The Southern Ocean plays a critical role in regulating global climate as a major sink for atmospheric carbon dioxide (CO₂), and in global ocean biogeochemistry by supplying nutrients to the global thermocline, thereby influencing global primary production and carbon export. Biogeochemical processes within the Southern Ocean regulate regional primary production and biological carbon uptake, primarily through iron supply, and support ecosystem functioning over a range of spatial and temporal scales. Here, we assimilate existing knowledge and present new data to examine the biogeochemical cycles of iron, carbon and major nutrients, their key drivers and their responses to, and roles in, contemporary climate and environmental change. Projected increases in iron supply, coupled with increases in light availability to phytoplankton through increased near-surface stratification and longer ice-free periods, are very likely to increase primary production and carbon export around Antarctica. Biological carbon uptake is likely to increase for the Southern Ocean as a whole, whilst there is greater uncertainty around projections of primary production in the Sub-Antarctic and basin-wide changes in phytoplankton species composition, as well as their biogeochemical consequences. Phytoplankton, zooplankton, higher trophic level organisms and microbial communities are strongly influenced by Southern Ocean biogeochemistry, in particular through nutrient supply and ocean acidification. In turn, these organisms exert important controls on biogeochemistry through carbon storage and export, nutrient recycling and redistribution, and benthic-pelagic coupling. The key processes described in this paper are summarised in the Graphical Climate-mediated changes in Southern Ocean biogeochemistry over the coming decades are very likely to impact primary production, sea-air CO₂ exchange and ecosystem functioning within and beyond this vast and critically important ocean region.

The deep waters of the Bransfield Strait receive considerable amounts of water from the Weddell Sea continental shelf. The restricted connections to the surrounding ocean and relatively easier access makes the Bransfield Strait an important proxy region for monitoring changes in the dense Weddell Sea shelf water masses, which are an important precursor of Antarctic Bottom Water (AABW). Long‐term hydrographic data from the period 1960s–2010s showed freshening and lightening of the deep water masses of the Bransfield Strait, which was likely caused by large freshwater inputs originating from the western shelf of the Weddell Sea. The rates of freshening and lightening were −0.0010 ± 0.0005 yr⁻¹ and −0.0016 ± 0.0014 kg m⁻³ yr⁻¹ for the central basin, respectively, and −0.0010 ± 0.0006 yr⁻¹ and −0.0029 ± 0.0013 kg m⁻³ yr⁻¹ for the eastern basin, respectively. The deep waters showed a high degree of interannual thermohaline variability, which appeared to be caused by changes in the proportions of source water mass mixing between the years. Statistically significant negative correlations between salinity/neutral density fields and the Southern Annular Mode (SAM) were observed (−0.56 and −0.62 for the central basin, respectively, and −0.58 and −0.68 for the eastern basin, respectively) between 1980 and 2014. During SAM positive phases, communication between the Weddell Sea and the Bransfield Strait is reduced, which leads to less saline and lighter water masses in the Bransfield Strait; however, the opposite trends are observed during SAM negative phases.

Regional formation of deep and bottom water masses around the Antarctic continental shelf is one of the most important processes contributing to variability of the global meridional overturning circulation deep cell. Southern Ocean hydrographic data collected during the years 1958–2010 indicate that dense shelf waters cooled and freshened during that period. In the surrounding open ocean, Antarctic Bottom Water (AABW) warmed, with no evidence of salinity change. As a result of source‐water property changes, AABW exported from the Southern Ocean to the deep world ocean became lighter over the period analyzed. The average rate of density change within the areas that experienced statistically significant change was −0.0019 kg m⁻³ yr⁻¹. For the last 20 years of the analysis, a negative AABW volume anomaly (relative to the half‐century average, 1958–2010) was indicated, possibly due to production of a lighter AABW variety or to changes in formation rates. Over the entire five decades, the upper isopycnal of the AABW layer deepened at a rate of −8.1 m yr⁻¹. Changes in fundamental hydrographic properties such as these can have important implications for long‐term global ocean circulation and climate.