Iron Fertilization Effects on Southern Ocean Carbon Cycling

The Southern Ocean is one of the largest high-nutrient, low-chlorophyll regions, where dissolved iron constrains phytoplankton growth despite abundant macronutrients. Natural iron fertilisation by upwelling or sediment inputs and deliberate enrichment experiments have demonstrated that even modest additions of bioavailable iron can stimulate diatom-dominated blooms, enhance carbon fixation and alter the structure of microbial communities. These blooms assemble large aggregates and faecal pellets that sink rapidly, strengthening the biological carbon pump and promoting long-term sequestration of atmospheric CO₂ in the deep ocean. Iron fertilisation also modulates nutrient remineralisation pathways, favouring silica utilisation in diatoms and influencing community successional dynamics, including the post-bloom dominance of smaller phytoplankton. Variability in bloom intensity, export efficiency and regional circulation leads to considerable heterogeneity in carbon flux across subpolar sectors. Improved quantification of particulate organic carbon export, transfer efficiencies and microbial interactions under naturally iron-replete conditions is critical for reducing uncertainties in global climate models and assessing the ecological implications of artificial iron fertilisation as a climate intervention.

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Iron Fertilization Effects on Southern Ocean Carbon Cycling publication trend

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Technical terms

Biological carbon pump: The process by which photosynthetic organisms convert CO₂ into organic matter and transport it to the deep ocean via sinking particles.

High-nutrient, low-chlorophyll (HNLC): Oceanic regions rich in macronutrients but with limited phytoplankton growth due to scarcity of micronutrients such as iron.

Particulate organic carbon (POC): Organic carbon contained in particles in the water column, central to carbon export and sequestration.

²³⁴Th tracer method: A radiochemical technique using the disequilibrium between thorium-234 and uranium-238 to estimate particulate export rates from surface waters.

Mesopelagic zone: The ocean layer between approximately 200 m and 1,000 m, where significant particle remineralisation and transfer efficiency shaping occur.

References

1. BioGeoChemical‐Argo Floats Reveal Stark Latitudinal Gradient in the Southern Ocean Deep Carbon Flux Driven by Phytoplankton Community Composition. Global Biogeochemical Cycles (2023).
2. Small phytoplankton contribute greatly to CO2-fixation after the diatom bloom in the Southern Ocean. The ISME Journal: Multidisciplinary Journal of Microbial Ecology (2021).
3. Carbon export in the naturally iron-fertilized Kerguelen area of the Southern Ocean based on the 234Th approach. Biogeosciences (2015).
4. The relative importance of phytoplankton aggregates and zooplankton fecal pellets to carbon export: insights from free-drifting sediment trap deployments in naturally iron-fertilised waters near the Kerguelen Plateau. Biogeosciences (2015).