A proposed seasonal cycle of dissolved iron-binding ligands in Antarctic sea ice

Abstract

Iron (Fe) is an essential micronutrient to oceanic microalgae, and its dissolved fraction (DFe) is retained in surface waters by Fe-binding ligands. Previous work has suggested that ligands may also bind Fe within sea ice, although supporting data are limited. This study investigates distribution, concentration, and potential drivers of Fe-binding ligands in Antarctic sea ice, considering the ice type, location and season. Results suggest that the concentration of ligands (CL) varies throughout the year, both spatially and seasonally. The lowest CL (3.3–8.0 nM) and DFe concentrations (0.7–3.5 nM) were recorded in newly formed winter sea ice in the Weddell Sea, likely due to the early stage of sea-ice growth and low biological activity. The highest CL (1.7–74.6 nM), which follows the distribution of DFe (1.0–75.5 nM), was observed during springtime, in the Eastern Antarctic Sector. There, consistently higher values for CL in bottom ice depths were likely associated with enhanced algal biomass, while aeolian deposition may have acted as an additional source of DFe and ligands near Davis station. In summer, the senescence of ice algae and advanced sea-ice melting led to intermediate CL (1.0–21.9 nM) and DFe concentrations (0.6–13.3 nM) both on and off the East Antarctic coast. Regardless of time and location, >99% of DFe was complexed, suggesting that CL controls the distribution of DFe in sea ice. This study represents a first attempt at a year-round investigation of CL in sea ice, providing results that support the premise that sea ice acts as a potential biogeochemical bridge between autumn and spring phytoplankton blooms.

Antarctic Landfast Sea Ice: A Review of Its Physics, Biogeochemistry and Ecology

“…Ligands may be of organic and/or inorganic origins, and include EPS (see below), humic substances (Hassler et al., 2020; Laglera et al., 2020; Powell & Wilson‐Finelli, 2003), and siderophores (Velasquez et al., 2011). Aeolian (i.e., wind‐carried) deposition and glacial meltwaters may also supply Fe‐binding ligands along the Antarctic coast and therefore, to fast ice (Genovese et al., 2023; Gerringa et al., 2012; Hassler et al., 2020). Regardless of their origin and nature, Antarctic fast ice exhibits high concentrations of Fe‐binding ligands (Genovese et al., 2023; Lannuzel et al., 2015) relative to Antarctic seawater (Southern Ocean Ligand Collection; A. J. Smith et al., 2022).…”

“…Aeolian (i.e., wind‐carried) deposition and glacial meltwaters may also supply Fe‐binding ligands along the Antarctic coast and therefore, to fast ice (Genovese et al., 2023; Gerringa et al., 2012; Hassler et al., 2020). Regardless of their origin and nature, Antarctic fast ice exhibits high concentrations of Fe‐binding ligands (Genovese et al., 2023; Lannuzel et al., 2015) relative to Antarctic seawater (Southern Ocean Ligand Collection; A. J. Smith et al., 2022). When fast ice melts in spring and summer, it has the potential to enrich seawater with DFe and ligands, thereby increasing Fe availability and boosting local phytoplankton productivity (De Jong et al., 2013; Duprat et al., 2019; van der Merwe et al., 2011).…”

“…Since DOC and POC (including EPS and iron‐binding ligands) are enriched in fast ice relative to seawater (Cozzi, 2014; Genovese et al., 2023; Lannuzel et al., 2015; van der Merwe et al., 2009), their release into seawater when fast ice melts has important ramifications on coastal productivity, export and carbon burial in sediments. Sediment trap deployments under pack ice are logistically challenging due to increased water depth and exposure to storms making moorings prone to failure.…”

“…Aeolian (i.e., wind-carried) deposition and glacial meltwaters may also supply Fe-binding ligands along the Antarctic coast and therefore, to fast ice (Genovese et al, 2023;Gerringa et al, 2012;Hassler et al, 2020). Regardless of their origin and nature, Antarctic fast ice exhibits high concentrations of Fe-binding ligands (Genovese et al, 2023;Lannuzel et al, 2015) relative to Antarctic seawater (Southern Ocean Ligand Collection; A. J. Smith et al, 2022).…”

Pack-ice seals contribute to biological transfers of iron in the Southern Ocean

“…Crabeater seals primarily use pack-ice (Southwell et al 2008;Bengtson et al 2011) whereas Weddell seals are primarily bound to fast-ice and nearby pack-ice, relying on breathing holes to navigate between ice-floes (Shirihai 2008;Thomas and Terhune 2009;Southwell et al 2012;Davis et al 2013;Goetz et al 2017). Whether iron is released on packice or on fast-ice -and thus seals different spatial contributions -has different implications, given the central role of sea-ice in retaining and releasing iron in SO waters (Lannuzel et al 2016b;Genovese et al 2023). Pack-ice, for instance, may grow and melt further offshore, releasing the iron it contains in more limited waters (Lannuzel et al 2016a) than fast-ice melting in coastal, enriched waters.…”