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Biological productivity in the Southern Ocean is modulated by iron availability. Every summer, a large phytoplankton bloom forms northwest of the Ross Sea, above the Antarctic Australian Ridge (AAR), due to a plume of iron‐rich waters. Here, we investigate the origin and trajectories of these iron‐rich waters by analyzing water mass observations and Lagrangian experiments. Output from the Southern Ocean State Estimate (SOSE) and in situ measurements reveal that iron‐rich AAR bloom waters share properties with Modified Circumpolar Deep Water (MCDW), which forms on the Antarctic shelf‐slope. The Lagrangian experiments are conducted using SOSE velocities. Bloom waters tracked with virtual Lagrangian particles highlight an along isopycnal pathway of MCDW from Antarctica's shelf‐slope to the AAR bloom site, illustrating advection of these waters by the Balleny Gyre. These results are supported by temperature‐salinity analyses, which show a correlation between waters advected northwards; MCDW properties; and high iron concentrations. Plain Language Summary: The biological productivity in the Southern Ocean is limited by the availability of iron and light. During the summer, when light is abundant, a current of iron‐rich waters allows the formation of a phytoplankton bloom on the Pacific sector of the Southern Ocean, above the Australian Antarctic Ridge (AAR). The AAR bloom has an average area of 266,000 km2 and has been observed by satellites for more than 20 years. The temporal and spatial consistency of this bloom has important ecological implications. For example, it is a persistent feeding ground for humpback whales. In this work, we use observations and model output to characterize the properties of these iron‐rich waters and their origin. Our analyses suggest that these iron‐rich waters form near the Antarctic shelf and are transported to the AAR bloom area by the local circulation. Quantifying the connectivity between iron sources and the surface is crucial to better monitor Southern Ocean ecosystems as well as their impact on carbon sequestration. Key Points: The Antarctic Australian Ridge phytoplankton bloom's cooler, iron‐rich waters share properties with Modified Circumpolar Deep WaterLagrangian simulations reveal pathways for iron supply from the Antarctic shelf‐slope to the bloom site following the Balleny GyreThe cooler, iron‐rich waters at the bloom site are correlated with northward Lagrangian displacements after a year of advection [ABSTRACT FROM AUTHOR]

The density of ice krill (Euphausia crystallorophias), a species of key ecological value, and related environmental factors were observed along two transects in the Amundsen Sea Coastal Polynya (ASCP) in Antarctica. The distribution of ice krill was processed using two-frequency acoustic backscatter data identification, and the target strength of ice krill was calculated via stochastic distorted-wave born approximation based on the ice krill sample. The ice krill density ranged from 0.96 to 11.01 g m−2 for each transect (mean value was 5.77 g m−2 for the entire survey). These results were of a lower order of magnitude than those of a previous study in the same waters, although the ice krill was still the dominant krill specie in this polynya. We contend that the extent of the phytoplankton bloom might be the factor behind this difference, while other hydrographic parameters such as surface temperature and salinity would have some effect on the spatial distribution of ice krill abundance. Meanwhile, the density and height of ice krill abundance suggested that the diurnal effect on the ice krill was almost negligible. According to the fitted results of regression model, the abundance of ice krill in ASCP in 2022 summer was estimated approximately 175,000 tons. • Ice krill density and related factors were observed in the Amundsen Polynya, Antarctica. • The ice krill density ranged from 0.96 to 11.01 g m−2 in two sampled transects. • Ice krill was still the dominant krill species considering previous findings. • The ice krill density, however, distinctly declined compared to the density in 2012. • The phenomenon of ice melting and eutrophication possibly influenced the density. [ABSTRACT FROM AUTHOR]