areas like the tropics. 

The data was compiled into a free online mapping tool that shows how fast different types of leaves decompose in local waterways.

Factors linked to increased decomposition rates 

Using predictive modeling, the scientists identified environmental factors responsible for increased decomposition rates, such as higher temperatures and increased nutrient concentrations. 

“Both of these factors are impacted by human activities,” said co-author David Costello, an associate professor of ecosystem biology at Kent State. “Reducing human impacts on decomposition will keep more carbon in rivers, preventing it from entering the atmosphere as carbon dioxide and contributing to climate change.”

These findings underscore the importance of taking urgent measures to mitigate human impact on waterways to protect biodiversity and help combat climate change.

How human activities impact waterways

Human activities have profound and diverse impacts on waterways, affecting everything from water quality to the health of aquatic ecosystems. 

Pollutants 

Industrial waste, agricultural chemicals, plastics, and untreated sewage are some common pollutants that degrade water quality and harm aquatic life. 

Overfishing 

Overfishing is another significant issue, leading to the depletion of fish populations and altering aquatic ecosystems, which impacts species diversity and ecosystem stability.

Unnatural barriers 

The construction of dams and other barriers is also a major factor, as it blocks the natural flow of rivers, affecting the migration and spawning of fish and other aquatic organisms, while altering sediment and nutrient flows. 

Deforestation 

Deforestation, especially along riverbanks, leads to increased erosion and sedimentation, which can smother habitats and reduce water quality. 

Urban development 

Urban development contributes as well, as it increases runoff from impermeable surfaces like roads and buildings, carrying pollutants into waterways and often involving the alteration or paving over of natural water bodies.

Climate change 

Climate change further exacerbates these impacts with rising temperatures and changing precipitation patterns affecting water availability and leading to more frequent extreme weather events. 

Invasive species

Additionally, the introduction of invasive species can disrupt local ecosystems, outcompete native species, and lead to a reduction in biodiversity. 

These impacts underscore the need for comprehensive management and policy interventions to protect and sustain waterways for future generations.

The study is published in the journal Science.

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