Arctic Ocean chemistry changes dramatically during winters

The Arctic winter is not a pause button for ocean carbon. New research shows that the carbon dissolved in seawater becomes easier for microbes to use late in winter, flipping a key part of the marine carbon cycle at a time many assumed was quiet.

That matters because the ocean’s pool of DOC is immense, with hundreds of billions of tons of carbon tied up in molecules too small to see. If winter conditions change how that pool behaves, the knock-on effects reach from microbes to climate.

Winter in the Arctic Ocean isn’t quiet

A team led by Maria G. Digernes at the Norwegian University of Science and Technology (NTNU) tracked what Arctic winter does to dissolved organic matter (DOM).

The researchers focused on the northern Barents Sea, a region that is warming unusually fast and sits at a gateway to the central Arctic.

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

The group sampled across early winter, late winter, and spring, then probed the chemical fingerprints of the organic molecules.

The study points to a simple takeaway for a complex Arctic system: late winter DOM looks more usable to microbes than early winter DOM.

Arctic Ocean shifts during winter

The researchers used high resolution mass spectrometry to sort thousands of individual formulas within the DOM pool. That approach let them track shifts in features that hint at how easy a molecule is to break down.

The team also compared surface waters and deep layers at selected sites along the shelf and slope. The picture that emerged linked season, light, mixing, and biology to changes in molecular makeup.

Late winter DOM contained more nitrogen bearing formulas than early winter, a sign of molecules that microbes tend to process more readily. It also showed a tilt toward smaller average molecular size, another flag for higher lability.

“This means that microbes can break down the material more easily,” said Digernes. By spring, the team found lower aromaticity and lower molecular weights in some surface samples, consistent with fresh production and continued transformation.

Those patterns together suggest a seasonal handoff in how DOM is stored, altered, and moved.

Winter microbes reshape Arctic carbon

The late winter signal of nitrogen rich DOM can arise when viruses rupture microbial cells and release their contents.

The authors propose that such viral processes earlier in the season could help load the water with more labile compounds.

Microbial activity then reworks that stock, with bacteria consuming some molecules and reshaping others. The chemistry bears those fingerprints without requiring a large bloom at the sampling moment.

Arctic fjords respond to winter

To test how these processes look in coastal Arctic waters, the team examined a sub-Arctic fjord near Tromsø.

In controlled incubations during the winter period, particulate carbon decreased while the DOM pool became more labile based on changes in saturation, aromaticity, and oxygenation.

During productive months, the same experiments showed rising particulate carbon and a trend toward less labile DOM. That contrast underscores how strongly the season steers the direction of carbon transformations.

The Barents Sea is warming fast

Rapid warming in the Barents area tightens the stakes. Observations show record high rates of surface air temperature increase in parts of the region, closely tied to sea ice loss.

Warmer seas alter mixing, light, and ice cover, each of which nudges DOM chemistry. A shift in the length or timing of winter and spring could therefore change how much carbon stays in surface waters versus gets transported downward.

The marine DOC pool is roughly on the same order as atmospheric CO2, and its residence time spans hours to millennia depending on molecular makeup. That size alone makes winter processes worth watching, because even small fractional shifts can represent large absolute amounts.

Late winter’s rise in labile DOM could prime the system by replenishing nutrients as microbes remineralize organic matter. It may also influence how much fresh DOM builds when spring light returns.

Models must account for winter

Most past work in the Arctic emphasized spring and summer when blooms are obvious. These new observations add the missing season and reveal that winter is chemically active, not dormant.

Climate models improve when they track the timing and quality of carbon, not just quantity. Adding winter DOM behavior, and the microbial and viral levers behind it, can help reconcile how carbon moves from surface waters to depth.

Open water expansion and stronger winds may alter vertical and lateral mixing in coming decades. Those physical shifts could either enhance or limit the export of oxygen rich or nitrogen rich DOM to deeper layers.

If late winter regularly tips DOM toward forms microbes can rapidly use, the surface carbon inventory might shrink while nutrient recycling grows. That tradeoff will shape both local ecosystems and the ocean’s role in storing carbon.

Future Arctic carbon dynamics

Fieldwork in polar night is hard, and it hinges on dedicated ship time and careful lab work. The payoffs reach beyond Arctic fjords because the same rules of chemistry and microbiology operate in cold oceans worldwide.

“It shows that the carbon cycle is active even in winter,” said Digernes. That simple sentence reframes a season many assumed did not matter much for ocean carbon.

The Barents Sea sits at a crossroads where Atlantic water meets polar water. That intersection amplifies sensitivity to climate signals and makes the region a bellwether for future Arctic carbon dynamics.

Changes in DOM chemistry here will echo across the basin as waters mix and flow north. Understanding winter is part of understanding where that story goes next.

The study is published in the journal Marine Chemistry.

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