RV Marion Dufresne
A seasonal carbon budget for a naturally iron fertilized bloom (Kerguelen I., Southern Ocean)
Jouandet, M.P.1, Blain, S.1, Metzl, N.2, Brunet, C.2, Trull, T.W.3, Obernosterer, I.4
Laboratoire d'Oceanographie et de Biogeochimie, Campus de Luminy, case 901, 13288 Marseille cedex 09-France/2 LOCEAN-IPSL, UMR 7159, CNRS, Universite P. et M. Curie Case 100, 4, place Jussieu - 75252 PARIS Cedex 5 – France/3 Antarctic Climate and Ecosystems CRC, University of Tasmania, CSIRO Marine and Atmospheric Research, Hobart, Tas 7001, Australia/4Universite Pierre et Marie Curie-Paris6, UMR7621, F66650 Banyuls-sur-Mer, France ; CNRS, UMR7621, F-66650 Banyuls-sur-Mer- France
1
Introduction
The Kerguelen Ocean and Plateau compared Study (KEOPS) was carried out from 19 January to 13 February 2005 South East of the Kerguelen Island. KEOPS was designed to study the impact of a natural iron fertilization on the biology and biogeochemistry. The occurrence of a deep source dissolved iron (DFe) above the Kerguelen Plateau promoted the massive three-month bloom detected by satellite every years. Here we focus on the CO2 parameters and we present the carbon budget for the mixed layer for station inside (A3) and outside (C11) the bloom. This allowed us to determine the daily carbon export for both contrasting areas.
KEOPS
Bloom localization and sea surface distribution of fCO2
Natural iron fertilization pattern and vertical carbon fluxes
Fertilization of the surface waters of the plateau resulted from - a higher winter stock at A3 than C11 - an enhancement of the on going DFe supply
gChla L-1
A3 C11
DFe (nmol L )
-1
A3
atm
0
0
0.1 0.2 0.3 0.4 0.5 0.6
Fatm (mmol m d )
-1
-20 0 20 40 60 80 100 120 140
A3
Air
C11
0
A3 C11
-2
C11
Cexp (mmol m d )
As result of the bloom developpement fCO2 was 77 µatm lower in waters above the Kerguelen Plateau (A3) than in offshore waters (C11).
-1
50 100 150 200 250 300 350 400 450 500
Source : Blain et al., in press
28 ±24
-3 ± 2
Depth (m)
Mixed layer
A3 C11
70 ±20 m
0 20 40 60 80 100 120 140
Seasonal carbon budget
The apparent seasonal depletion of DIC (∆Csdep) is the difference between the concentration of DIC in the mixed layer (ML) during winter and the concentration of DIC in the mixed layer during summer: ∆Csdep= (DICwinter-DICsummer) zMLρML The DIC in winter was estimated from the concentration of DIC at the depth of the minimum of temperature (zTmin) (Figure below).
DICwinter
θ (° C)
-1 0 50 100 0 1 2 3 4 5
2050 0 50 100 150 200 250 300 350 400
A3-18/01/05 A3-23/01/05 A3-04/02/05 C11
-2
85 ± 33
23 ± 8
z
Bloom waters (A3) were a large CO2 sink contrasting with HNLC waters (C11) which were a quite CO2 source. The carbon export is more than 3 fold higher in the bloom than outside.
DIC(µmol kg )
2100 2150 2200 2250
-1
Comparison of KEOPS results with artificial iron enrichment experiments
140 120 100 80 KEOPS
Depth(m)
zTmin
150 200 250 300 350 400
Sites of the artificial iron enrichment experiments in the Southern Ocean
EisenEx
SOFex-Sud
We derived the daily net community production (NCPdaily) correcting ∆Csdep from vertical, horizontal inputs and CO2 air-sea exchanges :
160 140 120
-1
60 40 20 0
Time (day) ∆fCO2 ( atm) Cexp.daily (mmol m-2 d-1)
SOIREE
KEOPS
EisenEx
SOFex-North SOFex-South
SOIREE
A03 C11
mmol m d
100 80 60 40 20 0 * -20 NCPdaily = ∆Csdep/∆T + Fatm
-2
Sources : Bozec et al., 2004; Boyd et al., 2004; Bakker et al., 2005
+ Fvert + Fhor
From the NCPdaily we deduced the carbon export (Cexp.daily) flux leaving the mixed layer subtracting the POC and DOC accumulation:
16 14 12 A3 C11
The longer bloom duration, (ie 3 months) above the Kerguelen Plateau, compared to the short period of observation of artificial blooms (ie few weeks) explains the deeper CO2 sink and the largest carbon export flux. The duration of the bloom above the Kerguelen plateau is due to iron fertilization but also due to the concomitant supply of major nutrients.
mmol m d
-1
10 8 6 4 2 0
Cexp.daily = NCPdaily - ∆POC/∆T * -∆DOC/∆T *
-2
Our results show that the biological pump triggered by a natural iron fertilization is 2 fold more intense compared to artificial iron enrichment experiments.
∆POC/∆T ∆DOC/∆T
∆T is the bloom duration and was determined base on chlorophyll satellite images (∆T = 90 days)
References Bakker, D.C.E., Bozec, Y., Nightingale, P.D., Goldson, L.E., Messias, M.J., de Baar, H.J.W, Liddicoat, M.I, Skjelvan, I., Strass, V., Watson, A.J., 2005. Iron and mixing affect biological carbon uptake in SOIREE and EisenEx, two Southern Ocean iron fertilization experiments. Deep-Sea Research I 52, 1001-1019. Blain et al, in press. Impacts of natural iron fertilization on carbon sequestration in the Southern Ocean Boyd, et al.,, 2000. A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization. Boyd et al., 2000. A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization. Nature 407, 695-702. Bozec, Y., Bakker, D C.E., Hartmann, C., Thomas, H., Bellerby, R.G.J., Nightingale, P.D., Riebesell, U., Watson, A.J., De Baar, H.J.W., 2004. The CO2 system in a Redfield context during an iron enrichment experiment in the Southern Ocean. Marine Chemistry 95, 89-105.
Acknowledgements KEOPS/OISO12 was supported by the Institut National des Sciences de L’Univers (INSU) Centre National de la Recherche Scientifique (CNRS), l’Institut Polaire Paul Emile Victor (IPEV). We thank the captain and the crew of the R/V Marion Dufresne. We thank also the Service National d’Analyse des Paramètres Océanique du CO2 (SNAPO-CO2) at LOCEAN/IPSL for DIC analyses.
*