This is how carbon flows – new theory combines decades of Swedish field research

The new framework, named Variable Scale Domains, is built on decades of research in the Krycklan catchment outside Vindeln in Västerbotten, Sweden. The model explains how different forms of carbon – both organic and inorganic – are transformed as they are transported through the stream network. All the way from precipitation to the outlet flowing into the ocean.

Earlier theories on the regulation of water ecology and water quality have generally leaned in two different directions. Some say that the size and scale of the water network is most important, while others say that local distinct features such as forests, wetlands and lakes carry the most explanatory power. The new theory combines these two perspectives into one.

– We show that both the general scale and the local environmental features are about equally as important but are regulated in different ways depending on the water flow, and the types of carbon compounds you are interested in, says Hjalmar Laudon, Professor of Forest landscape Biogeochemistry at the Swedish University of Agricultural Sciences.

Carbon contents are constantly changing

The carbon contents of a river as it reaches the ocean does not tell the full story. Through its journey, the amounts and forms of carbon are constantly in flux. Organic carbon is swept into the stream as it runs through vegetation, but is over time broken down to inorganic forms, releasing as carbon dioxide. Add seasons, weather and water flow, and you get an array of factors shaping how much carbon is moving and where it ends up.

To see the whole picture, you need to study the different landscape segments and how they are connected. This is where the Krycklan Catchment Study provides a unique strength. For many decades, researchers at the Swedish University of Agricultural Sciences have been monitoring the water flowing from forests and mires to the smallest streams, lakes and rapids toward the Vindel River, ultimately building a robust database.

– The focus on carbon in our water has increased dramatically in the last years. This is because carbon affects not only water quality, but also the pH and transportation of heavy metals in our streams. Being able to compile all of our data from these separate features into a complete model for the system is a great achievement, says Hjalmar Laudon.

Redrawn more than 60 times

Work on the model started as early as 2018. Hjalmar Laudon and colleague Ryan Sponseller at Umeå University drafted a first vision, and over time more colleagues have joined in on the effort. The model that is now presented in Nature Water has come a long way since then and is the result of more than 60 revisions.

– I would say this is by far the most work I have ever put into a paper, says Hjalmar Laudon.

The researchers hope the new framework will be tested by colleagues around the world working in other types of catchments and river systems. 

– We have been able to create a model that that takes into consideration a large array of mechanisms controlling the carbon balance. I hope and believe it can be useful for others to see how it works at their sites, especially those working in large systems with a lot of landscape variation, says Hjalmar Laudon.