Sea-ice ridge formation fuels Arctic pelagic food webs during the polar night

Abstract There is no light for photosynthesis by phytoplankton and sea-ice algae during the polar night, but microbial grazers remain active through the dark winter months in the Arctic Ocean. Where the energy to sustain these organisms comes from is unknown. Here we observed active tintinnid ciliates during the polar night, heterotrophic protists known to feed on phytoplankton and smaller heterotrophic protists. Our calculations indicate that the pelagic microbial loop transferring energy from bacterial production through microbial grazers was not sufficient to sustain the observed tintinnid biomass. However, the sea ice contained frozen-in particulate organic carbon produced during the previous growth seasons. We show that enough food particles can be released by mechanical break-up during sea-ice ridging, which together with bacterial production, sustained the observed tintinnid biomass in the water column. This is an important but overlooked mechanism for winter survival of plankton in the Arctic Ocean.