1Alfred-Wegener-Institute for Polar and Marine Research, Bremerhaven, Germany
2Max-Planck-Institute for Biogeochemistry, Jena, Germany
Biological response to iron fertilization in the Polar Frontal Zone of the Southern Ocean (EisenEx)
Philipp Assmy
1, Joachim Henjes
1, Christine Klaas
2& Victor Smetacek
1Chlorophyll a (mg m-3)
Fig. 1: Satellite picture of the chlorophyll patch.
During EisenEx - the second in situ iron fertilization experiment conducted in HNLC (High Nutrient Low Chlorophyll) waters of the Southern Ocean - an iron-enriched eddy was followed over a period of three weeks in austral spring 2000. Artificial iron infusion simulates an aeolian dust input into the surface water of the open ocean and its impact on the pelagic community and the biogeochemical processes driven by changes in plankton distribution.
Three weeks after the first iron release the SeaWIFS satellite picture showed an algal bloom 30 km in diameter (Fig. 1).
Diatom abundance increased 6-fold inside the fertilized patch compared to control values (Fig. 2). Pseudonitzschia lineola was the dominant species and accounted numerically for 51% of the diatom assemblage at the end of the experiment.
Our results confirm the stimulation of diatom growth by iron addition in the Southern Ocean.
Fig. 2: Temporal development of the diatom abundance in the upper 150m in and outside the fertilized patch.
0 50.000 100.000 150.000 200.000 250.000 300.000
-4 -2 0 2 4 6 8 10 12 14 16 18 20 22
Days since first Fe-release In-Patch
Out-Patch
Inside the patch diatom standing stock was dominated by medium-sized (Pseudonitzschia spp., Chaetoceros spp. and Fragilariopsis kerguelensis) and large diatoms (Corethron pennatum, large discoid and cylindrical diatoms) whereas outside large diatoms accounted for most of the biomass build-up (Fig. 3). In situaccumulation rates (Fig. 4) - the balance between growth and mortality rates - are higher inside the patch, except for cylindrical diatoms. The variations in accumulation rates of different species cannot be attributed to bottom-up factors alone but are also strongly influenced by the size of the seed population and the protection against grazing. Broken diatom frustules and copepod faecal pellets (Fig. 5 and 6) are indicators for grazing pressure and their stronger increase inside the patch indicate that a large amount of the accumulated phytoplankton biomass was used by higher trophic levels. Small grazers like nauplii and copepodites showed very high abundance (Fig. 7 and 8) with Oithonaspp. being the dominant species. Only the copepodites showed a response to the rapidly increasing food supply inside the patch and therefore could have exerted a significant grazing pressure.
Fig. 3: Diatom standing stock at the end of the experiment integrated over 150m depth.
Fig. 4:In situaccumulation rates over the course of the experiment.
Fig. 5: Mean number of broken diatom frustules at the end of the experiment.
0 50 100 150 200 250
-4 -2 0 2 4 6 8 10 12 14 16 18 20 22
Days since first Fe-release in patch
out patch 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5
Total Diatom
s Pse udo
nitzsch ia lin
eola Ps
eud onitzsc
hia turg idula Ps
eud onitzsc
hia s pp.
Fr agilario
psis ker
gue lens
is Chae
toc eros
spp . Th
alas sio
nem a ni
tzsc hoi
des Cy
lind rothe
ca clo
ster ium Co
reth ron
penna tum disco
id d iato
ms pe
nnate dia
tom s cy
lin dric
al dia tom
s
gC/m2 In-Patch
Out-Patch
-0,050 0,000 0,050 0,100 0,150 0,200
Total Diat
oms Ps
eud onitz
sch ia lin
eola Pseu
don itzsc
hia t urgi
dula Pseud
onit zsch
ia sp p.
Frag ilari
opsi s ker
gue lensi
s Chae
toce ros s
pp.
Thal assio
nem a ni
tzsc hoid
es Cylin
drot heca
clo steri
um Co
reth ron pe
nnat um disco
id d iat
om s pe
nna te di
atom s cyli
ndric al d
iatom s
accumulation rate, 1/t*ln(Pt/P0)
In-Patch Out-Patch
0 2.000 4.000 6.000 8.000 10.000 12.000
Tota l Dia
tom s Pseud
onitzs chia lin
eola Pseu
donitz schia
turg idula Ps
eudo nitzsc
hia sp p.
Fra gilario
psis ke rguele
nsis Chaetoc
eros s pp.
Th alas
sio nem
a n itzsch
oid es Core
thron pennat
um disco
id d iatoms
pennat e di
ato ms cylin dric
al diat oms
broken frustules/l
In-Patch Out-Patch
0 3.000 6.000 9.000 12.000 15.000
Tot al cop
epodi tes Oi thon
a spp.
Oncaea sp
p.
Ct enoca
lan us spp.
Me tridia sp
p.
Micr os
etella sp p.
Sm all cal
anoi d copepo
dites
Ind./m_
In-Patch Out-Patch
0 5.000 10.000 15.000 20.000 25.000 30.000
Tot al naupl
ii Oi
thona s pp.
On caea
spp.
Ctenoca
lanus spp.
Me tridia sp
p.
Mi crose
tella spp.
Ca lano
id naupl ii
Ind./m_
In-Patch Out-Patch
Fig. 6: Temporal development of copepod faecal pellet carbon integrated over 150m depth.
Fig. 7: Mean nauplii abundance averaged over all in and out patch CTD-stations.
Fig. 8: Mean copepodite abundance averaged over all in and out patch CTD-stations.
gC/m2 Accumulation rate, 1/t * ln(Pt/P0) Broken frustules/l
Cells/l
mgC/m2 Ind./m3 Ind./m3
