Threatened species drive the strength of the carbonate pump in the northern Scotia Sea

C Manno,G A Tarling,F Giglio,S Fielding,P Enderlein,G Stowasser

doi:10.1038/s41467-018-07088-y

Abstract

The efficiency of deep-ocean CO2 sequestration is regulated by the relative balance between inorganic and organic carbon export respectively acting through the biological carbon pump (BCP) and the carbonate counter pump (CCP). The composition and abundance of calcifying species in the prevailing oceanic plankton community plays a major role in driving the CCP. Here we assess the role of these calcifying organisms in regulating the strength of the CCP in a Southern Ocean region (northern Scotia Sea) known to be a major hotspot for the drawdown of atmospheric CO2. We show that, when shelled pteropods dominate the calcifying community, the total annual reduction of CO2 transferred to the deep ocean doubles (17%) compared to when other plankton calcifiers dominate (3–9%). Furthermore, predation enhances their contribution through the removal of organic soft tissue. Pteropods are threatened in polar regions by ocean warming and acidification. We determine that their potential decline would have major implications to the comparative strengths of the BCP and CCP.

Highlights

The efficiency of deep-ocean CO2 sequestration is regulated by the relative balance between inorganic and organic carbon export respectively acting through the biological carbon pump (BCP) and the carbonate counter pump (CCP)
The BCP is counteracted by the carbonate counter pump (CCP), which causes an increase in surface ocean CO2 through the calcification and precipitation of carbonate and the resulting export of particulate inorganic carbon (PIC)
In order to determine the efficiency of deep-ocean CO2 sequestration and, in turn, how this may affect the concentrations of CO2 in the surface ocean, it is crucial to understand the dynamics of the export of both PIC and particulate organic carbon (POC), their relative balance, termed the PIC:POC or rain ratio[2]

Summary

Introduction

The efficiency of deep-ocean CO2 sequestration is regulated by the relative balance between inorganic and organic carbon export respectively acting through the biological carbon pump (BCP) and the carbonate counter pump (CCP). The fate of exported POC can be influenced through zooplankton grazing, repackaging of organic matter into faecal pellets (FP), and the vertical migration and transport of carbon and nutrients into the mesopelagic zone (e.g., 3,4) Calcifying zooplankton such as pteropods, ostracods and foraminifera can influence the PIC flux through removing calcium (Ca2+) from the pelagic surface ocean waters to form shells and structures of calcium carbonate (CaCO3). These calcifiers play an important role in PIC sequestration to the deep ocean because their relatively large size and high density makes them sink rapidly[5,6]. Despite understanding variability in the relative abundance and composition of calcifying species is a crucial part of accounting for the fate of CO2 uptake and sequestration, little work has been done examining seasonal dynamics of calcifying communities

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