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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 μM). We postulate that the recycling of the high particulate Fe pool in summer sea ice supplies sufficient (∼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 μg 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 (∼4.3 μM) 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. Plain Language Summary: Iron (Fe) plays a crucial role in microalgal physiology and can control their growth in the Southern Ocean, where Fe concentrations are naturally low. Antarctic sea ice can incorporate high levels of Fe during its formation triggering phytoplankton blooms at the sea‐ice edge during the melt season. No studies to date have assessed sea‐ice Fe distributions in East Antarctica during mid‐ to late summer. Here we discuss Fe distribution in parallel with key sea‐ice physical and biological parameters measured during an expedition to East Antarctica in summer 2016/2017 to answer our central question: is Fe limiting sea‐ice primary productivity during summer? Results suggest nitrate, rather than Fe is the key nutrient controlling sea‐ice algal growth at this time of the year. We also found Fe‐rich platelet ice incorporated underneath pack ice sampled near the Moscow University Ice Shelf which suggests the potential accretion of Fe‐rich ice shelf waters under the sea ice. As climate change is expected to accelerate Antarctic ice shelve melting, a better understanding of how increased rates of glacial meltwater discharge will impact the distribution of Fe within the sea ice during summer is needed. Key Points: Primary production in East Antarctic fast and pack sea ice is not Fe‐limited during summerLow nitrate and high exopolysaccharide concentrations suggest heterotrophic dominance in Antarctic summer sea iceFe‐rich platelet sea ice near the Moscow University Ice Shelf indicates an influence of glacial meltwater in the coastal distribution of Fe [ABSTRACT FROM AUTHOR]

A likely important feature of the poorly understood aerosol‐cloud interactions over the Southern Ocean (SO) is the dominant role of sea spray aerosol, versus terrestrial aerosol. Ice nucleating particles (INPs), or particles required for heterogeneous ice nucleation, present over the SO have not been studied in several decades. In this study, boundary layer aerosol properties and immersion freezing INP number concentrations (nINPs) were measured during a ship campaign that occurred south of Australia (down to 53°S) in March–April 2016. Ocean surface chlorophyll a concentrations ranged from 0.11 to 1.77 mg/m3, and nINPs were a factor of 100 lower than historical surveys, ranging from 0.38 to 4.6 m−3 at −20 °C. The INP population included organic heat‐stable material, with contributions from heat‐labile material. Lower INP source potentials of SO seawater samples compared to Arctic seawater were consistent with lower ice nucleating site densities in this study compared to north Atlantic air masses. Plain Language Summary: The Southern Ocean is known for a prevalence of clouds that contain both liquid and ice, which are one of the most poorly understood cloud regimes in the climate system. A large gap in understanding important processes in these clouds is a lack of knowledge regarding particles (e.g., sea spray) required for forming ice crystals, termed ice nucleating particles. In a ship‐based monthlong field study, several instruments were deployed in efforts to characterize the ice nucleating particles present over the Southern Ocean for the first time in over four decades. Abundances of ice nucleating particles throughout the voyage were extremely low compared to other ocean regions, and concentrations were 2 orders of magnitude lower than the most recent survey conducted in the 1970s. We report that the ocean‐derived ice nucleating particles observed in this study were organic in nature, supporting a hypothesized link between ice nucleating particles and organic particles associated with phytoplankton blooms. The data from this study provide a desperately needed benchmark for constraining the number of ice crystals that may form in the remote and poorly understood clouds occurring over the Southern Ocean. Key Points: Number concentrations of ice nucleating particles over the Southern Ocean in March 2016 were a factor of 100 lower than historical surveysThe ice nucleating particle source strength of Southern Ocean seawater was lower than previous measurements in northern hemisphere seawaterIce nucleation site densities were lower over the Southern Ocean compared to measurements of pristine air masses from other ocean basins [ABSTRACT FROM AUTHOR]