An annual cycle of diatom succession in two contrasting Greenlandic fjords: from simple sea-ice indicators to varied seasonal strategists

Understanding environmental factors affecting diatom species composition at seasonal resolution can contribute to the improvement of paleo sea-ice reconstruction. We recorded diatom species succession over one full year (May 2017‒May 2018) using automated sediment traps installed in two contrasting Greenlandic fjords: seasonally ice-covered Young Sound in high arctic NE Greenland and nearly sea-ice free Godthåbsfjord in subarctic SW Greenland. The two study sites had distinct seasonal regimes in terms of both sediment and diatom fluxes. In Young Sound, diatom fluxes peaked during the ice-melt in June–July (max. 880×10⁶ valves m^-2 d^-1), but were very low (0.11­–12.7×10⁶ valves m^-2 d^-1) for the rest of the year. The pattern was very different in Godthåbsfjord, where diatom fluxes were more stable throughout the year and at maximum 320×10⁶ valves m^-2 d^-1 in summer. A total of 60 diatom taxa were present in Young Sound and 50 in Godthåbsfjord, with 19 and 22 sympagic or pelagic species, respectively. The diatom assemblage in Young Sound is strongly dominated by the pennate sea-ice species Fragilariopsis oceanica, Fragilariopsis reginae-jahniae and Fossula arctica, which exhibited pulse-like deposition in the trap during and after the ice melt. In Godthåbsfjord, the fluxes were dominated by resting spores of centric Chaetoceros, while the rest of the assemblage was characterized by the cold-water indicator species Detonula confervacea spore, Fragilariopsis cylindrus and Thalassiosira antarctica var. borealis spore accompanied by some warmer-water species. Some sea-ice indicator species were also observed in Godthåbsfjord, but at very low counts and throughout the year, likely transported from the inner fjord, which experiences seasonal sea-ice coverage. We show that F. oceanica, F. reginae-jahniae and F. arctica exhibit similar seasonal behaviour and are clearly linked to sea ice. On the other hand, Fragilariopsis cylindrus seems to have a more flexible niche, and is not an unequivocal ice indicator. Similarly, Pauliella taeniata has a differing niche, and does not favour our study locations probably due to its preference for lower salinities. We underscore the importance of taking into account ecological and seasonal preferences of the individual diatom species when reconstructing past sea-ice conditions qualitatively or quantitatively.