New study chronicles strong seasonal variations in Arctic plankton communities

A new study shows how light, sea ice and ocean circulation together shape a strong seasonal rhythm in protist plankton communities in the northwestern Barents Sea.

The study led by PhD student Lucie Goraguer from the iC3 Polar Research Hub reveals that very different communities dominate Arctic waters in spring, summer and winter. 

Credit: Lucie Goraguer (NPI, iC3)

A year in the life of Arctic protists

Protists are single-celled organisms that include phytoplankton and many microscopic grazers. These tiny organisms sit at the base of the marine food web.

The study tracks how these communities change throughout the year in the northwestern Barents Sea, which is part of the Arctic Ocean. The team analysed the abundance, biomass and composition of protist plankton across seasons and across different water masses.

The results show a clear seasonal succession.

In spring, when sunlight returns and nutrients are still plentiful, fast-growing diatoms dominate the system. These algae form the classic Arctic spring bloom that fuels the marine food web.

In summer, the picture changes. Surface waters become more stratified and nutrients become scarcer. At this stage, a more diverse group of organisms takes over. Many of these protists are mixotrophs and heterotrophs. They can both photosynthesise and feed on other organisms.

In winter, communities look different yet again. When the Arctic enters months of darkness, dinoflagellates become the dominant group.

Lead author of the study Lucie Goraguer says:

“The seasonal turnover was striking. Each part of the year hosts a distinct community with different ecological strategies.”

Credit: Karoline Barstein (NTNU)

Atlantic waters shape Arctic plankton

The research team also found that ocean circulation plays an important role.

The Barents Sea is strongly influenced by warm Atlantic water flowing northwards. This inflow has an influence on temperature, nutrient supply and sea-ice conditions.

The study shows that these physical drivers shape which protist groups thrive. For example, ciliates were particularly abundant in warmer Atlantic-influenced waters, which typically combine higher temperatures with lower nutrients and reduced sea ice, during the summer and late summer months.

“These organisms respond quickly to environmental conditions,” Lucie explains. “By looking at plankton communities,we can see how physical changes in the ocean shape Arctic ecosystems.”

One surprising result was the presence of potentially active protist communities even during the polar night.

“The Arctic winter is often seen as a quiet period for marine life,” Lucie says. “But our results show that diverse protist communities remain highly present, especially mixo- and heterotrophic species,  even during the dark months.”

Credit: Tim Kalvelage

Why these tiny organisms matter

Protists may be microscopic, but their influence is enormous. They form the base of Arctic marine food webs and play an important role in the ocean carbon cycle.

Changes in their seasonal dynamics can ripple through the ecosystem, affecting zooplankton, fish and the transfer of carbon from the ocean surface to the deep sea.

The Barents Sea is currently one of the fastest warming regions of the Arctic Ocean. Atlantic water is becoming more dominant, and sea-ice cover is declining.

“As the Arctic becomes more Atlantic-influenced, we expect plankton communities to change,” Lucie says. “Understanding today’s seasonal patterns helps us anticipate how Arctic ecosystems may evolve in the future.”

Read more

The study “Environmental drivers shaping protist plankton seasonality in the northwestern Barents Sea” has been published in Progress in Oceanography.

Lead author Lucie Goraguer is a PhD student with the iC3 Polar Research Hub. She is based at the Norwegian Polar Institute in Tromsø. Her research focuses on Arctic and Antarctic plankton ecology and how microscopic organisms respond to environmental change in polar seas. To find out more about her work, please check out her list of publications, or contact her by email.

Co-author Philip Assmy is a researcher at the Norwegian Polar Institute and at the Department of Geosciences at UiT The Arctic University of Norway. His work focuses on understanding the complex interactions between ice and ocean systems and their impact on marine ecosystems, particularly in the rapidly changing Arctic and Antarctic regions.

Philipp is looking for a candidate interested in applying for an MSCA Postdoctoral Fellowship in 2026 to research harmful algae and their toxins in Svalbard fjords. Please see here for more details.