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An under-ice bloom of mixotrophic haptophytes in low nutrient and freshwater-influenced Arctic waters

New publication by Dorte H. Søgaard, Brian K. Sorrell, Mikael K. Sejr, Per Andersen, Søren Rysgaard, Per Juel Hansen, Annaliina Skyttä, Signe Lemcke & Lars Chresten Lund-Hansen


The pelagic spring bloom is essential for Arctic marine food webs, and a crucial driver of carbon transport to the ocean depths. A critical challenge is understanding its timing and magnitude, to predict its changes in coming decades. Spring bloom onset is typically light-limited, beginning when irradiance increases or during ice breakup. Here we report an acute 9-day under-ice algal bloom in nutrient-poor, freshwater-influenced water under 1-m thick sea ice. It was dominated by mixotrophic brackish water haptophytes (ChrysochromulinaPrymnesium) that produced 5.7 g C m−2 new production. This estimate represents about half the annual pelagic production, occurring below sea ice with a large contribution from the mixotrophic algae bloom. The freshwater-influenced, nutrient-dilute and low light environment combined with mixotrophic community dominance implies that phagotrophy played a critical role in the under-ice bloom. We argue that such blooms dominated by potentially toxic mixotrophic algae might become more common and widespread in the future Arctic Ocean.