Giant iceberg behaviour impacts regional biogeochemical cycling in the Southern Ocean.

Giant Antarctic iceberg calving is projected to increase with climate change, affecting ocean circulation, nutrient supply, and carbon cycling. These icebergs can stimulate primary production and influence Southern Ocean carbon fluxes through modification of upper ocean physics and biogeochemistry, yet the underlying mechanisms remain poorly constrained. We investigate the coupled effects of meltwater input and nutrient dynamics around two of the largest known icebergs, A-76A and A-23A, using silicon isotopes alongside hydrographic, meltwater, and macronutrient observations to examine nutrient cycling. Around A-76A, enhanced glacial meltwater input coincides with macronutrient variability and strong silicon isotope fractionation, indicating diatom utilisation sustained by continued macronutrient supply. In contrast, waters around A-23A show minimal glacial meltwater enhancement and remain macronutrient-rich, with no silicon isotope fractionation, indicating limited biological uptake despite favourable background conditions. These contrasting regimes reveal that iceberg influence on ocean biogeochemistry is highly heterogeneous, reflecting the combined effects of micronutrient fertilisation, macronutrient resupply, and environmental context. Our findings demonstrate that giant icebergs exert dual controls on productivity by initiating blooms through micronutrient delivery and sustaining biomass accumulation through resupply from depth. This mechanistic understanding is critical for assessing the role of increasing iceberg discharge in future Southern Ocean productivity and carbon cycling.