Response of small grazers to an iron-induced diatom bloom in the Polar Frontal Zone of the Southern Ocean

1Alfred-Wegener-Institute for Polar and Marine Research, Bremerhaven, Germany

2Max-Planck-Institute for Biogeochemistry, Jena, Germany

Response of small grazers to an iron-induced diatom bloom in the Polar Frontal Zone of the Southern Ocean

J. Henjes

, P. Assmy

, C. Klaas

& V. Smetacek

Results of the second in situ iron fertilization experiment (EisenEx) conducted in the HNLC (High Nutrients Low Chlorophyll) waters of the Southern Ocean in austral spring (November) 2000 are presented here. A 10 km diameter spiral was fertilized in the centre of an eddy with 4 tonnes of FeSO₄on three occasions at weekly intervals. The response of the pelagic community and the processes within the food web were studied in detail and compared with processes in the surrounding water for over three weeks.

Fig. 4: Temporal development of copepod faecal pellet carbon integrated over 150m depth

Fig. 6: Accumulation rate of broken diatom frustules over the course of the experiment.

-0,06 -0,02 0,02 0,06 0,10 0,14 0,18 0,22 0,26

Tot al Diatoms

Pseudoni

tzschia lineo

la Ps

eudoni tzsch

ia turg idul

a Pseudoni

tzschia spp.

Fragilariops

is k ergue

lensis Tha

lassione

ma ni tzschoi

des othe

r penna te dia

toms Corethr

on pe

nnat um othe

r cylindr

ical di atom

s In-Patch Out-Patch

Accumulation rate d-1

0 50 100 150 200 250 300

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

days since first Fe-release

mgC/m²

IN-PATCH OUT-PATCH

0 200 400 600 800 1000 1200

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

days since first Fe-release

mgC/m²

IN-PATCH OUT-PATCH

0 5.000 10.000 15.000 20.000 25.000 30.000

Total n aup

lii Oithona

spp.

Onca ea sp

p.

Cte nocalan

us spp . Met ridia

spp.

Mic rose

tella sp p.

Cal anoid nau

plii

Ind./m³

In-Patch Out-Patch

Fig. 5: Mean abundances of (A) nauplii, (B) copepodites and small copepods (<1.5mm) averaged over all in and out patch CTD-stations.

0 3.000 6.000 9.000 12.000 15.000

Tot al c

opepod ites

Oithon a spp.

Onca ea s

pp.

Ctenoc alanu

s spp . Me

tridia spp .

Micro sete

lla sp p.

Sm all calan

oid

Ind./m³

In-Patch Out-Patch

A

B

Fig. 1:The complex phytoplankton-based food web (Fig. modified after graph from Z. Johnson)

A

B

Fig. 2: Temporal development of acantharian abundance inside and outside the fertilized patch.

Fig. 3: Temporal development of (A) nauplii, (B) copepodite and small copepod (<1.5mm) biomass integrated over 150m depth and (C) broken diatom frustules abundance over the course of the experiment.

In-Patch

Out-Patch

C

Out-Patch In-Patch

Although growth rates of all phytoplankton groups increased within days, only diatoms accumulated biomass which, because of heavy grazing, increased significantly (4-fold) only after 2 weeks. Acantharians, but not other protozooplankton, showed a marked increase in abundance by a factor of ~2.7 in the fertilized patch, but only slightly outside the patch (Fig. 2). This is of major interest, since acantharians are suggested to be responsible for the formation of barite found in sediments and which is a paleo- indicator of high productivity regimes.

With regard to small mesozooplankton, copepod grazing pressure increased inside the patch as indicated by the numbers of broken diatom frustules and recognizable faecal pellets (Fig. 3C, 4 and 6). Nauplii and copepodites were extremely abundant (Fig. 5A and 5B) with Oithona spp. the dominant genus. Whereas nauplii numbers did not change significantly in patch, their increase in biomass is due to individual growth (Fig. 3B). Copepodite and small copepod (<1.5 mm) numbers and biomass increased by a factor of 3.1 and 3.5 inside the patch and 2.0 outside, respectively, suggesting that a significant portion of the fertilized phytoplankton biomass was channelled to higher trophic levels (Fig. 3B and 5B).

These results indicate that small copepods and their nauplii rapidly responded to increasing food supply due to iron addition in this Southern Ocean HNLC region. In unfertilized waters, the slight increase of grazing pressure by small grazers matched the seasonal increase in diatom biomass.

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0

-4 -2 0 2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0 2 2

d a ys sin ce first F e -re le a se

mgC/m²

In -Pa tc h Out- Pa tc h