Zooplankton production may diminish under the ice pack | Polarjournal
Buoys sensitive to temperature, depth and sound, and able of detecting the presence of zooplankton 50 metres below the surface of the ice, were installed in the Central Arctic Ocean in 2020 during the MOSAiC expedition. Image: Folke Mehrtens

The results of a study published in Nature suggest that more attention should be paid to Arctic zooplankton. Sea ice retreat could reduce the productivity of these invertebrates that are at the base of the food chain.

A pair of feelers, a segmented body and jerky movements. Zooplankton in the Arctic Ocean could well be deprived of food during increasingly austere winters, with dramatic consequences for predators such as cod and seals, and for the lives of the Inuit.

These findings were published in the journal Nature on 28 August by a team of scientists from the Alfred Wegener Institute, the British Antarctic Survey and the Universities of Newfoundland, Plymouth and College of London

During autumn, winter and spring – seasons that have been very poorly documented in this region – the migration in the water column of copepods, krill and other small crustaceans that live in planktonic form should change rhythm in the future. They would obviously spend more time at greater depths.

Animals such as the copepod Metridia longa dive to depths of more than 100 metres in the Arctic Ocean during the summer and rise again during the winter. Image: Hopcroft / UAF / NOAA / CoML

The cause is daylight, which reaches deeper and deeper layers. These animals are highly sensitive to sunlight and can detect variations in light intensity as small as 0.00024 watts per m2, or 50,000 times less than the light recommended in a bathroom.

The sun is in no way responsible for this influx of light. The warming of the ocean and the atmosphere is thinning the ice pack and the snow cover; the ice disappears earlier in the spring, allowing light to penetrate more easily.

A second mechanism should be triggered. The early retreat of the pack ice and its late reconstitution will encourage phytoplankton blooms. The formation of these microalgae will slow down before the ice forms again, preventing them from reseeding the autumn ice.

“This, in combination with their delayed rise to the surface, could lead to more frequent food shortages for the zooplankton in winter”, says Hauke Flores, the study’s principal investigator. “If that comes to pass, it will have fatal consequences for the entire ecosystem, including seals, whales and polar bears”.

The average ice cover of the sea is decreasing by 13% per decade. According to the latest studies, the North Pole will be ice-free in summer by 2030 – previously the deadline was underestimated. In 30 years’ time, zooplankton will only be able to live under the ice for one month a year. Image: Michael Wenger

The authors stress the importance of conducting studies in the Arctic in winter “to be able to predict whether the Arctic Ocean will become a new oasis or a desert if the climate crisis is not controlled”.

The researchers worked on data taken by observation buoys deployed during the Polarstern‘s drift. This German oceanographic vessel was trapped in the ice and currents of the Arctic Ocean between 2020 and 2021.

The French project Tara Polar Station should drift in the Arctic during the winter of 2025-2026. Research programs wish to continuously observe zooplankton and phytoplankton through a vertical shaft accessing the water under the ice and around which the drifting scientific station will be built. Drawing: Louison Wary / Tara Foundation

Other research organisations could also look into the matter, given the scale of the current results. Information that is in line with the concerns of the Arctic Council, non-fishing agreements in international waters and treaties aimed at reducing greenhouse gas emissions.

Camille Lin, PolarJournal

Source Flores, H., Veyssière, G., Castellani, G., Wilkinson, J., Hoppmann, M., Karcher, M., Valcic, L., Cornils, A., Geoffroy, M., Nicolaus, M., Niehoff, B., Priou, P., Schmidt, K., Stroeve, J., 2023. Sea-ice decline could keep zooplankton deeper for longer. Nat. Clim. Chang. 1–9. https://doi.org/10.1038/s41558-023-01779-1

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