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

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.

Potential interactions between diatoms and bacteria are shaped by trace element gradients in the Southern Ocean

“…It is also possible that siderophores are cycled rapidly amongst the microbial community, and their concentrations therefore represent a single snapshot in time. Overall, our observed concentration of siderophores was an order of magnitude less than total strong Fe ligand concentrations previously reported from the same region in the SO (Buck et al, 2010;Ardiningsih et al, 2021), which is consistent with previous work showing siderophores comprise less than 10% of the ligand pool that is measured in seawater (Bundy et al, 2018).…”

Trace Element (Fe, Co, Ni and Cu) Dynamics Across the Salinity Gradient in Arctic and Antarctic Glacier Fjords

“…Extrapolation between Antarctic and Greenlandic systems is not straightforward because the geographic and biogeochemical context of Antarctic fjords is very different due to lower freshwater discharge rates, lower turbidity, higher macronutrient:micronutrient ratios and different patterns of primary production in Antarctica (Vernet et al, 2008;Hopwood et al, 2020;Forsch et al, 2021). Nevertheless, the physical/biogeochemical factors responsible for buffering of dFe concentrations in fjords (i.e., the interaction between turbidity, ligand concentration and properties) warrant further investigation in a global context in order to better constrain how a changing cryosphere (Meredith et al, 2019) will affect biogeochemical cycling in the ocean (Thuroczy et al, 2012;Ardiningsih et al, 2020;Ardiningsih et al, 2021).…”

Atmospheric Iron and Aluminium Deposition and Sea-Surface Dissolved Iron and Aluminium Concentrations in the South China Sea off Malaysia Borneo (Sarawak Waters)

“…In addition, highest average concentration of Chl-a was analysed at Area III. Organic ligand was also biologically produced by bacteria and phytoplankton (Blanco-Ameijeiras et al, 2020;Ardiningsih et al, 2021) as well as during the plankton cell lysis (Poorvin et al, 2004). Thus, high abundance of Chl-a at Area III could support the production of more strong class of organic ligands than Area I and Area II.…”

“…Log K obtained at coastal and open ocean water was 19.5-22.3 and 21.4-22.7, respectively (Mohamed et al, 2011;Cullen et al, 2006;Croot et al, 2004;Croot & Johansson, 2000). However, there were some locations that analysed with strong Fe organic ligands such as Sulu and adjacent Seas (log K 22.0-24.0), Southern Ocean and equatorial Pacific Ocean (log K > 23) (Kondo et al, 2007;Nolting et al, 1998;Rue & Bruland, 1995), due to the intensidied light exposure and located at the shelf (Ardiningsih et al, 2021).…”