depth (m)depth (m)

(1)

Species-specific growth performance and grazing mortality of the diatom assemblage during an in situ iron-induced bloom in the

Southern Ocean

P. Assmy, J. Henjes, C. Klaas & V. Smetacek

Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany

(2)

In situ iron fertilization experiments in „HNLC“-areas

EisenEx (2000)

EIFEX

(2004) SOIREE

(1999) SOFeX South

(2002)

IronEx I (1993)

SEEDS (2001)

IronEx II (1995) SERIES

(2002)

SOFeX North (2002)

(3)

Chla [mg m^-3]

SeaWIFS satellite image

Day 21 (In-patch)

Day 21 (Out-patch)

EisenEx target area

(4)

Temporal evolution of the iron-stimulated

phytoplankton bloom

100 80 60 40 20 00

depth (m)

0.35 0.40 0.45 0.50 0.55

Fv/Fm

100 80 60 40 20 00

depth (m)

0.2 0.6 1.0 1.4 1.8 2.2 2.6

mg/m³

Fv/Fm

Chl-a (mg m^-3)

Fv/Fm and Chl-a provided by M.Gorbunov

Diatom biomass (µmol C l^-1)

(5)

Corethron pennatum

Pseudo-nitzschia lineola

Thalassiosira lentiginosa

Growth performance of different diatom species during EisenEx

Fragilariopsis kerguelensis

(6)

Pseudo-nitzschia lineola

0 50000 100000 150000 200000 250000

0 2 4 6 8 10 12 14 16 18 20 22

Cells l-1

In-patch Out-patch

0 5.000 10.000 15.000 20.000

0 2 4 6 8 10

Exceptionally high growth rates throughout the experiment

In-patch Out-patch

(7)

Corethron pennatum

0 1000 2000 3000 4000 5000

0 2 4 6 8 10 12 14 16 18 20 22 Days since first Fe-release

Cells l-1

In-patch Out-patch

Continuous increase albeit linearly

(8)

Fragilariopsis k erguelensis

0 5000 10000 15000 20000 25000

0 2 4 6 8 10 12 14 16 18 20 22

Cells l-1

In-patch Out-patch

Linear increase but for only a short spurt phase

(9)

Nitzschia sp.

0 500 1000 1500 2000 2500 3000

Cells l-1

In-patch Out-patch

Initial linear increase and decline thereafter

(10)

Thalassiosira lentiginosa

0 100 200 300 400 500 600 700

0 2 4 6 8 10 12 14 16 18 20 22

Cells l-1

In-patch Out-patch

No consistent trend in response to Fe-addition

(11)

Intact empty and broken diatom frustules as mortality indicators Fragilariopsis kerguelensis

Discoid diatoms

(12)

Vertical distribution of intact empty and broken diatom frustules outside the patch

Empty frustules l^-1

Broken frustules l^-1

(13)

Vertical distribution of intact empty and broken diatom frustules inside the patch

Empty frustules l^-1

Broken frustules l^-1

(14)

Species-specific ratios of live cells vs. empty and broken frustules in three Pseudo-nitzschia species

Pseudo-nitzschia lineola

0 5 10 15 20 25 30

0 2 4 6 8 10 12 14 16 18 20 22

Days since first Fe-release

Pseudo-nitzschia heimii

0 5 10 15 20 25 30

0 2 4 6 8 10 12 14 16 18 20 22

Pseudo-nitzschia turgidula

0 5 10 15 20 25 30

(15)

0 100 200 300 400 500 600 700 800 900 1.000

0 2 4 6 8 10 12 14 16 18 20 22

Days since first Fe-release Broken and intact empty frustules l-1 broken frustules

empty frustules

Corethron pennatum Fragilariopsis kerguelensis

Species-specific grazing pressure

Crawford et al. 1998

0 500 1.000 1.500 2.000 2.500 3.000 3.500 4.000

0 2 4 6 8 10 12 14 16 18 20 22

Broken and intact empty frustules l-1

broken frustules

empty frustules EF:BF = 2.1 EF:BF = 0.2

(16)

Conclusions

Species-specific growth performance

• Pseudo-nitzschia lineola maintained exceptionally high growth rates throughout the experiment and contributed 25% of total biomass.

• Most other species were growing below or well below these rates.

→

The population size of the majority species to survive and evolve is well below that achieved by bloom-forming species.

(17)

Species-specific grazing selection

• Pseudo-nitzschia lineola dominated despite heavy grazing pressure because of its exceptionally high growth rates

• Grazers discriminated between morphological similar species of the same genus (Pseudo-nitzschia)

• Large spiny Corethron pennatum were eaten mainly by copepods

• The heavily silicified Fragilariopsis kerguelensis withstands grazing by copepods and showed no clear trend

(18)

Thank you for your attention

Thanks to the crew we survived these conditions

(19)

(20)

(21)

Experimental site of EisenEx

Chl a [mg m^-3]

SeaWIFS satellite image

Day 21 (In-patch)

Day 21 (Out-patch)

(22)

Marked differences in the growth behaviour of the species populations present during EisenEx were identified:

1. Exceptionally high growth rates throughout the experiment 2. Slow growth, albeit continuously

3. Rapid growth for short periods

4. Initial increase and decline thereafter 5. No response

6. Continuous decline

(23)

Category IV

Prorocentrum spp.

0 2000 4000 6000 8000 10000 12000 14000

Cells l-1

In-patch Out-patch

(24)

Category VI

Emiliania huxleyi

0 40000 80000 120000 160000 200000

0 2 4 6 8 10 12 14 16 18 20 22

Cells l-1

In-patch Out-patch

(25)

The accumulation of intact empty and broken diatom frustules indicate how mortality acted on individual

species populations:

(26)

Corethron pennatum

0 1 2 3 4 5 6

0 2 4 6 8 10 12 14 16 18 20 22

Fragilariopsis kerguelensis

0 1 2 3 4 5 6

0 2 4 6 8 10 12 14 16 18 20 22

Thalassionema nitzschioides

0 1 2 3 4 5 6

Species-specific ratios of live cells vs. empty and broken frustules in three other significant diatom species

(27)

Emiliania huxleyi

Prorocentrum sp.