Temporal changes in size distributions of the Southern Ocean diatom Fragilariopsis kerguelensis through high-throughput microscopy of sediment trap samples

Michael Kloster,Andrés S Rigual-Hernández,Leanne K Armand,Gerhard Kauer,Thomas W Trull,Bánk Beszteri

doi:10.1080/0269249x.2019.1626770

Abstract

Some aspects of the life cycle of the Southern Ocean diatom Fragilariopsis kerguelensis have been investigated previously, but many of its details have not been surveyed in nature. We investigated material from a two-year sediment trap time series by high-throughput imaging and image analysis, looking for morphometric signals of life cycle stages. Valve length distributions appeared close to unimodal but positively (right-) skewed. Size cohorts resulting from synchronized sexual reproduction events were not clearly distinguishable. Nevertheless, based on changes in valve length distributions, we found three general seasonal phases. These corresponded to periods of proliferation (with higher proportions of smaller cells during late spring/early summer), cessation of growth (relative loss of smaller cells during late summer/early autumn), and overwintering (little change in size distributions, with an increased proportion of large cells). We discuss possible causes of these signals, and their relevance to growth, sexual activity and adaption to environmental conditions, such as grazing pressures and the need for an overwintering strategy.

Highlights

The pelagic pennate diatom Fragilariopsis kerguelensis (O’Meara) Hustedt is endemic to the Southern Ocean (Hart 1942, Zielinski & Gersonde 1997, Cortese & Gersonde 2008)
The species usually occurs in chains of up to dozens of cells throughout the Southern Ocean, and the Polar Frontal Zone (PFZ) of the Antarctic Circumpolar Current (ACC) is considered to be its optimal habitat (Zielinski & Gersonde 1997, Cortese & Gersonde 2007, Pinkernell & Beszteri 2014)
Changes in F. kerguelensis valve length distribution throughout the two sampling years were plotted as histograms (Figs 2–3) and as violin plots (Figs 4–5)

Summary

Introduction

The pelagic pennate diatom Fragilariopsis kerguelensis (O’Meara) Hustedt is endemic to the Southern Ocean (Hart 1942, Zielinski & Gersonde 1997, Cortese & Gersonde 2008). High abundances of F. kerguelensis in Southern Ocean sediments are regarded as an indication of low-carbon high-silica exporting regimes (Smetacek et al 2004, Abelmann et al 2006, Assmy et al 2013) For these and other reasons, the species has generated interest in several fields of research, spanning biological oceanography (DeBaar et al 1997, Hoffmann et al 2007, Assmy et al 2013), biogeography (Pinkernell & Beszteri 2014), paleoceanography (Cortese & Gersonde 2007, 2008, Esper et al 2010, Cortese et al 2012, Shukla et al 2013, Shukla & Crosta 2017, Kloster et al 2018, Shukla & Romero 2018), ecophysiology (Timmermans & Van Der Wagt 2010, Trimborn et al 2013, 2014, Beszteri et al 2018b), and biomechanics (Hamm et al 2003, Wilken et al 2011)

Methods

Results

Discussion

Conclusion