LIQUID TREES

Recurring natural analogue + deep traps

Ocean iron fertilisation: South Georgia natural iron-fertilised bloom

A recurring diatom-rich bloom, island and glacial iron sources, repeat cruises and deep traps make South Georgia an unusually rich carbon-fate research context.

Geographic relationship

The UK administers South Georgia and the South Sandwich Islands; Argentina claims the islands and surrounding maritime spaces. The whole SGSSI maritime zone is an MPA under the UK administration.

Evidence class

Recurring natural analogue + deep traps

Central research question

What share of deep resting-spore flux is additional to natural production, and how do food webs, circulation and protected-area constraints shape the interpretation?

The scientific case

Why it matters

South Georgia sustains one of the Southern Ocean's largest recurring phytoplankton blooms. Shelf, glacial and other island-derived iron sources, upwelling and downstream circulation maintain a diatom-rich, iron-replete system. Repeat cruises, satellite records, BAS science and deep sediment traps make it a strong natural analogue for studying measurement and carbon fate.

Physical setting

Oceanography

Blooms develop mainly northwest/downstream of South Georgia in the Georgia Basin. Drifters and hydrography identify divergent flows, upwelling and shelf-water pathways that supply nutrients and iron. Interannual bloom extent depends on circulation as well as iron. Recent source work shows that the bioavailability of glacial, groundwater and other iron varies with both source chemistry and receiving water.

Biological response

Phytoplankton and algae

The natural bloom is strongly diatom-dominated, with the assemblage and silica drawdown changing through the season. Deep traps show that diatom resting spores can be disproportionate vectors of bathypelagic POC even when vegetative cells contribute little to the sinking assemblage. Krill and other grazers are central to South Georgia's food web and biogeochemistry.

Evidence record

Experiments and observations

  • Multi-cruise work described a large, sustained, naturally iron-replete bloom and its circulation controls.
  • Comparative modelling/observations across Kerguelen, Crozet and South Georgia support shelf-water advection as a major bloom mechanism.
  • Deep sediment traps downstream of South Georgia found strong resting-spore contributions to carbon transfer and measurable differences among trap sites.
  • New uptake experiments show glacially derived iron can be especially bioavailable, but bioavailability depends on source and seawater chemistry.

Accounting boundary

Carbon fate and permanence

Deep resting-spore flux shows carbon transfer to depth; storage duration remains unresolved, but it does not quantify the incremental result of deliberate OIF. The accounting separates recurring natural flux, additional export, atmospheric equilibration and avoided/downstream production. South Georgia's island/shelf iron supply, seed bank and food web are not recreated by soluble iron alone.

Observation system

Measurement and MRV priorities

  • Glacial, groundwater, shelf-sediment and upwelling iron sources, speciation and bioavailability.
  • Drifters, shelf-to-basin pathways, fronts and interannual bloom geometry.
  • Diatom/resting-spore species, krill and zooplankton, fish acoustics, seabirds and marine mammals.
  • Deep moored traps, sediment cores, bathypelagic degradation and benthic response.
  • Silica, carbonate, oxygen/N2O and year-round circulation/ventilation.

Field reality

Operations and cost drivers

Remote Southern Ocean ship time; narrow weather windows; SGSSI regulated-activity and MPA review; foreign-vessel MSR process where applicable; long deep-trap deployments and recovery; BAS/MPA data integration; glacial/shelf source sampling; biosecurity; fisheries, krill, seabird and marine-mammal monitoring; and dispute-sensitive legal review.

Uncertainty and exposure

Ecological and social risk pathways

Large protected food webs; krill/fisheries interactions; species and toxin change; silica depletion; non-local nutrient effects; carbonate counter-pump; deep oxygen/N2O; glacial-source transfer error; difficult attribution in a naturally iron-rich system; deep/benthic change; protected-area disturbance and sovereignty dispute.

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 UK administers South Georgia and the South Sandwich Islands; Argentina claims the islands and surrounding maritime spaces. Both positions are material; treating either as the sole uncontested route would be inaccurate. The Government of SGSSI's MPA covers the entire 1.24 million km² maritime zone, with expanded no-take zones in 2025. Scientific work under the UK administration requires a Regulated Activity Permit; non-UK-flagged marine scientific research also uses the FCDO Article 246 process. None of these facts establishes that OIF would be permissible.

Research agenda

Open questions

  1. How much of the deep resting-spore flux is caused by iron versus shelf seeding, bloom duration and food-web structure?
  2. Which iron sources are bioavailable across seasons and water masses?
  3. How do krill, fish, seabirds and mammals alter or respond to carbon pathways?
  4. How much natural export reaches water masses isolated on climate-relevant timescales?
  5. Can any intervention question be asked without unacceptable MPA, ecological or dispute-related consequences?

Evidence trail

Primary and official sources

Results vary by location, season, intervention, method and observation window.