Winter organic carbon export could feed zooplankton in the High Arctic

In the High Arctic, winter is often portrayed as a season when the ocean’s biological machinery shuts down beneath darkness, freezing temperatures, and sea ice. A new study challenges that simplified picture by highlighting a potentially important source of food moving through the polar ocean during the coldest months: organic carbon exported from surface waters. The research, published in Communications Earth & Environment, suggests that this wintertime carbon could help sustain zooplankton, the small drifting animals that form a crucial link between microscopic producers and larger marine life.

The study by Wu, Jin, Ji and colleagues focuses on a process that is central to the ocean’s carbon cycle. Organic carbon refers to carbon contained in biological material, including the remains of algae, cellular compounds released into seawater, and particles formed when microscopic organisms die or aggregate. When this material sinks or is transported away from the surface, scientists describe the process as organic carbon export. In most discussions of the Arctic, attention has traditionally centered on the summer bloom, when returning sunlight stimulates intense phytoplankton growth. The new work draws attention to what happens after that brief productive season ends.

The High Arctic receives little or no sunlight during part of the winter, depending on latitude. This polar night sharply restricts photosynthesis, the process by which phytoplankton convert light, carbon dioxide, and nutrients into organic matter. Yet the absence of sunlight does not mean that the ecosystem becomes chemically or biologically inactive. Organic particles produced during earlier months can remain in the water column, be transformed by microbes, or be redistributed by currents, turbulence, ice formation, and melting. These processes can determine whether carbon is stored, remineralized back into carbon dioxide, or made available to organisms that survive through winter.

Zooplankton are especially important in this chain. Although many species are tiny, they consume phytoplankton, bacteria, and decaying organic material, and they are eaten by fish, seabirds, seals, and whales. Some Arctic zooplankton species have evolved strategies for surviving long periods of limited food availability, including slowing their metabolism, storing lipids, or moving vertically through the water column. Even so, winter nutrition can influence their survival, reproduction, and condition when the next productive season begins. A supply of organic carbon during winter could therefore have consequences far beyond the microscopic particles themselves.

The phrase “potential food source” is scientifically significant. It indicates that exported organic carbon may be available to zooplankton, but it does not necessarily mean that every particle is directly consumed or that the carbon immediately becomes animal biomass. Organic matter can enter the food web through several pathways. Zooplankton may ingest particles directly, while bacteria may first decompose complex material into smaller compounds. Zooplankton can then feed on those microbes, creating a microbial loop that transfers otherwise inaccessible carbon back into the grazing food web. The nutritional value of the carbon may also depend on its chemical composition, age, and origin.

In Arctic waters, the physical environment strongly controls these pathways. Sea ice can act as both a barrier and a habitat, influencing light penetration, gas exchange, nutrient movement, and the distribution of organisms. When seawater freezes, salts and dissolved substances are expelled into the surrounding water, changing its density and circulation. During freezing and melting, organic particles can become trapped in or released from ice. Meanwhile, storms and shifting ice fields can mix surface and deeper waters, potentially redistributing carbon and zooplankton across large areas. These interactions make winter export a dynamic process rather than a simple downward rain of particles.

The findings also matter because the Arctic is changing rapidly. Warmer temperatures are reducing sea-ice coverage and altering the timing of freeze-up and melt. Increased open water can allow more light and wind-driven mixing into the ocean, while changes in freshwater input and circulation can affect nutrient availability and plankton communities. Such shifts may change when organic carbon is produced, how long it remains suspended, and where it is exported. A process that is relatively minor in one region or season could become more important as the balance between ice-covered and ice-free waters changes.

Understanding winter carbon export is also essential for improving estimates of the Arctic Ocean’s role in the global carbon system. Marine organisms absorb carbon dioxide through photosynthesis, and some of the resulting organic carbon can be transported into deeper waters or sediments, where it may remain isolated from the atmosphere for extended periods. This mechanism, often called the biological carbon pump, is one of the ocean’s natural pathways for moving carbon away from the surface. However, carbon that is consumed and respired by organisms may return to the water as carbon dioxide before reaching depth. The ecological and climate significance of export therefore depends on both its quantity and its fate.

By connecting winter carbon movement with zooplankton nutrition, the study adds a biological dimension to a process often considered mainly in terms of carbon storage. It suggests that the Arctic winter should not be treated as an ecological blank period between two summer blooms. Instead, organic matter produced earlier may continue circulating through food webs beneath the darkness and ice. The research points toward a more continuous view of polar productivity, in which winter processes help shape the condition of organisms and the efficiency of carbon transfer throughout the year.

The study’s broader message is that the Arctic ecosystem may be sustained by hidden seasonal connections. Carbon exported during winter could support zooplankton, zooplankton could nourish higher predators, and the movement of this material could influence how much carbon remains in the ocean or returns to the atmosphere. As climate change transforms the High Arctic, measuring these links will become increasingly important. Future observations and experiments will need to determine how much exported carbon zooplankton actually consume, which organisms benefit most, and how changes in sea ice and circulation will reshape this wintertime food pathway.