Abstract

In the Arctic Ocean ice algae constitute a key ecosystem component and the ice algal spring bloom a critical event in the annual production cycle. The bulk of ice algal biomass is usually found in the bottom few cm of the sea ice and dominated by pennate diatoms attached to the ice matrix. Here we report a red tide of the phototrophic ciliate Mesodinium rubrum located at the ice-water interface of newly formed pack ice of the high Arctic in early spring. These planktonic ciliates are not able to attach to the ice. Based on observations and theory of fluid dynamics, we propose that convection caused by brine rejection in growing sea ice enabled M. rubrum to bloom at the ice-water interface despite the relative flow between water and ice. We argue that red tides of M. rubrum are more likely to occur under the thinning Arctic sea ice regime.

Highlights

  • In the high Arctic the relative contribution of ice algae to total primary production can be up to 60% because the snow covered perennial pack ice cover efficiently shades the under-ice water column, limiting phytoplankton growth[1,2,3]

  • The 7–117 mg chlorophyll a (Chl a) m−3 we measured in the slurp samples from the young ice (YI) interface layer is 14–234 times higher than the concentration in the water column (

  • The interface bloom can be likened to the red tides of M. rubrum often observed at lower latitudes, where the Chl a concentration can be >100 mg m−3 and abundance up to 106 cells L−1 28

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Summary

Introduction

In the high Arctic the relative contribution of ice algae to total primary production can be up to 60% because the snow covered perennial pack ice cover efficiently shades the under-ice water column, limiting phytoplankton growth[1,2,3]. The bloom of M. rubrum at the ice-water interface can be likened to a red tide, well known for this species at lower latitudes[18]. To our knowledge this is the first observation of an ice-associated red tide of M. rubrum in the Arctic Ocean. M. rubrum is a motile planktonic species that, unlike e.g. diatoms and surface associated ciliates, cannot attach to the ice matrix[19,20]. How can these ciliates remain stationary at the ice-water interface despite the flow? The proposed hydrodynamic model provides a mechanistic explanation for the occurrence of blooms of motile algae below drifting pack ice

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