The organic complexation of iron in the Ross sea

Abstract

The Ross Sea Polynya (RSP) has the highest primary production of Antarctic waters. Iron (Fe) is one of the most important growth limiting factors in the Southern Ocean. Dissolved iron (DFe)binding organic ligands play an important ecological role because they increase the residence time of the scarce Fe. Therefore, we studied the DFe-binding organic ligands in the vicinity of the Ross Sea during a cruise between 20 December 2013 and 5 January 2014. The DFe-binding organic ligands were measured using Competing Ligand Exchange Cathodic Stripping Voltammetry (CLE-CSV) with TAC as competing ligand. The DFe-binding organic ligand concentrations always exceeded the DFe concentrations except in the bottom nepheloid layer of the RSP. No relationship was found between depth and DFe-binding organic ligand concentrations in the RSP indicating that these ligands are resistant to degradation and are probably exported by high salinity shelf water into the circumpolar current. DFe-binding organic ligand concentrations were highest in the RSP and the Antarctic Circumpolar Current (ACC) west of the Ross Sea, in association with seasonal phytoplankton blooms, although no correlation was found with parameters reflecting phytoplankton abundance or species. Phytoplankton sources and sinks of DFe-binding organic ligands are likely related to the seasonal progression of the bloom. In 39% of the samples, two DFe-binding organic ligand groups were distinguished based on the difference in binding strength. The distinction was especially clear in the RSP and in the ACC west of the RSP (54 and 77% of the samples, respectively) where blooms occurred and much less in the low biomass waters of the ACC east of the RSP and ice covered eastern part of the Ross Sea (15 and 10% of the samples, respectively). In these waters, other environmental factors, like sea ice melt, probably explain the absence of distinct relationships between primary production and ligand characteristics.

Dissolved Trace Metals in the Ross Sea

“…Apart from direct sources and sinks, the competition between complexation by dissolved organic ligands and scavenging can be important in determining dissolved metal concentrations, notably DFe concentrations. When the Fe-binding dissolved organic ligands are saturated, the excess DFe concentration is not stable due to the low solubility product of Fe-oxyhydroxides (Liu and Millero, 2002) and Fe will be scavenged or precipitated, as has been demonstrated in the Ross Sea for this cruise data (Gerringa et al, 2019).…”

“…As described in previous papers on data from the same cruise (Gerringa et al, 2015(Gerringa et al, , 2019, four main water masses were present in the study area, based on definitions of Tomczak and Godfrey (2001) and Orsi and Wiederwohl (2009). Antarctic Surface Water (AASW, potential temperatures ( ) > −1.85 • C and neutral density < 28 kg m −3 ) covered the surface, below which, in the center of the polynya, remnants of cold Winter Water (WW) were present north of 76 • S (St. 48) ( Figure 2).…”

“…Samples were taken during expedition NBP13-10 on the RVIB Nathaniel B. Palmer in the Ross Sea, entering from the northeast on 20 December 2013 and leaving to the northwest on 5 January 2014 ( Figure 1) (for full details, see Gerringa et al, 2015Gerringa et al, , 2019Alderkamp et al, 2019).…”

“…Measurements were made on four channels: PO 4 3− , Si(OH) 4 , NO 3 − and NO 2 − combined, and NO 2 − separately. For details, see Gerringa et al (2015Gerringa et al ( , 2019. The data can be found at https://doi.org/10.25850/nioz/7b.b.g.…”

“…MSW is formed through mixing of northward flowing SW with southward flowing AASW and mCDW, and has a broad range of salinities and temperatures (S > 34.5 and > −1.85 • C, Orsi and Wiederwohl, 2009). A BNL was observed above the bottom in the Ross Sea Polynya in HSSW and at two stations in MSW (Gerringa et al, 2015(Gerringa et al, , 2019.…”