The northern Barents Sea region has experienced rapid warming and increased advection of warm Atlantic water in recent decades resulting in winter sea ice loss larger than in any other part of the Arctic Ocean. Scientists want to know what these changes mean for Barents Sea ecosystems.
By: Zoe Koenig, Morven Muilwijk, Øyvind Foss, Philipp Assmy, Agneta Fransson, Sebastian Gerland and Mats A Granskog // Norwegian Polar Institute,
Karen M Assmann, Melissa Chierici, Elizabeth M Jones and Angelika HH Renner // Institute of Marine Research
Map of the Barents Sea with Nansen Legacy transect stations (yellow dots). Diamonds indicate stations where sea ice and extensive biological sampling was performed. Red arrows represent the inflow of Atlantic Water into the Barents Sea. Blue arrows highlight the circulation of Polar Water in the northwestern Barents Sea (delineated by the black dashed lines). Purple/green/orange/red lines show the sea ice edge (SIE) in
March/May/July/September, respectively.
Credit: Zoe Koenig / Norwegian Polar Institute
Air temperatures over the northern Barents region have increased 5-7 times the global average, and warm water flowing in from the Atlantic not only brings heat, but also causes changes in the physical and chemical environment of the Barents Sea. How these factors and their impacts on the marine ecosystem unfold on a seasonal basis is still poorly understood because of inadequate data coverage in autumn, winter, and spring in the northwestern Barents Sea shelf, which is still seasonally ice-covered.
Between the late 1990s and early 2000s, primary production (synthesis of organic material by algal photosynthesis) in the northern Barents Sea was mainly driven by the length of the open water season, determining light availability in the water column. With warmer conditions and less sea ice in summer, the system shifted in 2009 from being light-limited to nutrient-limited, meaning that nutrient availability now plays a more important role in primary production and phytoplankton growth. This change could alter the timing and magnitude of primary production and, ultimately, energy transfer in the marine food web, but we still do not fully understand how these processes vary by season.
In the Nansen Legacy project, we collected physical, chemical, and biological data from the northwestern Barents Sea during four cruises in 2021, from late winter to late summer, with conditions ranging from fully ice-covered to ice-free. The cruises studied a swath northward across the central Barents Sea shelf into the adjacent Nansen Basin.
The data show that sea ice meltwater and the timing of ice-free conditions during summer play crucial roles in controlling heat accumulation, light availability, nutrient levels, and biological activity throughout different depths and across seasons.
Some specific seasonal findings include
March (late winter)
The ocean was cold and ice-covered, with a deep mixed layer (the upper ocean layer where mixing occurs) extending to 90–120 m. Nutrient levels were high, but phytoplankton biomass (the amount of microscopic algae) was very low, likely in part due to low light levels below the ice.
May (early spring)
As ice continued to grow between March and May, the mixed layer deepened and became more saline. Although the ocean was still mostly ice-covered, the levels of chlorophyll-a (an indicator of phytoplankton biomass) began to increase in the upper 50 m, marking the start of the spring bloom. This early bloom initiation under deeply mixed conditions suggests that phytoplankton can begin growing under low light conditions and before the onset of surface stratification.
Summer
Meltwater from sea ice formed a fresh, low-density surface layer that created a strong barrier for mixing, which meant that phytoplankton consumed all the available nutrients in the layer. As summer progressed and light penetrated farther into the water, phytoplankton growth occurred deeper below the surface layer, where nutrients were available.
The study’s findings suggest that in a future ice-free Barents Sea (as current observed trends and climate models suggest), reduced freshwater input (from melting sea ice), with less surface stratification, would increase vertical mixing and surface nutrients, likely boosting annual nitrate-based (new) production and enhancing resources throughout the pelagic food chain, from zooplankton to fish and potentially also carbon transport to the deep ocean. On the other hand, ice-associated production would be reduced, and its timing would change, negatively impacting species that depend on sea ice as a habitat and food source. Either way, the presence of sea ice as a habitat or source of freshwater is a key aspect of the functioning of the Barents Sea system.
Given the Barents Sea’s role as an important entry point for Atlantic Water into the central Arctic Ocean, understanding its seasonal changes is essential. The four cruises from March to September 2021 highlighted the close link between sea ice, ocean conditions, and primary production, showing how climate change is reshaping the Barents Sea marine ecosystem as the Barents Sea transitions to an ice-free state.
Acknowledgement
This study was part of the Nansen Legacy project (RCN#276730), funded by the Research Council of Norway and the participating research institutions.
Further reading
Koenig Z et al (2024) From winter to late summer in the northwestern Barents Sea shelf: Impacts of seasonal progression of sea ice and upper ocean on nutrient and phytoplankton dynamics. Progress in Oceanography 220: 103174.