Fe-binding organic ligands in coastal and frontal regions of the western Antarctic Peninsula

Indah Ardiningsih,Loes J A Gerringa,Kevin R Arrigo,Sylvia G Sander,Gert-Jan Reichart,Kyyas Seyitmuhammedov,Claudine H Stirling,Rob Middag

doi:10.5194/bg-18-4587-2021

Abstract

Abstract. Organic ligands are a key factor determining the availability of dissolved iron (DFe) in the high-nutrient low-chlorophyll (HNLC) areas of the Southern Ocean. In this study, organic speciation of Fe is investigated along a natural gradient of the western Antarctic Peninsula, from an ice-covered shelf to the open ocean. An electrochemical approach, competitive ligand exchange – adsorptive cathodic stripping voltammetry (CLE-AdCSV), was applied. Our results indicated that organic ligands in the surface water on the shelf are associated with ice-algal exudates, possibly combined with melting of sea ice. Organic ligands in the deeper shelf water are supplied via the resuspension of slope or shelf sediments. Further offshore, organic ligands are most likely related to the development of phytoplankton blooms in open ocean waters. On the shelf, total ligand concentrations ([Lt]) were between 1.2 and 6.4 nM eq. Fe. The organic ligands offshore ranged between 1.0 and 3.0 nM eq. Fe. The southern boundary of the Antarctic Circumpolar Current (SB ACC) separated the organic ligands on the shelf from bloom-associated ligands offshore. Overall, organic ligand concentrations always exceeded DFe concentrations (excess ligand concentration, [L′] = 0.8–5.0 nM eq. Fe). The [L′] made up to 80 % of [Lt], suggesting that any additional Fe input can be stabilized in the dissolved form via organic complexation. The denser modified Circumpolar Deep Water (mCDW) on the shelf showed the highest complexation capacity of Fe (αFe'L; the product of [L′] and conditional binding strength of ligands, KFe'Lcond). Since Fe is also supplied by shelf sediments and glacial discharge, the high complexation capacity over the shelf can keep Fe dissolved and available for local primary productivity later in the season upon sea-ice melting.

Highlights

The Southern Ocean is a high-nutrient low-chlorophyll (HNLC; e.g., Sunda et al, 1989) region where the phytoplankton biomass is relatively low despite high ambient macronutrient concentrations, i.e., nitrogen (N), phosphorus (P) and silicon (Si) (e.g., Martin et al, 1991; Schoffman et al, 2016)
Two distinct horizontal currents exist in the study area: the coastal current (CC) and the Antarctic Circumpolar Current (ACC) (Fig. 1)
Our results indicate that organic Fe-binding ligands in surface water on the continental shelf of the western Antarctic Peninsula (WAP) are associated with ice-algal exudates and the addition of ligands from melting sea ice

Summary

Introduction

The Southern Ocean is a high-nutrient low-chlorophyll (HNLC; e.g., Sunda et al, 1989) region where the phytoplankton biomass is relatively low despite high ambient macronutrient concentrations, i.e., nitrogen (N), phosphorus (P) and silicon (Si) (e.g., Martin et al, 1991; Schoffman et al, 2016). The generally limited availability of light and the micronutrient iron (Fe) prevents phytoplankton from depleting P and N in the vast majority of HNLC areas (de Baar et al, 2005; de Baar, 1990; Martin et al, 1991; Viljoen et al, 2018). In the HNLC Southern Ocean, the availability of Fe has a direct impact on the early spring phytoplankton bloom and on primary productivity (Moore et al, 2013).

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Results

Discussion

Conclusion