Millennial-scale fingerprint of macroalgae in Arctic marine sediments

Macroalgae are the most widely distributed marine vegetated habitats and contribute to marine carbon cycling and storage but with limited empirical documentation of long-term burial. To evaluate long-term burial of macroalgal-derived carbon in Arctic sediments, we analyzed eDNA from six dated sediment cores from off the coast of West Greenland (79°N-60°N). We applied metabarcoding of 18S rRNA genes to selected sediment layers covering the past ∼2600 years, assessed spatio-temporal patterns of macroalgal taxa, and evaluated climatic drivers of macroalgal change using proxies for past sea surface conditions. Macroalgal DNA was present in all cores and 86.5 % of samples. Orders of brown algal (Laminariales, Fucales) were more prevalent than red or green orders and we found no consistent changes in the identity of macroalgal taxa, despite significant changes in sea surface conditions (temperature, sea ice, salinity) over this period. However, locally, we observed a significant decline in richness of macroalgal taxa buried over the last c. 800 years (Qeqertarsuup Tunua, Disko Bay). This decline coincided with the onset of the Little Ice Age and greater fluctuations in sea surface conditions. Overall, we demonstrate that macroalgae can be preserved for millennia in marine sediments along Greenland's west coast, documenting that macroalgae contribute to long-term carbon burial in the Arctic although their quantitative significance is still unknown.