However, it remains unclear what ecological effects an earlier and a stronger retreat of the ice-edge will have for carbon export on the Arctic shelf seas and margins. Primary production models suggest that with the northward propagation of ice-edge blooms along leads and polynyas, the impact of ice-algae and under-ice phytoplankton blooms on productivity is likely to increase 18, with ecological consequences for pelagic and benthic ecosystems 16, 19.

For example in 2012, the release of fast-sinking ice-algae, the Melosiraarctica, from melting sea ice delivered up to 9вЂ‰g of carbon per square meter of seafloor, which was more than 85% of the total carbon export that year 16. When sea ice melts, such ice-algae and under-ice phytoplankton blooms that are mostly composed of diatoms can also deliver substantial pulses of carbon and nutrients to benthic ecosystems 8, 16, 17. Accordingly, field observations show an increased spatial and temporal extent of sea-ice algae and under-ice pelagic phytoplankton in the Arctic basins, for which ocean color based primary production assessments are not available 6, 13, 14, 15. Furthermore, the transformation from thick multiyear to thin first-year sea ice is increasing light transmission through it 12. Remote sensing of ocean color in the Arctic shelf seas suggests an increase in net primary production by 57% since 1998 10, which likely enhances vertical carbon and nutrient fluxes 11. However, the complexity of factors driving Arctic productivity regionally makes it difficult to generalize to the future carbon flux in the entire Arctic Ocean 9. 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 1981вЂ“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-iceВ 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 deepВ 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 seaВ ice cover is shrinking due to warming.

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