Climate change is causing Arctic sea ice to recede. A look into the past now shows the drastic consequences this can have for the ecosystems there: The entire biocoenosis can change during the transition from seasonally icy to ice-free conditions, as confirmed by the analysis of ancient DNA from the Arctic seabed. Scientists warn that this could have serious consequences for fisheries and the global climate.
The seas of the polar regions are still covered with a frozen shell of sea ice for weeks or months each winter. But this is changing as a result of climate change: the sea ice, especially in the Arctic, is becoming thinner and its area is shrinking. “Up until now, it has been difficult to assess the long-term consequences of the low summer sea ice cover for sea creatures because there were no corresponding long-term studies,” explains Ulrike Herzschuh, head of the Polar Terrestrial Environmental Systems research group at the Alfred Wegener Institute, Helmholtz Center for Polar and Polar Sciences Marine Research (AWI) in Potsdam.
Marine sediment as an archive
That’s why Herzschuh and her team have now taken a look back around 20,000 years to the last ice age. At that time, too, there was strong warming of the climate and the previously thick ice sheets on land and oceans retreated and shrank. This changed the extent and distribution of the sea ice and, with it, the conditions in the sea below. Information about the changes is provided by deposits that accumulated at the bottom of the sea over the course of thousands of years. “These sediments are a natural archive of climate history,” says Herzschuh.
If you bring the material to the surface with a drill, you can find traces of long-dead sea creatures in the form of their genomes in the layers of different ages. In order to find out which organisms lived in the ocean during the transition from the glacial to the interglacial period, Herzschuh and her colleagues used what is known as shotgun sequencing to analyze the DNA from sediment samples from various drilling sites around 70 kilometers off the coast of Kamchatka in the Bering Sea. This region was covered by seasonal sea ice at the peak of the last ice age, but was ice-free during the warmer periods before and after.
Drastic change in the species community
Analysis of the sediments revealed the DNA of sea creatures from 167 different families. “We were surprised ourselves that these old sediments contain information about the entire ecosystem,” says Herzschuh. Diatoms and other algae living in or under the sea ice were typical of the colder phases of the last ice age. These tiny oxygen producers were a popular food source for copepods, which in turn were eaten by cod-family fish such as Pacific cod, Alaska pollock, and arctic cod.
In the warmer epochs without ice, on the other hand, there were significantly fewer diatoms and copepods, but all the more cyanobacteria. On the seabed, seagrass meadows spread out in sheltered bays and instead of cod, more salmon and Pacific herring swam in the Bering Sea. “We can now show for the first time how the entire ecosystem is being restructured as the sea ice recedes,” summarizes Herzschuh. “It starts with the algae and goes all the way to the fish and whales.”
Preview of the future
The team expects similarly profound changes in the warmer and largely ice-free future we are facing today. This could have massive ecological impacts. “A look into the past shows us that we have to expect substantial shifts in the species composition in open water and on the seabed for the Arctic regions,” the researchers explain. This would also affect ocean productivity and fisheries in particular. As a result, some popular food fish such as pollock and cod may no longer be commercially viable in the Bering Sea and other parts of the Arctic Sea. On the other hand, pink salmon and Pacific herring could advance further north.
In addition, under ice-free conditions, the plankton communities transport less carbon into the depths and deposit it in the sediments. The oceans may then no longer be able to store as much carbon dioxide, which would further fuel climate change. The disappearance of the sea ice could also mean that these ecosystems can no longer provide important services to the usual extent.
Source: Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research; Specialist article: Nature Communications, doi: 10.1038/s41467-023-36845-x