Past subarctic marine food web shifts recovered by sedaDNA and network analysis.

Subarctic marine ecosystems are highly sensitive to current climate and sea-ice change but long-term evidence for food web shifts is lacking. Sedimentary ancient DNA (sedaDNA) data covering the last 124,000 years combined with network analysis provides evidence for a shift from glacial (GLC) bottom-up to a deglacial-interglacial (IG) top-down food web structure. A consensus network approach, calibrated on generalized Lotka-Volterra simulations, delivers distinct modules that potentially resemble interglacial and glacial trophic structures. The glacial food web with a dominance of sea-ice adapted primary producers, densely linked with each other, supports a high module robustness and resource-driven bottom-up control. In contrast, the deglacial-interglacial food web with predominant consumers, like salmon, herring and small whales, are trophically diverse but rather fragile. Our study assumes that mobile consumers are facilitated by habitat expansion through submerged shelves and ice-free conditions during deglacial-interglacial periods altering the composition and structure of higher trophic levels. Our study implies that future warming, sea-ice decline and sea level rise may affect the structure and stability of subarctic consumer-driven, top-down food webs with adverse consequences for ecosystem services.