As sea ice declines, Arctic sunlight penetrates deeper into the ocean, boosting primary production in the water and even within the ice. This shift carries serious consequences for seals, polar bears, and whales whose food depends on these microscopic producers.
The response of marine zooplankton to available light also changes their behavior, especially how small organisms move up and down the water column.
An international research team led by the Alfred Wegener Institute in Germany has shown that future conditions could bring more frequent food shortages for zooplankton and negative effects on larger species such as seals and whales, findings published in Nature Climate Change .
The extent and thickness of Arctic sea ice is decreasing due to human-caused climate change; today the average sea ice extent is falling by about 13% each decade. By 2030, simulations indicate the North Pole could experience its first summer without ice.
As a consequence, the physical conditions for organisms in the Arctic Ocean are changing. Shrinking and thinning sea ice allow sunlight to reach far below the surface.
Arctic melting has a range of consequences for ecosystems and communities. The primary production of microalgae in water and ice can rise under certain conditions, yet how these changing light patterns affect higher trophic levels, such as zooplankton that feed on microalgae, remains an area of active study .
In this context, an international team of researchers, led by Dr. Hauke Flores of the Alfred Wegener Institute, has gathered valuable new data to illuminate these dynamics .
Mass movement of microorganisms
Flores notes that the largest daily biomass movement on the planet comes from the seasonal migration of zooplankton, including small copepods and krill. At night they rise to the surface to feed; with daylight they descend again, seeking safety from predators.
Even though individual organisms are tiny, together they drive a vast daily vertical transport of biomass in the water column. In polar regions, however, this migration follows a seasonal pattern: zooplankton spend months in the depths, and during the polar night some rise and stay just under the ice, he explains.
Surface-level, near-surface, and under-ice ecosystems all depend on this daily rhythm. Tiny organisms generally prefer twilight conditions, seeking light levels that are low and just above darkness.
When sunlight intensity shifts with the time of day or season, zooplankton move to locations that meet their preferred light conditions, which means a continual adjustment of their vertical position in the water column.
There was limited data on zooplankton in the upper 20 meters just below the sea ice, a zone considered highly interesting because microalgae feeding on zooplankton grow inside and just beneath the ice .
To access readings in this challenging region, the team designed a self-contained biophysical observatory and anchored it under the ice at the end of the MOSAiC expedition with the research icebreaker Polarstern in September 2020. The setup enabled continuous measurements of light intensity under the ice and the movements of zooplankton in a relatively light-pollution-free environment.
Light depends on the thickness of the ice
From these readings, researchers determined a very low critical radiation level for zooplankton: 0.00024 watts per square meter. This parameter was incorporated into computer simulation models of the sea ice system to project, under various climate scenarios, how the depth of this radiation threshold will shift as sea ice becomes thinner later this century.
The experts found that a continuous reduction in ice thickness pushes the critical light level deeper in the water, and it stays below the surface for longer periods each year. Since zooplankton largely inhabit waters beneath this level, their movements will mirror this deeper distribution.
In future warmer climates, later ice formation in autumn will reduce algae production on the ice, and the delayed rise of light to the surface may lead to more frequent ice formation and food shortages for zooplankton during winter .
If zooplankton rise earlier in spring, larvae of ecologically important deeper-dwelling zooplankton species may be endangered, and many of these larvae could be prey for adults, Flores notes.
Taken together, the study points to a mechanism that further lowers the survival chances of Arctic zooplankton in the near term. If warming progresses unchecked, the consequences could be severe for the entire ecosystem, including seals, whales, and polar bears. Yet simulations also show that staying close to the 1.5-degree target matters: reducing anthropogenic warming helps limit impacts on vertical migration, underscoring the importance of climate action for the Arctic ecosystem .
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Notes from the environment department are not included here in order to maintain focus on the scientific findings.