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

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

doi:10.1038/s41598-021-82413-y

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.

Highlights

The pelagic spring bloom is essential for Arctic marine food webs, and a crucial driver of carbon transport to the ocean depths
We demonstrate for the first time an under-ice bloom driven by mixotrophic brackish-water haptophytes beneath 1-m thick sea ice in a nutritionally dilute environment
The indication of a mixotrophic-based bloom suggests that mixotrophic algae may play an important role in driving the Arctic spring bloom and the ecosystem and carbon dynamics in this area

Summary

Introduction

The pelagic spring bloom is essential for Arctic marine food webs, and a crucial driver of carbon transport to the ocean depths. An improved insight into the extent and frequency of these extensive blooms driven by potentially mixotrophic haptophyte species underneath thick sea ice is fundamental for understanding and modelling future changes in Arctic Ocean pelagic net primary productivity.

Results

Conclusion