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.
Review of “Highly branched isoprenoids for Southern Ocean semi-quantitative sea ice reconstructions: a pilot study from the Western Antarctic Peninsula” By Vorrath and co-authors.
