Effects of phytoplankton community composition and productivity on sea surface pCO2 variations in the Southern Ocean

Shintaro Takao,Shin-Ichiro Nakaoka,Fuminori Hashihama,Keishi Shimada,Hisayuki Yoshikawa-Inoue,Toru Hirawake,Jota Kanda,Gen Hashida,Koji Suzuki

doi:10.1016/j.dsr.2020.103263

Abstract

The Southern Ocean is a vast net sink for atmospheric carbon dioxide (CO2), with marine phytoplankton playing a crucial role in CO2 fixation. We assessed how changes in the dominant phytoplankton community and net primary productivity (NPP) affected variations in the partial pressure of CO2 in surface water (pCO2sw) in the Indian sector of the Southern Ocean during austral summer. pCO2sw was negatively correlated with total phytoplankton and diatom abundances, as estimated from pigment signatures, in the zone south of the Antarctic Circumpolar Current; however, pCO2sw was not correlated with haptophyte abundance. Additionally, a stronger correlation was found between pCO2sw and total phytoplankton NPP than between chlorophyll a concentration and pCO2sw. We reconstructed pCO2sw at inter-annual scale using satellite data and assessed the inter-annual variability of air-sea CO2 flux. Over the period from 1997 to 2007, the integrated CO2 fluxes over the study region showed very large variations from a small source to a strong sink. Variations in the integrated CO2 fluxes were also correlated with changes in satellite-derived phytoplankton community in the Indian sector of the Southern Ocean and changes in the dominant phytoplankton community may control CO2 dynamics in the marginal ice zone.

Highlights

The Southern Ocean is one of the most important regions for the global carbon cycle, as it is a vast net sink for atmospheric carbon di oxide (CO2) (e.g., Landschützer et al, 2015; Lenton et al, 2013; McNeil et al, 2007), and can absorb a large amount of anthropogenic CO2 (Gruber et al, 2009; Khatiwala et al, 2009; Ito et al, 2010; Mikaloff Fletcher et al, 2006)
This study describes changes in the abundance of phytoplankton groups determined from pigment signatures and net primary productivity (NPP) and their effects on variations in pressure of CO2 in surface water (pCO2sw) in the Indian sector of the Southern Ocean
We found that pCO2sw in the SACCZ during our cruises was negatively correlated with chl a concentrations derived from total phytoplankton or diatoms, but not correlated with that of type 8 haptophytes

Summary

Introduction

The Southern Ocean is one of the most important regions for the global carbon cycle, as it is a vast net sink for atmospheric carbon di oxide (CO2) (e.g., Landschützer et al, 2015; Lenton et al, 2013; McNeil et al, 2007), and can absorb a large amount of anthropogenic CO2 (Gruber et al, 2009; Khatiwala et al, 2009; Ito et al, 2010; Mikaloff Fletcher et al, 2006). During the austral summer in particular, phyto plankton productivity plays a crucial role in the carbon cycle of the Southern Ocean (e.g., Metzl et al, 2006; Takahashi et al, 2012). The non-calcifying haptophyte Phaeocystis antarctica regularly forms huge colonies with high sinking rates in

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