The changing Arctic Ocean

Research vessel Polarstern returns to its home port of Bremerhaven at the weekend
[29. September 2023] 

After eventful and busy months, the Arctic season ends this weekend with the Polarstern expedition called ArcWatch-1. The team of almost 100 crew and scientists measured sea ice thickness and properties, recorded the currents and chemical properties of the ocean and investigated life in and under the ice, in the open water and at the bottom of the deep sea. Their data show significant changes compared to previous expeditions. On 7 September 2023, Polarstern reached the North Pole, and on 20 September there was the world's first livestream of an ROV under-ice dive from the Central Arctic.

The summer of 2023 marks the globally hottest summer since weather records began, glaciers are melting faster than ever, huge forest fires in Canada and Siberia are leaving their mark, sea ice was already melting faster than before in May and June 2023. Therefore, the expedition team expected particularly little sea ice during the investigations in the central Arctic. The first results were surprising: The sea ice of the central Arctic Ocean did not melt as much as expected in August and September, and it was also thicker than in previous years.

“The melt ponds, the sediment inclusions, the ice ridges, which are otherwise so characteristic of the Arctic sea ice landscape in summer, were missing. The ice was exceptionally flat and heavily melted from below. An unusual amount of snow on the floes ensured that they were protected from surface melt and there was little light directly under the ice,” reports AWI sea ice physicist Dr Marcel Nicolaus. He and his team used an underwater robot (Remotely Operated Vehicle, ROV) at nine ice stations. A special highlight of the expedition was the live broadcast of such a dive on the AWI YouTube channel on 20 September, the first ROV under-ice dive to be broadcast live from the Central Arctic on the internet and followed by several hundred people.

The large-scale deployment of the EM-Bird,  a sensor system deployed from the Polarstern helicopter, as well as parallel aircraft campaigns showed: the thickness of the level sea ice was still 1.2 metres at the beginning of September - more than in the summer of the MOSAiC expedition in 2020 or at the largest sea ice minimum in 2012. Thanks to real-time telecommunications via new satellites with polar coverage, the expedition’s data could be fed directly into models. Sea ice physicist Dr Thomas Krumpen explains the observed anomaly as follows: “In the last decades the floes drifted mainly from the Siberian shelves into the Eurasian Basin, but this year and also last year the sea ice came from the Canadian Basin, without contact to the shallow shelf. This is a very unusual transpolar drift pattern.” The cause is probably a phenomenon of unusually stable low-pressure systems that held the ice together on the Siberian shelf over the summer and was linked to a supply of cold polar air.

“Accordingly, we found hardly any ice algae on the underside of the sea ice. Especially Melosira arctica was missing, which can form metre-long chains and is an important nutrient supplier for the entire ecosystem. The ice looked rather dead this year. Due to the darkening by snow, algae floated up from the water and formed a film under the ice, to get some light,” reports chief scientist Prof. Dr Antje Boetius, Director of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). The physical oceanographers on board also discovered changes in the uppermost ocean layer, which was saltier than in previous years, due to the lack of ice melt and to lower inputs of Siberian shelf water.

The comparison with the previous survey years 2012 and 2020 also showed differences for the planktonic life floating in the water. In August and September, the algal bloom was long gone, and no ice algal biomass built up under the ice. Instead, the researchers found swarms of animals such as arrow worms, tunicates, ice amphipods, copepods, winged snails and comb jellies. The team led by the expedition co-leader Dr Christina Bienhold therefore found the biotic communities in the deep sea had changed: “Hardly any ice-algae sank into the deep sea this year. Nevertheless, the overall activity of living organisms on the bottom has increased somewhat compared to the sea ice minimum in 2012.” Images taken with a deep-sea camera showed that the composition of the community has changed substantially. The once smooth seafloor became heavily colonised and burrowed by annelids and bristle worms, crawling sea anemones and sea cucumbers. “It is amazing how quickly Arctic life responds to changes in sea ice cover,” says Antje Boetius. The team was able to obtain samples of all size classes of life in the Arctic deep-sea to study their diversity and distribution as well as changes compared to past decades.

The research of the ArcWatch-1 expedition also included seafloor mapping of previously unknown seamounts, one of which turned out to be a biodiversity hotspot. In addition, the chemists on board obtained large quantities of water and ice samples to record changes in the carbon pump to the deep sea and to detect non-degradable chemicals. For a European project, they are assessing the distribution of pollutants in the Arctic. The Polarstern team also successfully deployed a range of new high-tech instruments such as robots, autonomous sensors and sampling modules, and high-resolution under-ice cameras. They built a large network of buoys and deployed novel moorings for year-round surveys. This will provide them with further data on the changes in the central Arctic Ocean, even after Polarstern now returns from the central Arctic.

Interested parties can get further insights into the expedition as early as the turn of the year: A documentary, produced by UFA Documentary, with the working title ARCWATCH - HOFFNUNG IM EIS will be broadcas on December 29 at 9:45 p.m. on the German TV channel ARD and will be available in the ARD-Mediathek. Polarstern will spend the next three weeks in the Lloyd Werft shipyard in Bremerhaven for standard maintenance and repair work before setting off for Antarctica at the end of October.


The ArcWatch-1 expedition in figures:

  • Start on August 3, 2023 in Tromsø, Norway, end on September 30, 2023 in Bremerhaven, Germany.
  • On board 54 scientists and 43 crew members from 15 countries
  • Distance travelled: 5311 nautical miles (9836 km), including 2578 nautical miles (4774 km) through the ice in 48 days
  • Overflights for ice thickness measurement: 1300 km
  • Buoys deployed: 73 for meteorology, oceanography, and sea ice physics for an international program; and 2 one-year moorings
  • Number of ice stations where researchers worked on the sea ice: 9.
  • Ice dives with ROV "Beast": 38 dives, over 50km total distance
  • Drilled ice cores: ca. 100
  • Number of newly discovered seamounts: 3
  • Area of newly mapped seafloor: 37,442 km²
  • Number of polar bear sightings: 3
  • Water samples collected: (11,808 liters of water)
  • Number of net hauls: 55
  • Seafloor images: 8,970, a total of 35,880 square meters of seafloor photographed, at depths ranging from 1,580 to 4,375m, and 30 kilometers of transects (with 200m width) surveyed with high-resolution sonar.
  • Biological samples from the seafloor: 56 with the epibenthic sled, and 250 with the box grab for meiofauna and macrofauna. For meiofauna, 173 samples from the multicorer.
  • Seafloor biogeochemistry: 196 cores from 20 deployments of the video-guided multigrab, and 13 successful lander deployments (one did not return from the deep sea).
  • Formation of a band: the "ArcWatchers".

Participating institutions

Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung
Albert-Ludwigs-Universität Freiburg
Dartmouth College
Deutscher Wetterdienst
Max-Planck-Institut für Marine Mikrobiologie
Otto-von-Guericke-Universität Magdeburg
Senckenberg Gesellschaft für Naturforschung
Universität Bremen
University of Colorado, Boulder
University of Exeter
University of Southern Denmark
University of Tasmania (UTAS)
University of Tokyo