An under-ice bloom of mixotrophic haptophytes in low nutrient and freshwater-influenced Arctic waters
Dorte Haubjerg Søgaard, Brian K. Sorrell, Mikael K. Sejr, Per Andersen, Søren Rysgaard, Per Juel Hansen, Annaliina Skyttä, Signe Lemcke, Lars Chresten Lund–Hansen
Abstract
Abstract 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.