The Crozet EEZ is within the French Southern Lands national nature reserve. TAAF controls access and activities across this protected natural laboratory.
Natural analogue + deep-export observations
Ocean iron fertilisation: Crozet Plateau
CROZEX connects a naturally fertilised bloom to seasonal export, deep sediment traps and seafloor delivery—and reveals how island-specific resting spores shape the result.
Natural analogue + deep-export observations
How much deep flux reflects island-specific seed populations and a longer export season rather than a transferable open-ocean pathway?
The scientific case
Why it matters
CROZEX was designed around a naturally iron-fertilized bloom north of Crozet and an HNLC control south of the plateau. It measured surface production, seasonal export, deep sediment-trap flux and seafloor delivery. Crozet is therefore one of the strongest contexts for understanding what deep export evidence looks like and why its species-specific island ecology limits extrapolation.
Physical setting
Oceanography
Iron and nutrient-rich waters associated with Crozet and the plateau are carried into the bloom region by circulation and horizontal supply. The annual bloom north of the islands contrasts with deeper mixed and low-iron waters south of the plateau. Light, fronts, water-mass origin and export-season duration all affect the comparison.
Biological response
Phytoplankton and algae
The bloom north of Crozet included substantial Phaeocystis antarctica; diatoms and Phaeocystis varied across plateau and control regimes. Deep export was strongly associated with resting spores of the island-linked diatom Eucampia antarctica var. antarctica. That is a key limitation: Crozet's carbon pathway was not simply generic open-ocean diatoms responding to iron.
Evidence record
Experiments and observations
- CROZEX confirmed natural iron supply and higher production and export in bloom waters than the HNLC control.
- Annual particulate carbon fluxes to deep water and the ocean floor were two to three times higher under the naturally fertilized bloom than the adjacent control.
- Seasonal 234Th budgets attribute much of the difference to a longer export season, not necessarily a higher instantaneous daily flux.
- Deep traps linked the enhanced flux to Eucampia resting-spore ecology and warned that island analogues may not transfer to open-ocean fertilization.
Accounting boundary
Carbon fate and permanence
Crozet offers unusually direct observations at deep and seafloor levels. Even so, natural bloom carbon, additional carbon caused by iron, net atmospheric drawdown and deliberate-OIF permanence are different quantities. Daily, seasonal, deep and seabed flux estimates differ, and remineralization and carbonate production enter the budget at distinct points.
Observation system
Measurement and MRV priorities
- Horizontal iron supply, frontal circulation, bloom residence time and north/south control comparability.
- Phaeocystis, diatom species and resting spores; island-derived seed populations.
- Paired 234Th, drifting traps and moored traps below 2,000 m, plus sediment and benthic response.
- Seasonal export duration, amino-acid/degradation state and POC/biogenic-silica stoichiometry.
- Carbonate counter-pump and changes in deep benthic ecology.
Field reality
Operations and cost drivers
Remote ship time; TAAF permission and protected-area review; repeated north/south sections; long mooring/trap deployments and recovery; trace-metal chemistry; taxonomy/resting-spore work; deep-sea and benthic sampling; autonomous platforms; biosecurity and independent ecosystem monitoring.
Uncertainty and exposure
Ecological and social risk pathways
Treating an island-seeded natural system as a generic open-ocean analogue; community shifts toward Phaeocystis or specific diatoms; silica/light/grazing controls; nutrient redistribution; carbonate counter-pump; deep oxygen and benthic change; protected seabirds/mammals and fisheries; full-reserve constraint and difficult recovery logistics.
Social and ocean-use baseline: fisheries, potentially affected coastal, Indigenous and local communities, protected species and existing ocean users follow the actual and downstream footprint rather than the nearest port alone.
Institutions and protection
Governance and protection context
The entire Crozet EEZ is included in the French Southern Lands national nature reserve under Decree 2022-157. TAAF controls access and reserve activities. Crozet is a protected natural laboratory with exceptional historic data, not as an available intervention site.
Research agenda
Open questions
- How much of Crozet's deep flux depends on island-specific Eucampia resting spores?
- What portion of the deep flux represents incremental atmospheric CO2 uptake rather than redistributed natural production?
- How do Phaeocystis, diatoms, carbonate and grazing change net climate effect?
- Can control waters match physical history well enough for attribution?
- What long-term benthic and food-web changes accompany enhanced deep flux?
Evidence trail
Primary and official sources
- Pollard et al. (2007), CROZEX overview, DOI 10.1016/j.dsr2.2007.07.023 ↗.
- Pollard et al. (2009), deep-water and seafloor export, DOI 10.1038/nature07716 ↗.
- Poulton et al. (2007), diatoms and Phaeocystis, DOI 10.1016/j.dsr2.2007.06.005 ↗.
- Morris et al. (2011), seasonal carbon budget, DOI 10.1029/2010GB003780 ↗.
- Salter et al. (2012), resting spores and deep export, DOI 10.1029/2010GB003977 ↗.
- Planquette et al. (2011), particulate iron and carbon, DOI 10.1029/2010GB003789 ↗.
- French Decree 2022-157 ↗ and TAAF reserve information ↗.
Results vary by location, season, intervention, method and observation window.