Anne-Julie Cavagna Frank Dehairs Stéphanie H.M. Jacquet Frédéric Planchon

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

2. 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) What do we learn from comparative study of Fe-replete / Fe-deplete areas & time series located in FeNX sites ? SAZ-Sense Jan.-Feb. 2007 SUMMER CROZEX leg 1 (Nov. 2004-mid-Dec. 2004) leg 2 (mid-Dec. 2004-Jan. 2005) KEOPS leg 1 (Jan.-Feb. 2005) SUMMER leg 2 (Oct.-Nov. 2011) SPRING

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

4. CROZEX (Spring – early Summer 2004/05) Morris and Sanders, 2011 (GBC) • Seasonal integration • Hide shorter timescale events • 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., 2009 - Nature)

5. CROZEX (Spring – early Summer 2004/05) N-S gradient Leg 1 Leg 2 No N-S gradient seen once the modest bloom occurred in the south ≈ 180 mgC m-2 d-1 234Th derived export rate: Post-bloom EP insensitive to size of bloom ≈ 60 mgC m-2 d-1 Mid-Dec.. => Jan. Nov. => mid-Dec. Morris et al., (2007) DSR2 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)

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

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

8. SAZ-Sense (Summer period 2007) 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 P1 Export 100 m vs. production P2 P3 Export 600 m vs. export 100 m P3 P1 P2 P3 => High Biomass Low Sequestration system ? Stable system less efficient than versatile system for carbon export + sequestration

9. KEOPS (KEOPS 1 Summer period - 2005) 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”

10. KEOPS (KEOPS 1 Summer period - 2005) Highly active bacterial community On-shelf 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 15.2% 28.3%

11. KEOPS (KEOPS 2 Spring period - 2011) 11 novembre 2011 E5 E1 F Courtesy from Y-H. Park (MODIS Chl-a biomass + data from surface buoy and altimetry (Nov. 2011) E4E E3 E4W R A3 From expedition & first workshop data analysis: 3 clusters + reference station: • 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

12. Toolbox data KEOPS 1 & KEOPS 2 C-export production 234Th proxy (mgC m-2 d-1) Meso-remin. Particulate Baxs proxy (mgC m-2 d-1) C-sequestration Efficiency (mgC m-2 d-1) Net PP (mgC m-2 d-1) 132 ± 22 300 3380 ± 145 3287 ± 83 2172 ± 230 1460 578 ± 54 748 ± 103 1037 ± 130 1064 ± 126 23 ± 17 120 53 ± 07 87 ± 12 47 ± 22 250 156 ± 18 159 ± 16 On going 99 ± 11 R C11 (summer) NPF E4W A3-2 (spring) A3 (summer) E1 (day 0) E3 (day 5) E4E (day 14) E5 (day 20) 86.3 36 41.2 65.5 17.6 28 42.0 32.3 57.4 62.0 To be investigated 85 16.9 32.9 21.7 222 115.6 130 On going 74.5 • 234Th derived integrated export below 100m exceeds 200m trap C-export (T. Trull pers. communic.) by 20 to 60%

13. Toolbox data KEOPS 2 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)

14. KEOPS Integrated Information A3 C11 (K1 HNLC) C11 Early spring E1 (day 0) ThE = EP/NetPP Spring E3 (day 5) A3 (K1) E5 (day 20) Summer Meander E E4W A3-2 NPF EP700/EP = 1 – MR/EP • 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

15. 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?

16. Putting the pieces together

17. 3 / Primary production & potential for carbon export 17 Natural Fe-availability and enhanced surface Chl a does not always reflect enhanced integrated production and deep carbon export Fe, nutrients, light, stratification POC (µM) different systems can have the same deep export efficiency 0 m Gross primary production remineralization 100 m Export remineralization 600 m Export remineralization POC attenuation curve

18. What do we learn from previous FeNXs inter-comparison ? 9 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 different systems can have the same export export efficiency and inversely Fe, nutrients, light, stratification POC (µM) End of the productive season, naturally Fe-fertilized sites seems to function as HBLE systems => Needs further investigations 0 m Gross primary production remineralization 100 m Export remineralization These 2 studies occurred at the end of the productive season 600 m Export remineralization POC attenuation curve

19. 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. 2002 – 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., 2004 - 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. 2013 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).

20. The toolbox – production / export / remineralisation 13C-assimilation (Net PP) and 15NO3 / 15NH4-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) • 15N-NO3- dilution experiment to measure nitrification in the euphotic zone Carbon export below the surface water using ISP sampling 234Th proxy Fe, nutrients, light, stratification POC (µM) • 234Th deficit / excess depth profile measurement • C-export conversion using C/234Th ratio in particle (2 size classes at each sampling depth) • (See Savoye et al. 2008 DSR2) 0 m Net primary production Remin. 100 m Export Mesopelagic carbon remineralisation using particulate Baxs proxy Remin. 700 m Export • Baxs ICP-MS measurements • Dehairset al. (1997) DSRII algorithm to convert Baxs content into final POC mineralization rate • (S.H.M. Jacquet – poster 361) Remin. POC attenuation curve

21. 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.

22. 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.

23. Planchon et al. 2013 BGH transect (summer period from South Africa to northern Weddell gyre) => same range thanExp. 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

24. Regime of production – surface water (euphotic zone) 11 f-ratio U-NO3/(U-NH4+U-NO3) Exportable prod. Net PP x f-ratio Net PP (mgC m-2 d-1) Euphotic Zone nitrification 132 ± 22 3380 ± 145 3287 ± 83 2172 ± 230 578 ± 54 748 ± 103 1037 ± 130 1064 ± 126 0.41 0.81 - 0.87 0.70 0.59 0.67 0.64 R NPF E4W A3-2 E1 (day 0) E3 (day 5) E4E (day 14) E5 (day 20) 50 2738 - 1890 405 441 695 681 yes yes no Yes no yes no no R station => control HNLC with low Net PP A3 => KEOPS 2 = 181.0 ± 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

25. Carbon export – below the surface water (100m horizon) 12 Euphotic layer depth (m) 0.3% (0%) PAR C-export production 234Th proxy (mgC m-2 d-1) Mixed layer depth (m) ThE-ratio (%) EP:NetPP Net PP (mgC m-2 d-1) 17 1.6 03 02 27 21 On going 09 116 (-) 33 (52) 42 (67) 49 (78) 80 (126) 86 (137) 42 (67) 69 (110) 107 29 57 163 64 27 70 58 = = < < > > < > R NPF E4W A3-2 E1 (day 0) E3 (day 5) E4E (day 14) E5 (day 20) 132 ± 22 3380 ± 145 3287 ± 83 2172 ± 230 578 ± 54 748 ± 103 1037 ± 130 1064 ± 126 23 ± 17 53 ± 07 87 ± 12 47 ± 22 156 ± 18 159 ± 16 On going 99 ± 11 • Evidence for carbon export in pre-bloom conditions. • 234Th 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)

26. Remineralisation – mesopelagic zone (MLD - 700m) 13 C-export production 234Th proxy (mgC m-2 d-1) Meso-remin. Particulate Baxs proxy (mgC m-2 d-1) Net PP (mgC m-2 d-1) Meso-remin:EP (0<value<1) 3.75 0.78 0.75 0.37 0.27 0.20 On going 0.63 R NPF E4W A3-2 E1 (day 0) E3 (day 5) E4E (day 14) E5 (day 20) 132 ± 22 3380 ± 145 3287 ± 83 2172 ± 230 578 ± 54 748 ± 103 1037 ± 130 1064 ± 126 86.3 41.2 65.5 17.6 42.0 32.3 57.4 62.0 23 ± 17 53 ± 07 87 ± 12 47 ± 22 156 ± 18 159 ± 16 On going 99 ± 11 • R : meso-remineralization strongly exceeds C-export below the euphotic zone / mixed layer. • Though same magnitude of temporal scale integration for 234Th and Baxs 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.