Less Sea Ice, More Herring

The disappearance of ice in the polar seas could be the beginning of far-reaching changes in the ecosystem. “Until recently, it was extremely difficult to gauge the long-term effects of low summertime sea-ice cover on marine organisms, as there were no corresponding long-term investigations to draw on,” explains Prof Ulrike Herzschuh, head of the research group Polar Terrestrial Environmental Systems at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) in Potsdam. Her team, including AWI colleagues Heike Zimmermann and Kathleen Stoof-Leichsenring, as well as experts from Jacobs University Bremen and the GEOMAR Helmholtz Centre for Ocean Research in Kiel, took a look 20,000 years back in our history, to the last ice age.

Information on the respective environmental conditions can be gleaned from the deposits that have accumulated on the ocean floor in the course of millennia. “These sediments are a natural archive of climatic history,” says Herzschuh. When this material is retrieved and brought to the surface, the different layers yield evidence of marine organisms long-since dead. Using a method known as shotgun sequencing, the team found DNA from representatives of 167 families of marine organisms that live(d) in the ice or open water. “Even we were surprised to find that these old sediments contained information on the entire ecosystem,” says Herzschuh.

According to their findings, diatoms and other algae that lived in or below the sea ice were typical for the colder phases of the last ice age. These tiny oxygen-producing organisms were a favourite food for copepods, which were in turn eaten by fish from the Gadidae family, like the Pacific cod, Alaska pollock and Arctic cod. In the warmer, ice-free phases, there were far fewer diatoms and copepods, and many more cyanobacteria. Seagrass meadows flourished in protected hamlets on the ocean floor, and instead of cod, the Bering Sea was home to more salmon and Pacific Herring.

“As such, now we can for the first time show how the loss of sea ice tranforms entire ecosystems,” Herzschuh summarises. “It starts with the algae and extends all the way to fishes and whales.” The team expects that a warmer and largely ice-free future would produce similarly far-reaching changes. This could have massive ecological and economic ramifications. For example, catching favourite food fishes like saithe and Atlantic cod in the Bering Sea might no longer be worthwhile. At the same time, the pink salmon and Pacific herring could spread farther north.

In addition, under ice-free conditions, plankton communities might transport less carbon to the ocean’s depths, where it is deposited in the sediments. If that happens, the ocean won’t be able to store as much carbon dioxide, exacerbating climate change. Consequently, the loss of the sea ice could mean that these ecosystems will no longer be able to provide their valuable services on the same scale.