“We have known for some time that while nutrient pollution can lead to more organic matter production and subsequently more carbon accumulation in the sediments of lakes and rivers. Nutrient pollution also leads to more greenhouse gas emissions, particularly in the form of the powerful greenhouse gas methane,” says Kosten. “An eye-opener about our new analyses is that even organic matter from outside of water systems (such as leaf litter) breaks down faster in these nutrient-rich waters reducing the carbon storage and potentially leading to extra emissions of greenhouse gases.”
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, the authors point out, is a major contributor to the increase in carbon emissions 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 co-lead author 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.”
“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 modeling, the researchers also 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 Tiegs.
