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The ocean holds gigantic amounts of carbon, much more than all land-based plants and soil. Scientists previously studied these carbon stocks in spring and summer. Now, in two published studies, they have looked at what happens in winter.

The ocean contains a lot of dissolved organic material. And because this is material that comes from plants and animals, it contains a lot of carbon.

Dissolved organic matter, abbreviated DOM, is a huge carbon store, much larger than all plants and soil on land. Therefore, it also plays a major role in reducing CO₂ in the atmosphere, and thus in climate regulation.

But changes in the climate, and especially increased temperatures in the Arctic oceans, can also change how the carbon in DOM is stored and released.

"Previously, researchers have concentrated on studying conditions in the Arctic in the spring and summer. But we looked at what happens to DOM in the winter, especially in the northern parts of the Barents Sea," says Ph.D. research fellow Maria Guadalupe Digernes. That paper is in the journal Marine Chemistry.

Using advanced chemical methods such as mass spectrometry, she and her colleagues looked at both the amount of DOM and how it was distributed in Arctic regions throughout the year. They also looked at how the material changes. And here's where the researchers found something surprising.

No downtime in winter

"In the past, we thought that winter is a dead time for dissolved organic material. But that's not the case at all," Digernes said.

This material actually becomes much more unstable during the late winter and early spring. "This means that microbes can break down the material more easily," she said.

That is exactly what happened. In a study of fjord water from Tromsø in Biogeosciences, they found that the size of the particles also became smaller. The organic particles thus disintegrate even more than at other times of the year.

In other words, in winter, the dissolved organic material decomposes more easily than in summer. It forms smaller particles.

This is important in our efforts to understand how carbon stores may change over time, especially as the climate warms.

"It shows that the carbon cycle is active even in winter. We have not known for sure until now. And not least, it is important because the ocean is getting warmer, and thus the entire balance can change. Warmer seas mean that seasonal variations can also be different," she said.

We do not yet know what consequences this may have.