Sea-ice algae - microscopic algae living at the ice-water interface - supply a carbon source that Koch et al. (2023) traced, via lipid biomarkers, into 96% of the organisms they investigated - over 2300 samples from 155 species collected in every month of the year, including through the polar night. This established year-round utilization of ice-associated (sympagic) carbon, retained in seafloor sediments and recycled through benthic communities, as an energetic backbone of Arctic marine food webs. The source paper's central open question is whether a future Arctic Ocean without summer sea ice, in which sea ice-benthic coupling may be reduced or entirely absent in some regions, can still supply sufficient primary production - in quantity, quality, and timing - to sustain this ecosystem through the extended dark winter. The question remains open: 2023-2026 literature resolves components (bloom phenology under thinning ice, benthic reliance on sympagic carbon, food-quality shifts) but no work quantifies the year-round, winter-inclusive carbon budget of a seasonally ice-free Arctic or the pan-Arctic buffering capacity of sedimentary ice-algal carbon.

Background

The term sympagic (literally 'together with ice') describes organisms living in association with sea ice. The bottom few centimeters of Arctic sea ice host ice algae - mainly diatoms that bloom in spring at the ice-water interface as light returns - forming lipid-rich aggregates that sink toward the seafloor. Because this ice-associated production precedes the open-water phytoplankton bloom and part of it is deposited on bottom sediments, it links the ice, the water column (pelagic compartment), and the seafloor (benthic compartment) of Arctic shelf ecosystems, a linkage termed sympagic-pelagic-benthic coupling. This carbon can be tracked chemically: IP25, a highly branched isoprenoid lipid (HBI) produced almost exclusively by Arctic sea-ice diatoms, serves as a biomarker allowing the fraction of ice-algal particulate organic carbon (iPOC) in consumer tissues or sediments to be estimated.

Using such biomarkers on over 2300 samples from 155 species - invertebrates, fish, seabirds, and marine mammals - collected across Arctic shelves from January to December, Koch et al. (2023) found ice-algal carbon signatures in 96% of investigated organisms in every month of the year. Even in the polar night (the months-long dark Arctic winter), when no new primary production occurs and consumers must live off stored energy and sediment-derived carbon, much of that carbon is ice-derived, presumably through benthic retention and recycling. Year-round utilization of sympagic carbon therefore supports a food web on which Indigenous communities, commercial and subsistence fisheries, and Arctic biodiversity depend.

The Arctic is projected to lose its summer sea ice within decades, leaving a seasonal-ice-only ocean. Whether the food web built on this year-round ice-carbon subsidy can survive that transition is the source paper's central unresolved question. The subsidy could only be replaced if several conditions hold jointly: the longer phytoplankton growing season must produce enough carbon in excess of what pelagic grazers consume to be exported and stored; over-winter nutrient availability must sustain the longer blooms; temperate species expanding into the Arctic must not outcompete Arctic consumers for this production; and food quality must remain adequate, since projected shifts toward smaller or flagellate-dominated algal communities can move production outside the preferred size and biochemical range of primary consumers. Regional contrasts matter, as Atlantic-influenced ('Atlantified') and Pacific-influenced shelves differ in nutrient supply, ice phenology, and benthic-pelagic partitioning. Finally, sediments themselves store ice-algal carbon - Koch et al. estimated 9-14 mg iPOC per gram of dry sediment in the upper 70 mm of a single Amundsen Gulf box core, accessible to burrowing infauna (animals living within the sediment) for roughly a century of deposits - but this buffering capacity and its lifetime have not been quantified across the different Arctic shelf regions.

Problem Statement

Will a future Arctic Ocean with no summer sea ice, in which sea ice-benthic coupling is reduced or entirely absent in at least some regions, supply sufficient primary production to sustain the sympagic ecosystem - the community associated with sea ice, here including the ice-associated invertebrates, fish, seabirds, marine mammals, and benthic organisms that depend on ice-derived carbon - throughout the extended dark winter period (the polar night, the months-long sunless Arctic winter)?

At issue is whether year-round carbon supply meets over-winter demand for the sympagic-dependent community, in quantitative and regionally resolved terms. The conditioning factors identified in the source include: (1) the sympagic (ice-algal) production remaining under a seasonal-ice-only regime and its phenology; (2) whether pelagic primary production in the extended open-water season exceeds the grazing capacity of pelagic consumers by enough to leave an exportable surplus for benthic storage; (3) whether over-winter nutrient availability can sustain the longer phytoplankton blooms; (4) the magnitude and duration of sedimentary and benthic ice-algal carbon stores that buffer the loss of new sympagic production, quantified across the different Arctic shelf regions rather than a single site; (5) the usable fraction of this supply given projected shifts in algal community size structure and composition (food quality) and competition from temperate-associated species expanding into the Arctic; and (6) how these terms differ between the Atlantic- and Pacific-influenced sectors of the Arctic, including what the differing migratory strategies, foraging plasticity, and degrees of specialization on sea-ice carbon of the affected species imply for their persistence. These factors are not independently mandatory requirements: they jointly bound the scope of the question, which is resolved by determining whether their combined effect keeps year-round carbon supply sufficient to maintain the sympagic-dependent populations through the dark winter.

Here, sufficiency means enough year-round carbon supply to maintain the sympagic-dependent populations - for example ice-associated amphipods, polar cod, benthic invertebrates, and the predators they support - through the dark winter, an energy-budget and biomass-maintenance criterion rather than total annual carbon mass alone.

References

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