Primary production and potential for carbon export in naturally iron-fertilized waters in the Southern Ocean Anne-Julie Cavagna Frank Dehairs Stéphanie H.M. Jacquet Frédéric Planchon Antarctic Session Gaining information on C-sequestration efficiency using a production / export / remineralisation toolbox: the S.O. naturally Fe-fertilized areas study-case

Natural Fe-fertilized open ocean zones in the S.O. Constraint of blooms by circulation & topography SeaWiFS chl-a images in October and December 1998 (from Pollard et al., 2007) KEOPS leg 1 (Jan.-Feb. 2005) SUMMER leg 2 (Oct.-Nov. 2011) SPRING CROZEX leg 1 (Nov mid-Dec. 2004) leg 2 (mid-Dec Jan. 2005) SAZ-Sense Jan.-Feb SUMMER What do we learn from comparative study of Fe-replete / Fe-deplete areas & time series located in FeNX sites ?

CROZEX (Spring – early Summer 2004/05) North area LARGE LONG EARLY BLOOM High surface chl-a high productivity zone Defined as “bloom / Fe-replete” South area SMALL SHORT LATE BLOOM Low surface chl-a low productivity zone Defined as “HNLC control / Fe-deplete” N S Surface Chl a (mg m -3 )

CROZEX (Spring – early Summer 2004/05) Morris and Sanders, 2011 (GBC) - Significant increased level of integrated PP in the N. compared to the S. -- shallow seasonally integrated export, annually integrated deep water POC flux and core-top organic carbon accumulation enhanced 2 to 3 fold as a result of the iron-fertilized bloom (Pollard et al., Nature) Seasonal integration  Hide shorter timescale events

CROZEX (Spring – early Summer 2004/05) Morris et al., (2007) DSR2 234 Th derived export rate: Post-bloom EP insensitive to size of bloom Leg 1Leg 2 Why similar export in high productive & low productive zone during Leg 2 ?  North = High Biomass Low Export zone ? (HBLE – Lam & Bishop, 2007 DSR II)  Miss the high export rate at bloom peak ?  New and export production are not equivalent, with this lack of equivalence being particularly pronounced in the north (Fe-replete area) ≈ 180 mgC m -2 d -1 ≈ 60 mgC m -2 d -1 No N-S gradient seen once the modest bloom occurred in the south N-S gradient Nov. => mid-Dec. Mid-Dec.. => Jan.

The toolbox – production / export / remineralisation NetPP (mgC m -2 d -1 ) EP (mgC m -2 d -1 ) MR (mgC m -2 d -1 ) 100 m 0 m 1000 m Export Net primary production POC (µM) POC attenuation curve Remin. Fe, nutrients, light, stratification Based on the idea of Buesseler & Boyd L&O (2009) Carbon sequestration efficiency (deep carbon export relative to surface netPP)

SAZ-Sense (Summer period 2007) Surface Chl-a (mg m -3 ) AZ PFZ SAZ-S SAZ-N STZ P1 P3 STF SAF-N SAF-S EAC ZC P1 #3 P3 P1 #3 P1 #2 P2 3 repeat measurement / station in 1 week -euphotic layer- P2

SAZ-Sense (Summer period 2007) P1 P3 P2 P1 = 929 ± 808 mgC m -2 d -1 => 70 mgC m -2 d -1 P2 = 424 ± 18 mgC m -2 d -1 => 32 mgC m -2 d -1 P3 = 680 ± 96 mgC m -2 d -1 => 5.4 mgC m -2 d -1 Export 600 m vs. export 100 m Export 100 m vs. production P3 => High Biomass Low Sequestration system ? Stable system less efficient than versatile system for carbon export + sequestration P1 P2 P3

KEOPS (KEOPS 1 Summer period ) A3 site INSIDE THE BLOOM High surface chl-a high productivity zone Defined as “bloom / Fe-replete” C11 site OUTSIDE THE BLOOM Low surface chl-a low productivity zone Defined as “HNLC control / Fe-deplete”

KEOPS (KEOPS 1 Summer period ) Highly active bacterial community On-shelf 15.2% 28.3% Prevalence of regenerated production and low uptake of NO3 above the Plateau  proportionally low export. Plateau surface waters operate as a High Biomass Low Export system, but since subsurface remineralisation is relatively limited there still is an important fraction of C left for deep sequestration. However overall the off- shelf system appears as the most efficient site for C sequestration

11 novembre 2011 R E5 E3 E1 F A3 E4E E4W KEOPS (KEOPS 2 Spring period )  Reference station (HNLC and low Fe) : R  Cluster 1 (productive sites south of PF) : A3-2 and E4W  Cluster 2 (stationary permanent meander south of PF): E stations: E1, E3, E4E, E5  Cluster 3 (productive site on to north of the Polar Front): NPF From expedition & first workshop data analysis: 3 clusters + reference station: Courtesy from Y-H. Park (MODIS Chl-a biomass + data from surface buoy and altimetry (Nov. 2011)

Toolbox data KEOPS 1 & KEOPS 2 Net PP (mgC m -2 d -1 ) 132 ± ± ± ± ± ± ± ± 126 R C11 (summer) NPF E4W A3-2 (spring) A3 (summer) E1 (day 0) E3 (day 5) E4E (day 14) E5 (day 20) C-export production 234 Th proxy (mgC m -2 d -1 ) 23 ± ± ± ± ± ± 16 On going 99 ± 11 Meso-remin. Particulate Ba xs proxy (mgC m -2 d -1 ) C-sequestration Efficiency (mgC m -2 d -1 ) To be investigated On going Th derived integrated export below 100m exceeds 200m trap C-export ( T. Trull pers. communic.) by 20 to 60%

In accordance with CROZEX (Morris et al., 2007 – DSR2), we observe for KEOPS 2 an evidence for a decoupling of new and export production. With also the effect being most apparent in the high productive area (for CROZEX the effect was most apparent within the northern bloom area) Toolbox data KEOPS 2

KEOPS Integrated Information ThE = EP/NetPP EP700/EP = 1 – MR/EP A3-2 E4W E3 (day 5) E1 (day 0) E5 (day 20) KEOPS 2 (spring period) and KEOPS 1 (summer period)  High surface chl-a sites = high production – low sequestration  Meander E & A3 site at keops 1 and 2 = highlight a seasonal cycle A3 (K1) C11 (K1 HNLC) Spring Summer Early spring NPF Meander E A3 C11

KEY-POINTS Deep carbon sequestration efficiency is related to the type of production regime Low Biomass systems (E stations at K2 in early season; C11 at K1) seem to be more efficient in terms of C-sequestration than High Biomass systems (K2: E5 cluster 1 and 3; K1: A3) ** High Biomass Low Sequestration vs. Low Biomass High Sequestration ** => Not in contradiction with Fe-replete areas exporting more than Fe-deplete areas Example for K1: PP at C11 (Fe-deplete area / HNLC) is only 20% of PP at A3 (Fe-replete area) => C11 sequestration = 38% A3 sequestration Is there evidence for a temporal succession from LBHS to HBLS over the season ?  LBHS at the early stage of the productive season  Rapid transition to HBLS was ongoing for E stations, while clusters 1 and 3 were already HBLS at the start of the study =>At the end of the season HBLS conditions (A3 Keops2) returned to LBHS (A3 Keops1) QUESTION : Do systems keep the ‘LBHS’ status during winter ? What is the strength of the biological pump in winter?

Putting the pieces together

Natural Fe-availability and enhanced surface Chl a does not always reflect enhanced integrated production and deep carbon export 3 / Primary production & potential for carbon export m 0 m 600 m Export Gross primary production POC (µM) POC attenuation curve different systems can have the same deep export efficiency remineralization Fe, nutrients, light, stratification

Natural Fe-availability and enhanced surface Chl a does not always reflect enhanced integrated production and deep carbon export, especially at the end of the productive season What do we learn from previous FeNXs inter-comparison ? m 0 m 600 m Export Gross primary production POC (µM) POC attenuation curve different systems can have the same export export efficiency and inversely remineralization Fe, nutrients, light, stratification End of the productive season, naturally Fe-fertilized sites seems to function as HBLE systems => Needs further investigations These 2 studies occurred at the end of the productive season

Key observations 12 High surface productivity in the Kerguelen Islands area is perhaps not only due to natural iron fertilization but also to vicinity with Polar Front (mesoscale frontal dynamics boost primary production- Strass et al – DSR II) If nutrient consumption efficiency is increased by iron artificial addition, what will remain for the low latitude regions nutriently supplied by Antarctic Intermediate Water (Sarmiento et al., Nature) ? Tamburini et al. (2009 –DSR II) demonstrate from 200 to 1500m that pressure decrease the number of prokaryotes attached to aprticles and the apparent activity of free-living prokaryotes. This helps to explain why fast sinking particles such as fecal pellets, but possibly also including fast sinking marine snow aggregates, can fall through the water column with minimal degradation. Looking on A station, we join one of the De Brauwère et al conclusion being that increasing analytical information throughout the duration of the bloom would strongly help to upgrade and tune models Deep carbon export efficiency using the proposed toolbox is an encouraging way to gain information on the biological carbon pump. The important point is to carefully take MLD and EZD into account in order to avoid dangerous misestimation. KEOPS 2: Raw information is available to mature the 3 flux estimation needed to obtain the relative global view of studied systems R station shows a peculiar functioning: leads to the question of winter primary production Preliminary results. Have to be carefully validated together (depth layers).

The toolbox – production / export / remineralisation 13 C-assimilation (Net PP) and 15 NO 3 / 15 NH 4 -uptake rates (f-ratio – New production)  Euphotic zone depth integrated parameters (7 depths measurements between 75 and 0% light attenuation)  24 h incubation experiments (daily Net PP = Gross PP + C-loss)  15 N-NO 3 - dilution experiment to measure nitrification in the euphotic zone Carbon export below the surface water using ISP sampling 234 Th proxy Mesopelagic carbon remineralisation using particulate Ba xs proxy 100 m 0 m 700 m Export Net primary production POC (µM) POC attenuation curve Remin. Fe, nutrients, light, stratification  Ba xs ICP-MS measurements  Dehairs et al. (1997) DSRII algorithm to convert Baxs content into final POC mineralization rate (S.H.M. Jacquet – poster 361)  234 Th deficit / excess depth profile measurement  C-export conversion using C/ 234 Th ratio in particle (2 size classes at each sampling depth) (See Savoye et al DSR2)

KEOPS CROZEX Isotopic model of oceanic silicon cycling: the Kerguelen Plateau case study (de Brauwere et al., in revision for DSR I)  Having additional measurement during the season would tremendously help to constrain the bloom peak and hence the rate parameters  A puzzling result of this modeling exercise is that seasonally-integrated Si-uptake flux above the plateau is lower than off the plateau while it might be expected that above the plateau more production occur due to the fertilization effect.

Natural Fe-fertilized open ocean zones in the S.O. xx S.O. species have overcome the antagonistic iron-light relationship by increasing size rather than number of photosynthetic units under low irradiance resulting in an acclimatation strategy that does not increase their cellular iron requirement.

Planchon et al BGH transect (summer period from South Africa to northern Weddell gyre) => same range than Exp. Prod. at KEOPS 2 R station C-flux at 100m (SS model) (mgC m -2 d -1 ) 21,6 10,8 20,4 27,6 31,2 39,6 42,0 56,4 61,2 51,6 39,6

Regime of production – surface water (euphotic zone) Net PP (mgC m -2 d -1 ) Exportable prod. Net PP x f-ratio f-ratio U-NO3/(U-NH4+U-NO3) 132 ± ± ± ± ± ± ± ± Euphotic Zone nitrification R station => control HNLC with low Net PP A3 => KEOPS 2 = ± 19.2 mmolC m -2 d -1 (f-ratio = 0.9) EARLY SPRING KEOPS 1 = 80.6 ± 5.6 mmolC m -2 d -1 (f-ratio = 0.6) SUMMER E stations => Effective temporal variation through 3 to 4 weeks monitoring R NPF E4W A3-2 E1 (day 0) E3 (day 5) E4E (day 14) E5 (day 20) yes no Yes no yes no 11

Carbon export – below the surface water (100m horizon) Net PP (mgC m -2 d -1 ) R NPF E4W A3-2 E1 (day 0) E3 (day 5) E4E (day 14) E5 (day 20) 132 ± ± ± ± ± ± ± ± 126 C-export production 234 Th proxy (mgC m -2 d -1 ) 23 ± ± ± ± ± ± 16 On going 99 ± 11 Evidence for carbon export in pre-bloom conditions. 234 Th derived integrated export below 100m exceeds 200m trap C-export ( T. Trull pers. communic.) by 20 to 60% K2 C-export fluxes (early spring) are generally smaller than during K1 (summer) ThE-ratio (%) EP:NetPP On going 09 Euphotic layer depth (m) 0.3% (0%) PAR 116 (-) 33 (52) 42 (67) 49 (78) 80 (126) 86 (137) 42 (67) 69 (110) Mixed layer depth (m) ==<<>><>==<<>><> 12

Remineralisation – mesopelagic zone (MLD - 700m) Net PP (mgC m -2 d -1 ) R NPF E4W A3-2 E1 (day 0) E3 (day 5) E4E (day 14) E5 (day 20) 132 ± ± ± ± ± ± ± ± 126 C-export production 234 Th proxy (mgC m -2 d -1 ) 23 ± ± ± ± ± ± 16 On going 99 ± 11 Meso-remin. Particulate Ba xs proxy (mgC m -2 d -1 ) Meso-remin:EP (0<value<1) On going 0.63 R : meso-remineralization strongly exceeds C-export below the euphotic zone / mixed layer.  Though same magnitude of temporal scale integration for 234 Th and Ba xs proxies (several weeks), EZ C-export & meso-remineralization seems to be decoupled.  If ambient mesopelagic water are saturated in BaSo3, barytine cristals will not be dissolved: to be checked. 13