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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-257
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-257
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  17 Jul 2020

17 Jul 2020

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A revised version of this preprint was accepted for the journal BG and is expected to appear here in due course.

Southern Ocean BGC-Argo Detect Under Ice Phytoplankton Growth Before Sea Ice Retreat

Mark Hague1 and Marcello Vichi1,2 Mark Hague and Marcello Vichi
  • 1Department of Oceanography, University of Cape Town, South Africa
  • 2Marine Research Institute, University of Cape Town, South Africa

Abstract. The seasonality of sea ice in the Southern Ocean has profound effects on the life cycle (phenology) of phytoplankton residing under the ice. The current literature investigating this relationship is primarily based on remote sensing, which often lacks data for half the year or more. One prominent hypothesis holds that following ice retreat in spring, buoyant melt waters enhance irradiance levels, triggering a bloom which follows the ice edge. However, an analysis of BGC-Argo data sampling under Antarctic sea ice suggests that this is not necessarily the case. Rather than precipitating rapid accumulation, we show that melt waters enhance growth in an already highly active phytoplankton population. Blooms observed in the wake of the receding ice edge can then be understood as the emergence of a growth process that started earlier under sea ice. Indeed, we estimate that growth initiation occurs, on average, 4–5 weeks before ice retreat, typically starting in August and September. Novel techniques using on-board data to detect the timing of ice melt were used. Furthermore, such growth is shown to occur under conditions of substantial ice cover (> 90 % satellite ice concentration) and deep mixed layers (> 100 m), conditions previously thought to be inimical to growth. This led to the development of several 0D model experiments in which we sought to investigate the mechanisms responsible for such early growth. The results of theses experiments suggest that a combination of higher light transfer (penetration) through sea ice and extreme low light adaptation by phytoplankton can account for the observed phenology.

Mark Hague and Marcello Vichi

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Mark Hague and Marcello Vichi

Mark Hague and Marcello Vichi

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Latest update: 23 Nov 2020
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Short summary
This paper examines the question of what causes the rapid spring growth of microscopic marine algae (phytoplankton) in the ice covered ocean surrounding Antarctica. One prominent hypothesis proposes that the melting of sea ice is the primary cause, while our results suggest that this is only part of the explanation. In particular, we show that phytoplankton are able to start growing before the sea ice melts appreciably, much earlier than previously thought.
This paper examines the question of what causes the rapid spring growth of microscopic marine...
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