Holocene marine ecosystem dynamics off central West Greenland traced by ancient DNA

The West Greenland shelf is a diverse and biologically unique arctic ecoregion harboring marine areas of international value like the North Water polynya and the Melville Bay Wildlife Sanctuary. Model projections suggest that the shelf will be increasingly affected by sea-ice loss and freshwater discharge from the Greenland Ice Sheet. Large uncertainties prevail on how the regional diversity and ecosystem structure will change in the future, partly because ecological responses are highly complex and because modern long-term observations cover timeframes already affected by climate change. This is raising questions on how to define baselines for evaluating observed changes, and on how to disentangle the effects of human impact and natural climate variability. As ocean sediments are natural archives of the past, they provide vital insights into the natural variability of ecosystems and long-term biotic responses to environmental change. Here, we present a Holocene sedimentary ancient DNA (sedaDNA) record to explore past marine ecosystem dynamics to climate variability off West Greenland over the past 8,500 years. We applied targeted amplicon sequencing of a diatom-specific (rbcL) and a universal eukaryotic (18S V7 region) marker to a well-dated marine sediment core retrieved on the shelf off the Upernavik Ice Stream (AMD14-204C; Lat. 73.261, Long. -57.899, 987 m water depth). Our record revealed a high taxonomic diversity, including many taxa whose body parts are usually not preserved in sediments, such as copepods, polychaetes or jellyfish. Sequence variants representing 251 families in 31 phyla were retrieved for 18S and 19 diatom genera in 14 families for the rbcL marker, respectively. Taxonomic assignments of diatom-derived sequences contain typical cold-water (Porosira glacialis, Chaetoceros socialis, Thalassiosira gravida) and sea-ice associated species (Nitzschia frigida), including species indicative of sea ice and brackish water conditions (Cylindrotheca closterium, Pauliella taeniata). Many sequences remain unassigned, indicating the need for a close collaboration between taxonomists and geneticists to fill this gap for unlocking the full potential of sedaDNA in the future. We will discuss the exceptional potential of ancient DNA to improve predictions of marine productivity and biodiversity, and to support ocean and cryosphere risk assessment and conservation efforts.