As primary producers form the base of the food web, such shifts are likely to have drastic consequences, not just in the pelagic realm, but also for pelagic-benthic coupling and biogeochemical cycling in the Arctic Ocean 8. It has been suggested that temperate phytoplankton species will become resident in the Eurasian Basin of the Arctic Ocean if the intrusion of warming Atlantic waters continues 7. For example, Atlantic phytoplankton species, such as Phaeocystis spp., are already seasonally reoccurring in the Fram Strait 5, and were also recently observed during a phytoplankton bloom in the central Arctic Ocean 6. Increasing temperature, combined with declining sea ice extent, ice thickness and multiyear ice 3 are impacting the composition of primary producers in the Arctic Ocean 4. Ice coverage is declining by ~13% per decade compared to the mean September extent for 1981–2010 1, and climate models project that business-as-usual scenarios will result in seasonally ice-free conditions by 2050 2. The Arctic Ocean is currently undergoing unprecedented changes due to advancing climate warming. Therefore, continuous sea-ice loss may decrease the vertical export efficiency, and thus the pelagic-benthic coupling, with potential repercussions for Arctic deep-sea ecosystems. Taken together, our results showed that ice-covered regions are responsible for high export efficiency and provide strong vertical microbial connectivity. Using microbial source tracking, we found that ice-covered regions were also associated with more surface-born microbial clades exported to the deep sea. In situ collected aggregates revealed two-fold higher carbon export of diatom-rich aggregates in ice-covered regions, compared to Phaeocystis aggregates in the ice-free region.

To investigate this sea-ice enhanced export, we compared how different early summer phytoplankton communities in seasonally ice-free and ice-covered regions of the Fram Strait affect carbon export and vertical dispersal of microbes. Long-term sediment trap data shows higher export efficiency of particulate organic carbon in regions with seasonal sea ice compared to regions without sea ice. Arctic Ocean sea ice cover is shrinking due to warming.

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