Abstract

As anthropogenic carbon continues to accumulate in the ocean–land–atmosphere system, constraining the future state of the ocean is increasingly urgent. The ocean absorbs roughly half of all anthropogenic CO₂ emissions, and the Southern Ocean alone accounts for about 40% of this global sink. This capacity depends strongly on the efficiency with which phytoplankton utilize available nutrients. The Southern Ocean also supports a diversity of endemic species that have evolved unique physiological adaptations to a region now undergoing rapid environmental change. Multiple lines of evidence—spanning historical observations to numerical modelling—indicate that profound, multifaceted shifts are already underway, posing risks to both ecosystems and the services they sustain. However, most available datasets extend back only a few decades, limiting our ability to disentangle natural variability from anthropogenic forcing.

New high-resolution archives provide deeper temporal context. Nitrogen isotope compositions of proteinaceous organics preserved within diatom frustules (DB-δ¹⁵N) from the West Antarctic region reveal unprecedented changes in nutrient consumption and food-web structure over the past century. In the same areas, we document a remarkable dwarfing of Antarctic benthic foraminifera over the last 150 years, observed consistently across multiple sites. This size reduction suggests increasing metabolic stress, likely driven by the combined impacts of warming and deoxygenation associated with enhanced incursions of Circumpolar Deep Water onto the Antarctic shelf. Together, a more efficient biological pump and shrinking body sizes in benthic communities may trigger cascading ecological consequences, ultimately affecting the resilience of Antarctic ecosystems to ongoing climate change.