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.

Polynyas represent regions of enhanced primary production because of the low, or absent, sea-ice cover coupled with the proximity of nutrient sources. However, studies throughout the Southern Ocean suggest elevated pri-mary production does not necessarily result in increased carbon export. Three coastal polynyas in East Antarctica and an off-shelf region were visited during the austral summer from December 2016 to January 2017 to examine the vertical distribution and concentration of particulate organic carbon (POC). Carbon export was also exam-ined using thorium-234 (234Th) as a proxy at two of the polynyas. Our results show that concentrations and integrated POC stocks were higher within the polynyas compared to the off-shelf sites. Within the polynyas, vertical POC concentrations were higher in the Mertz and Ninnis polynyas compared to the Dalton polynya. Similarly, higher carbon export was measured in the diatom-dominated Mertz polynya, where large particles (>53 mu m) represented a significant fraction of the particulate 234Th and POC (average 50% and 39%, respec-tively), compared to the small flagellate-dominated Dalton polynya, where almost all the particulate 234Th and POC were found in the smaller size fraction (1-53 mu m). The POC to Chlorophyll-a ratios suggest that organic matter below the mixed layer in the polynyas consisted largely of fresh phytoplankton at this time of the year. In combination with a parallel study on phytoplankton production at these sites, we find that increased primary production at these polynyas does lead to greater concentrations and export of POC and a higher POC export efficiency.

Iron (Fe) has been shown to limit growth of marine phytoplankton in the Southern Ocean, regulating phytoplankton productivity and species composition, yet does not seem to limit primary productivity in Antarctic sea ice. Little is known, however, about the potential impact of other metals in controlling sea-ice algae growth. Here, we report on the distribution of dissolved and particulate cadmium (Cd), cobalt (Co), copper (Cu), manganese (Mn), nickel (Ni), and zinc (Zn) concentrations in sea-ice cores collected during 3 Antarctic expeditions off East Antarctica spanning the winter, spring, and summer seasons. Bulk sea ice was generally enriched in particulate metals but dissolved concentrations were similar to the underlying seawater. These results point toward an environment controlled by a subtle balance between thermodynamic and biological processes, where metal availability does not appear to limit sea-ice algal growth. Yet the high concentrations of dissolved Cu and Zn found in our sea-ice samples raise concern about their potential toxicity if unchelated by organic ligands. Finally, the particulate metal-to-phosphorus (P) ratios of Cu, Mn, Ni, and Zn calculated from our pack ice samples are higher than values previously reported for pelagic marine particles. However, these values were all consistently lower than the sea-ice Fe:P ratios calculated from the available literature, indicating a large accumulation of Fe relative to other metals in sea ice. We report for the first time a P-normalized sea-ice particulate metal abundance ranking of Fe >> Zn ≈ Ni ≈ Cu ≈ Mn > Co ≈ Cd. We encourage future sea-ice work to assess cellular metal quotas through existing and new approaches. Such work, together with a better understanding of the nature of ligand complexation to different metals in the sea-ice environment, would improve the evaluation of metal bioavailability, limitation, and potential toxicity to sea-ice algae.

Antarctic sea ice can incorporate high levels of iron (Fe) during its formation and has been suggested as an important source of this essential micronutrient to Southern Ocean surface waters during the melt season. Over the last decade, a limited number of studies have quantified the Fe pool in Antarctic sea ice, with a focus on late winter and spring. Here we study the distribution of operationally defined dissolved and particulate Fe from nine sites sampled between Wilkes Land and King George V Land during austral summer 2016/2017. Results point toward a net heterotrophic sea-ice community, consistent with the observed nitrate limitation (<1 mu M). We postulate that the recycling of the high particulate Fe pool in summer sea ice supplies sufficient (similar to 3 nM) levels of dissolved Fe to sustain ice algal growth. The remineralization of particulate Fe is likely favored by high concentrations of exopolysaccharides (113-16,290 mu g xeq L-1) which can serve as a hotspot for bacterial activity. Finally, results indicate a potential relationship between glacial meltwater discharged from the Moscow University Ice Shelf and the occurrence of Fe-rich (similar to 4.3 mu M) platelet ice in its vicinity. As climate change is expected to result in enhanced Fe-rich glacial discharge and changes in summer sea-ice extent and quality, the processes influencing Fe distribution in sea ice that persists into summer need to be better constrained.

Antarctic sea ice is an important temporal reservoir of iron which can boost primary production in the marginal ice zone during the seasonal melt. While studies have reported that Antarctic fast ice bears high concentrations of iron due to the proximity to coastal sources, less clear are the biogeochemical changes this iron pool undergoes during late spring. Here we describe a 3-week time series of physical and biogeochemical data, including iron, from first-year coastal fast ice sampled near Davis Station (Prydz Bay, East Antarctica) during late austral spring 2015. Our study shows that dissolved and particulate iron concentrations in sea ice were up to two orders of magnitude higher than in under-ice seawater. Furthermore, our results indicate a significant contribution of lithogenic iron from the Vestfold Hills (as deduced from the comparison with crustal element ratios) to the particulate iron pool in fast ice after a blizzard event halfway through the time series. Windblown dust represented approximately 75% of the particulate iron found in the ice and is a potential candidate for keeping concentrations of soluble iron stable during our observations. These results suggest that iron entrapped during ice formation, likely from sediments, as well as local input of coastal dust, supports primary productivity in Davis fast ice. As ice-free land areas are likely to expand over the course of the century, this work highlights the need to quantify iron inputs from continental Antarctic dust and its bioavailability for ice algae and phytoplankton.

Nutrient input from icebergs can fertilize productivity in the ocean. Ten years of satellite measurements reveal that giant icebergs could be responsible for up to 20% of carbon export to depth in the Southern Ocean. Primary productivity is enhanced within a few kilometres of icebergs in the Weddell Sea1,2 owing to the input of terrigeneous nutrients and trace elements during iceberg melting. However, the influence of giant icebergs, over 18 km in length, on marine primary production in the Southern Ocean is less well studied1,3. Here we present an analysis of 175 satellite images of open ocean colour before and after the passage of 17 giant icebergs between 2003 and 2013. We detect substantially enhanced chlorophyll levels, typically over a radius of at least 4–10 times the iceberg’s length, that can persist for more than a month following passage of a giant iceberg. This area of influence is more than an order of magnitude larger than that found for sub-kilometre scale icebergs2 or in ship-based surveys of giant icebergs1. Assuming that carbon export increases by a factor of 5–10 over the area of influence, we estimate that up to a fifth of the Southern Ocean’s downward carbon flux originates with giant iceberg fertilization. We suggest that, if giant iceberg calving increases this century as expected4, this negative feedback on the carbon cycle may become more important.