Marine diatoms record Late Holocene regime shifts in the Pikialasorsuaq ecosystem.

The Pikialasorsuaq (North Water polynya) is an area of local and global cultural and ecological significance. However, over the last decades, the region has been subject to rapid warming, and in some recent years, the seasonal ice arch that has historically defined the polynya's northern boundary has failed to form. Both factors are deemed to alter the polynya's ecosystem functioning. To understand how climate-induced changes to the Pikialasorsuaq impact the basis of the marine food web, we explored diatom community-level responses to changing conditions, from a sediment core spanning the last 3800 years. Four metrics were used: total diatom concentrations, taxonomic composition, mean size, and diversity. Generalized additive model statistics highlight significant changes at ca. 2400, 2050, 1550, 1200, and 130 cal years BP, all coeval with known transitions between colder and warmer intervals of the Late Holocene, and regime shifts in the Pikialasorsuaq. Notably, a weaker/contracted polynya during the Roman Warm Period and Medieval Climate Anomaly caused the diatom community to reorganize via shifts in species composition, with the presence of larger taxa but lower diversity, and significantly reduced export production. This study underlines the high sensitivity of primary producers to changes in the polynya dynamics and illustrates that the strong pulse of early spring cryopelagic diatoms that makes the Pikialasorsuaq exceptionally productive may be jeopardized by rapid warming and associated Nares Strait ice arch destabilization. Future alterations to the phenology of primary producers may disproportionately impact higher trophic levels and keystone species in this region, with implications for Indigenous Peoples and global diversity.