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

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 (Chrysochromulina/ Prymnesium) 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.

Automated summary

The pelagic spring bloom in the Arctic Ocean is crucial for marine food webs and carbon transport. An acute under-ice algal bloom dominated by mixotrophic brackish water haptophytes was reported, suggesting that potentially toxic mixotrophic algae blooms may become more common and widespread in the future Arctic Ocean.