Humans are increasing the rate at which organic matter decomposes in freshwater ecosystems around the world, speeding up natural processes that contribute to greenhouse gas emissions and threaten biodiversity. This is according to a first-of-its-kind study published recently in the journal Science that combines predictive modeling with field data from 550 rivers around the world.
The multi-year project was led by a team of five researchers from the University of Georgia, Oakland University, Kent State University and William & Mary’s Virginia Institute of Marine Science (VIMS). It also leveraged the work of the Celldex Consortium, a global network of 150 collaborators from 40 countries.
By taking advantage of the global consortium’s ability to collect field samples from around the world, including from underrepresented tropical regions, the team measured the rate of cellulose decomposition in each river using a standardized cotton assay. Then, using machine-learning algorithms that combined cellulose decomposition rates, satellite-derived environmental conditions, and observations of leaf litter decay, they were able to predict the decomposition rates of different types of leaves at the scale of individual streams under current or future conditions anywhere in the world.
“Leaves differ among individual plants, populations and species in terms of chemistry, such as nutrient content, and structure, such as cellulose and lignin content,” said Chris Patrick, associate professor at VIMS and one of the corresponding authors on the study. “To take our measurements of cellulose decomposition rates to the next level, we built a leaf trait library from all the published trait data we could find and then used that to link the baseline predictions to observations of real-world leaf breakdown in a model that works remarkably well.”
The data was clear: Humans are impacting decomposition rates in rivers on a global scale, with temperature and nutrients identified as some of the most important drivers of the patterns.
“Everyone in the world needs water,” said Krista Capps, co-principal investigator on the study and an associate professor at the University of Georgia. “When human activities change the fundamental ways rivers work, it’s concerning. 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.”
“This has huge management implications,” said Patrick. “It allows us to forecast from regional to global scales how decomposition rates and carbon cycling will respond to climate change, environmental alterations and species invasion.”
Nutrients and temperature act as catalysts in changing landscapes
When a leaf falls into a river, processes are set in motion that will either recycle the organic carbon through the food chain, store it in sediment deposits or release it into the atmosphere. Insects may find forage in the leaf litter, providing nutrients for growth and reproduction before themselves becoming a meal for fish or other animals.
However, the study found that decomposition rates were highest in densely populated, agricultural areas. In places such as the U.S., Europe and Southeast Asia, faster decomposition caused by rising temperatures and nutrients from runoff are increasing carbon dioxide emissions and altering the food chain.
“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 Scott Tiegs, co-principal investigator on the study and professor at Oakland University. “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.”
Things get even more complex when species are considered in addition to environmental conditions. Humans are changing forest species composition, both directly by introducing invasive species and indirectly through environmental changes. These changes alter the dominant plant and animal species in the environment.
One of Patrick’s major contributions was developing a way to link the decomposition of the cotton strip, the baseline assay, to different species of leaves. On this work, he collaborated closely with co-principal investigator David Costello, associate professor at Kent State University. Costello developed a website that maps predicted leaf decomposition rates by species—users anywhere in the world can view decomposition rates in their local waterways.
“Our work demonstrates that changes in forest tree species due to climate change and species invasion can be just as important to decomposition rates as pure environmental change,” said Patrick.
Ultimately, the authors note that reducing human impacts on decomposition will prevent the release of organic carbon into the atmosphere as CO2.
“We need to minimize human impacts on fresh waters to more effectively manage our global carbon cycle,” said Tiegs.