Late-summer hotspot of particulate organic matter and essential omega-3 fatty acids at the sea-ice bottom in the Central Arctic Ocean.

The Central Arctic Ocean is undergoing rapid environmental change associated with sea-ice retreat. The sea-ice bottom and adjacent under-ice waters are key zones for particulate organic matter (POM) accumulation and zooplankton foraging, yet the characteristics of POM across this interface remain unclear. Here, we analyzed particulate organic carbon (POC) and fatty-acid (FA) profiles of POM from these two habitats to quantify organic matter enrichment and characterize POM sources, omega-3 FA availability, and potential trophic significance. The sea-ice bottom represented an organic-matter hotspot, where POC and essential omega-3 FAs (eicosapentaenoic acid, EPA, and docosahexaenoic acid, DHA) were enriched relative to the under-ice water, with station-specific enrichment factors approaching two orders of magnitude. FA signatures indicate predominantly algal-derived POM in both habitats; the elevated diatom index at the sea-ice bottom suggests a greater diatom contribution to ice-bottom POM. In contrast to the concentration enrichments at the sea-ice bottom, the sum of EPA and DHA proportions (EPA% + DHA%) was similar between the two habitats. Copepod-marker FA concentrations also peaked at the sea-ice bottom, coinciding with the maxima in organic matter and essential omega-3 FAs, suggesting the potential ecological relevance of this interface for Arctic copepods. Our findings suggest the sea-ice bottom is a hotspot of POC and essential omega-3 FAs, and may be ecologically significant for Arctic copepods. These results provide a reference point for evaluating how sea-ice loss may alter the availability of these resources and foraging conditions for consumers in the Central Arctic Ocean.