The Ecology of Iron Enhanced Ocean Productivity

Michael R. LandryIntegrative Oceanography DivisionScripps Institution of OceanographyUniversity of California, San Diego

Funding: National Science Foundation Grants OCE-9908808 and -9911765

Focus: Upper-ocean ecology, not carbon sequestration

Mechanisms & implications

Units of Biomass Response

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PatchControl

Time (year day)

Y = 68,900 * X-1.79

R2 = 0.98

IronEx II SOFeXChl a 16-20 X 20 X

Phyto-C 4-5 X 2 X

SOFeX South PatchPatch Increase

Overstating the caseGrowth interpretations

Phytoplankton Community Response

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PROSYNPEUKPRYMPHAEODIATOMDINO

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< 2 2-5 5-10 10-20 > 200

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IronEx II

Phytoplankton Community Structure

% Diatoms

Cont PatchIronEx II 4 74

N-SOFeX 5 38

S-SOFeX 66 80

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nkt

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g C

m-3)

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Size Fraction (µm)

SOFEX - South

flagellates ==> pennates IronEx, N-SOFeXpennates ==> centrics SEEDS+ silicified ==> less silicified EisenEx

Growth and Grazing in IronEx II

Landry et al. (2000)

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Time (Julian Day)

C

2 nM 1 nM 1 nMIron additions

µg

Heterotrophic Protists

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HFLAGCHOANOHDINOCILIATE

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Grazing Regulation

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IronEx II

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SOFeX Grazers

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SOFeX - South

Bio

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% PP Grazed: SOFeX IronExInitial 44 38Bloom Peak 90 + 94

Microbial Community Interactions& Sequestration Potential

Strong µ => g enhances nutrient cycling

Diminishes “structural boost” to export ratio

Quality of export -- single cell egesta

Community shifts -- C:Si export ratio, ballast Different suite of diatoms -- high µ, low Si Variable silicification: SOFeX -- 50% Si:C decr

Rollwagen Bollens & Landry (2001)

Growth rate implications:

from Chl ingested/mgC and C:Chl ratio and 20% GGE

Double C biomass d-1

MesozooplanktonIronEx II: Biomass-specific ingestion of phytoplankton increased ~ 20X

MesoZoo Grazing ≈ 10% µ

Explanations ?

H1: Tightly coupled predatory control

H2: Predators find the patch (scale artifact)

H3: Diatom inhibition of egg hatching success

These are examples of ecological issues thatcould be reasonably addressed by larger or longer experiments.

Many are not

Full population and numerical responses, complex life histories

Phenotypic/genotypic selection & adaptations

Cascade and trickle-down effects of larger and longer-lived consumers

Down-stream effects on adjacent ecosystems

Neocalanus in the Subarctic Pacific

Decreasing depth of mid winter Mixed-layer

Monthly-averaged surface density anomaly

Freeland et al. (1998)Whitney et al. (1999) Mackas et al. (1999)

Timing: > 2 month variability in date of maximum biomass, ~1975 trend reversal

Southern Ocean Krill

Recruitment success -- sea ice & diatom blooms

Foraging migrations

Summary

Fe-fertilization experiments have greatly advanced our understanding of open-ocean production ecology. There are clear and recurrent patterns in microbial community response.

Effects on “macro” components of the food web (aka “animals”) are poorly known. Extrapolation to relevant temporal & spatial scales is difficult.

Beyond sequestration, we need to better understand the ecology of HNLC regions in the context of a changing ocean.