Approaches, material and methods

The modes and efficiency of carbon export at the early stage of the Kerguelen bloom were explored by collecting natural sinking particles at sites presenting various biomass levels and contrasting planktonic community structures. Free-drifting standard sediment traps (PPS3/3) and polyacrylamide gel–filled sediment traps were deployed at several depths below the mixed layer (between 100 and 400 m depth) in HNLC waters and at sites under variable iron-fertilisation influences. PPS3/3 traps (Technicap, La Turbie, France) consist of a cylinder with an internal conical funnel, which collects the sinking flux in individual cups over a pre–determined duration and according to a pre–programmed sequence. Once in a cup, all particles collected can break–

up, collide and aggregate, thus this type of trap is used to measure the bulk chemical composition of the flux but cannot inform on the categories of particle constitutive of the flux. To isolate the sinking particles and preserve their initial structure by a slow deceleration, cups filled with a highly viscous medium (polyacrylamide gel) were placed at the base of cylindrical sediment traps (Ebersbach and Trull, 2008; Lundsgaard, 1995). These were deployed at the same sites and depths as the PPS3/3 traps to relate biogeochemical composition of the sinking flux to its characteristics in term of particle type, number and size. This work is presented in Chapter 2.

Controls on the sinking velocity of large phytodetrital aggregates (‘marine snow’) forming the main numerical fraction of the flux collected in traps, were investigated by an experimental approach. At similar sites to the trap deployments, phytoplankton assemblages were sampled from Niskin bottles mounted on a CTD rosette deployed in the euphotic zone, and incubated in cylindrical tanks placed on a rotating table. By always keeping the particles in suspension, this ‘roller tank’ technique developed by Shanks and Edmondson (1989) simulates the continuous fall of particles in the water column and allow their aggregation by differential settling as it would occur in the water column (i.e. the large fast-sinking particles scavenge the smaller suspended or slow-sinking particles). An extensive set of morphological, chemical and biological properties were measured on the aggregates formed and explored as potential controls on the sinking velocity. Particular focus was made on relating particle sinking velocity determinant of carbon export efficiency and variation of phytoplankton mixtures composing the aggregates. This experimental study is shown in Chapter 3.

The relationship identified between the proportion of different phytoplankton morphologi- cal types composing the aggregates and their sinking velocity, was introduced in a generic 3–D coupled physical-biogeochemical model. The model was used to explore further the influence of plankton community variations on carbon export efficiency. The physical module was the Sparse Hydrodynamic Ocean Code or ‘SHOC’ part of the Ecosystem Modelling Suite (EMS) developed at the Commonwealth Scientific and Industrial Research Organisation, division of Marine and Atmospheric Research (CSIRO CMAR, Hobart, Australia). Because of the complexity of the physical context around the Kerguelen Islands, mainly due to the proximity of the polar and subantarctic fronts, the physics was resolved at the Southern Ocean Time Series (SOTS) lo- cated South of Tasmania (between the sub–tropical and sub–antarctic fronts near 140°E, 47°S) where an extensive dataset is provided by the Integrated Marine Observing System (IMOS). The

biogeochemical module of the model is a classical Nutrient Phytoplankton Zooplankton Detri- tus (NNPPZZDD) model where physiological rates are scaled on temperature. Phytoplankton growth depends on dissolved nutrients and light availability, and small and large zooplankton graze respectively on small and large phytoplankton. The relative contributions of small and large phytoplankton and zooplankton to the flux directed toward the detritus are used to com- pute a variable detritus sinking velocity. The carbon flux is then calculated below the euphotic zone. Seasonal and inter–annual variability in planktonic community structure induced by the physical forcing from SHOC, were related to the changes of carbon flux and thus assessed in term of carbon export efficiency. This is presented in Chapter 4.

Conclusions from these field, experimental and model observations are then summarised in Chapter 5, to provide an overall perspective on future research needs.

Acu˜na, J. L., L´opez-Alvarez, M., Nogueira, E., and Gonz´alez-Taboada, F.: Diatom flotation at the onset of the spring phytoplankton bloom: anin situ experiment, Marine Ecology Progress Series, 400, 115–125, doi:10.3354/meps08405, 2010.

Alldredge, A. L.: The chemical composition of macroscopic aggregates in two neretic seas, Limnology and Oceanography, 24, 855–866, doi:10.2307/2835325, 1979.

Alldredge, A. L.: Particle Aggregation Dynamics, Encyclopedia of Ocean Sciences, 4, 2090–2097, 2001.

Alldredge, A. L. and Gotschalk, C.: In Situ Settling Behavior of Marine Snow, Limnology and Oceanography, 33, 339–351, doi:10.2307/2837008, 1988.

Alldredge, A. L. and Gotschalk, C. C.: Direct observations of the mass flocculation of diatom blooms: characteristics, settling velocities and formation of diatom aggregates, Deep Sea Research Part A. Oceanographic Research Papers, 36, 159–171, doi:10.1016/0198-0149(89) 90131-3, 1989.

Alldredge, A. L. and Silver, M. W.: Characteristics, dynamics and significance of marine snow, Progress in Oceanography, 20, 41–82, doi:10.1016/0079-6611(88)90053-5, 1988.

Amante, C. and Eakins, B.: ETOPO1 1 Arc-Minute Global Relief Model: Procedures, Data Sources and Analysis. NOAA Technical Memorandum NESDIS NGDC-24, National Geophys- ical Data Center, NOAA, 2009.

Armand, L. K., Cornet-Barthaux, V., Mosseri, J., and Queguiner, B.: Late summer diatom biomass and community structure on and around the naturally iron-fertilised Kerguelen Plateau in the Southern Ocean, Deep Sea Research Part II: Topical Studies in Oceanography, 55, 653–676, doi:10.1016/j.dsr2.2007.12.031, 2008.

Armstrong, R. A., Lee, C., Hedges, J. I., Honjo, S., and Wakeham, S. G.: A new, mechanistic model for organic carbon fluxes in the ocean based on the quantitative association of POC with ballast minerals, Deep-Sea Research Part II: Topical Studies in Oceanography, 49, 219–236, doi:10.1016/S0967-0645(01)00101-1, 2001.

Asper, V.: Accelerated settling of particular matter by marine snow aggregates, Ph.D. the- sis, Woods Hole Oceanographic Institution and Massachusetts Institute of Technology, Mas- sachusetts, 1986.

Bishop, J. K. B., Edmond, J. M., Ketten, D. R., Bacon, M. P., and Silker, W. B.: The chemistry, biology, and vertical flux of particulate matter from the upper 400 m of the equatorial Atlantic Ocean, Deep Sea Research, 24, 511–548, doi:10.1016/0146-6291(77)90526-4, 1977.

Blain, S., Qu´eguiner, B., Armand, L., Belviso, S., Bombled, B., Bopp, L., Bowie, A., Brunet, C., Brussaard, C., Carlotti, F., Christaki, U., Corbi`ere, A., Durand, I., Ebersbach, F., Fuda,

J. L., Garcia, N., Gerringa, L., Griffiths, B., Guigue, C., Guillerm, C., Jacquet, S., Jeandel, C., Laan, P., Lef`evre, D., Lo Monaco, C., Malits, A., Mosseri, J., Obernosterer, I., Park, Y. H., Picheral, M., Pondaven, P., Remenyi, T., Sandroni, V., Sarthou, G., Savoye, N., Scouarnec, L., Souhaut, M., Thuiller, D., Timmermans, K., Trull, T., Uitz, J., van Beek, P., Veldhuis, M., Vincent, D., Viollier, E., Vong, L., and Wagener, T.: Effect of natural iron fertilization on carbon sequestration in the Southern Ocean, Nature, 446, 1070–1074, doi:10.1038/nature05700, 2007.

Blain, S., Queguiner, B., and Trull, T.: The natural iron fertilization experiment KEOPS (KEr- guelen Ocean and Plateau compared Study): An overview, Deep-Sea Research Part Ii-Topical Studies in Oceanography, 55, 559–565, doi:10.1016/j.dsr2.2008.01.002, 2008a.

Blain, S., Sarthou, G., and Laan, P.: Distribution of dissolved iron during the natural iron- fertilization experiment KEOPS (Kerguelen Plateau, Southern Ocean), Deep-Sea Research Part Ii-Topical Studies in Oceanography, 55, 594–605, doi:10.1016/j.dsr2.2007.12.028, 2008b. Bopp, L., Resplandy, L., Orr, J. C., Doney, S. C., Dunne, J. P., Gehlen, M., Halloran, P., Heinze, C., Ilyina, T., S´ef´erian, R., Tjiputra, J., and Vichi, M.: Multiple stressors of ocean ecosystems in the 21st century: projections with CMIP5 models, Biogeosciences, 10, 6225– 6245, doi:10.5194/bg-10-6225-2013, 2013.

Boyd, P. and Newton, P.: Evidence of the potential influence of planktonic community structure on the interannual variability of particulate organic carbon flux, Deep Sea Research Part I: Oceanographic Research Papers, 42, 619–639, doi:10.1016/0967-0637(95)00017-Z, 1995. Boyd, P. and Trull, T.: Understanding the export of biogenic particles in oceanic waters: Is

there consensus?, Progress In Oceanography, 72, 276–312, doi:10.1016/j.pocean.2006.10.007, 2007.

Boyd, P. W., Sherry, N. D., Berges, J. A., Bishop, J. K. B., Calvert, S. E., Charette, M. A., Giovannoni, S. J., Goldblatt, R., Harrison, P. J., Moran, S. B., Roy, S., Soon, M., Strom, S., Thibault, D., Vergin, K. L., Whitney, F. A., and Wong, C. S.: Transformations of biogenic particulates from the pelagic to the deep ocean realm, Deep Sea Research Part II: Topical Studies in Oceanography, 46, 2761–2792, doi:10.1016/S0967-0645(99)00083-1, 1999.

Boyd, P. W., Watson, A. J., Law, C. S., Abraham, E. R., Trull, T., Murdoch, R., Bakker, D. C. E., Bowie, A. R., Buesseler, K. O., and Chang, H.: A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization, Nature, 407, 695–702, doi:10.1038/ 35037500, 2000.

Buesseler, K. O.: The decoupling of production and particulate export in the surface ocean, Global Biogeochemical Cycles, 12, 297–310, doi:10.1029/97GB03366, 1998.

Buesseler, K. O. and Boyd, P. W.: Shedding light on processes that control particle export and flux attenuation in the twilight zone of the open ocean, Limnology and Oceanography, 54, 1210–1232, doi:10.4319/lo.2009.54.4.1210, 2009.

Buesseler, K. O., Lamborg, C. H., Boyd, P. W., Lam, P. J., Trull, T. W., Bidigare, R. R., Bishop, J. K. B., Casciotti, K. L., Dehairs, F., Elskens, M., Honda, M., Karl, D. M., Siegel, D. A., Silver, M. W., Steinberg, D. K., Valdes, J., Van Mooy, B., and Wilson, S.: Revisiting Carbon Flux Through the Ocean’s Twilight Zone, Science, 316, 567–570, doi:10.1126/science.1137959, 2007.

Burd, A. B. and Jackson, G. A.: Particle Aggregation, Annual Review of Marine Science, 1, 65–90, doi:10.1146/annurev.marine.010908.163904, 2009.

Cavagna, A.-J., Dehairs, F., Bouillon, S., Woule-Ebongu´e, V., Planchon, F., Delille, B., and Bouloubassi, I.: Water column distribution and carbon isotopic signal of cholesterol, brassi- casterol and particulate organic carbon in the Atlantic sector of the Southern Ocean, Biogeo- sciences, 10, 2787–2801, doi:10.5194/bg-10-2787-2013, 2013.

Coale, K. H., Johnson, K. S., Chavez, F. P., Buesseler, K. O., Barber, R. T., Brzezinski, M. A., Cochlan, W. P., Millero, F. J., Falkowski, P. G., Bauer, J. E., Wanninkhof, R. H., Kudela, R. M., Altabet, M. A., Hales, B. E., Takahashi, T., Landry, M. R., Bidigare, R. R., Wang, X., Chase, Z., Strutton, P. G., Friederich, G. E., Gorbunov, M. Y., Lance, V. P., Hilting, A. K., Hiscock, M. R., Demarest, M., Hiscock, W. T., Sullivan, K. F., Tanner, S. J., Gordon, R. M., Hunter, C. N., Elrod, V. A., Fitzwater, S. E., Jones, J. L., Tozzi, S., Koblizek, M., Roberts, A. E., Herndon, J., Brewster, J., Ladizinsky, N., Smith, G., Cooper, D., Timothy, D., Brown, S. L., Selph, K. E., Sheridan, C. C., Twining, B. S., and Johnson, Z. I.: Southern Ocean Iron Enrichment Experiment: Carbon Cycling in High- and Low-Si Waters, Science, 304, 408–414, doi:10.1126/science.1089778, 2004.

Corner, E. D. S., Head, R. N., and Kilvington, C. C.: On the nutrition and metabolism of zoo- plankton. VIII. the grazing ofBiddulphiacells byCalanus helgolandicus, Journal of the Marine Biological Association of the United Kingdom, 52, 847–861, doi:10.1017/S0025315400040595, 1972.

De La Rocha, C. L. and Passow, U.: Factors influencing the sinking of POC and the efficiency of the biological carbon pump, Deep Sea Research Part II: Topical Studies in Oceanography, 54, 639–658, doi:10.1016/j.dsr2.2007.01.004, 2007.

Diercks, A.-R. and Asper, V. L.: In situ settling speeds of marine snow aggregates below the mixed layer: Black Sea and Gulf of Mexico, Deep Sea Research Part I: Oceanographic Research Papers, 44, 385–398, doi:10.1016/S0967-0637(96)00104-5, 1997.

Doty, M. S. and Oguri, M.: The Island Mass Effect, Journal du Conseil, 22, 33–37, doi:10.1093/ icesjms/22.1.33, 1956.

Ducklow, H., Steinberg, D., and Buesseler, K.: Upper ocean carbon export and the biological pump, Oceanography, 14, 50–58, doi:10.5670/oceanog.2001.06, 2001.

Ebersbach, F. and Trull, T. W.: Sinking particle properties from polyacrylamide gels during the KErguelen Ocean and Plateau compared Study (KEOPS): Zooplankton control of carbon export in an area of persistent natural iron inputs in the Southern Ocean, Limnology and Oceanography, 53, 212–224, doi:10.2307/40006162, 2008.

Ebersbach, F., Trull, T. W., Davies, D. M., and Bray, S. G.: Controls on mesopelagic particle fluxes in the Sub-Antarctic and Polar Frontal Zones in the Southern Ocean south of Australia in summer—Perspectives from free-drifting sediment traps, Deep Sea Research Part II: Topical Studies in Oceanography, 58, 2260 – 2276, doi:10.1016/j.dsr2.2011.05.025, 2011.

Engel, A. and Schartau, M.: Influence of transparent exopolymer particles (TEP) on sinking velocity of Nitzschia closterium aggregates, Marine Ecology Progress Series, 182, 69–76, doi: 10.3354/meps182069, 1999.

Fowler, S. W. and Knauer, G. A.: Role of large particles in the transport of elements and organic compounds through the oceanic water column, Progress in Oceanography, 16, 147–194, doi: 10.1016/0079-6611(86)90032-7, 1986.

Fowler, S. W. and Small, L. F.: Sinking rates of euphausiid fecal pellets, Limnology and Oceanog- raphy, 17, 293–296, doi:10.1016/0079-6611(86)90032-7, 1972.

Fran¸cois, R., S., H., P., K., and S., M.: Factors controlling the flux of organic carbon to the bathypelagic zone of the ocean, Global Biogeochemical Cycles, 16, doi:10.1029/2001gb001722, 2002.

Giering, S. L. C., Sanders, R., Lampitt, R. S., Anderson, T. R., Tamburini, C., Boutrif, M., Zubkov, M. V., Marsay, C. M., Henson, S. A., Saw, K., Cook, K., and Mayor, D. J.: Rec- onciliation of the carbon budget in the ocean’s twilight zone, Nature, 507, 480–483, doi: 10.1038/nature13123, 2014.

Gross, F. and Zeuthen, E.: The Buoyancy of Plankton Diatoms: A Problem of Cell Physiology, Proceedings of the Royal Society of London. Series B, Biological Sciences, 135, 382–389, doi: 10.2307/82472, 1948.

Gruber, N. and Sarmiento, J. L.: Large-scale biogeochemical-physical interactions in elemental cycles, in: The sea, edited by Robinson, A. R., McCarthy, J. J., and Rothschild, B. J., vol. 12, chap. 9, pp. 337–399, John Wiley & Sons, 2002.

Hart, T.: Phytoplankton periodicity in Antarctic surface water, Discovery Reports, 21, 261–356, 1942.

Henson, S. A., Sanders, R., Madsen, E., Morris, P. J., Le Moigne, F., and Quartly, G. D.: A reduced estimate of the strength of the ocean’s biological carbon pump, Geophysical Research Letters, 38, doi:10.1029/2011gl046735, 2011.

Henson, S. A., Sanders, R., and Madsen, E.: Global patterns in efficiency of particulate organic carbon export and transfer to the deep ocean, Global Biogeochemical Cycles, 26, GB1028, doi:10.1029/2011GB004099, 2012.

Henson, S. A., Yool, A., and Sanders, R.: Variability in efficiency of particulate organic carbon export: A model study, Global Biogeochemical Cycles, 29, 33–45, doi:10.1002/2014GB004965, 2015.

Honjo, S.: Fluxes of particles to the interior of the open oceans, in: Particle flux in the Ocean, edited by Ittekkot, V., Schafer, P., Honjo, S., and Depetris, P., pp. 91–154, Wiley, 1996. Honjo, S., Manganini, S. J., and Cole, J. J.: Sedimentation of biogenic matter in the deep

ocean, Deep Sea Research Part A. Oceanographic Research Papers, 29, 609–625, doi:10.1016/ 0198-0149(82)90079-6, 1982.

Honjo, S., Manganini, S. J., Krishfield, R. A., and Francois, R.: Particulate organic carbon fluxes to the ocean interior and factors controlling the biological pump: A synthesis of global sediment trap programs since 1983, Progress in Oceanography, 76, 217–285, doi:10.1016/j. pocean.2007.11.003, 2008.

Iversen, M. H. and Ploug, H.: Ballast minerals and the sinking carbon flux in the ocean: carbon- specific respiration rates and sinking velocity of marine snow aggregates, Biogeosciences, 7, 2613–2624, doi:10.5194/bg-7-2613-2010, 2010.

Iversen, M. H. and Poulsen, L. K.: Coprorhexy, coprophagy, and coprochaly in the cope- pods Calanus helgolandicus, Pseudocalanus elongatus, and Oithona similis, Marine Ecology Progress Series, 350, 79–89, doi:10.3354/meps07095, 2007.

Jackson, G. A.: A model of the formation of marine algal flocs by physical coagulation processes, Deep-Sea Research Part I: Oceanographic Research Papers, 37, 1197–1211, doi: 10.1016/0198-0149(90)90038-W, 1990.

Jahnke, R. A.: The global ocean flux of particulate organic carbon: Areal distribution and magnitude, Global Biogeochemical Cycles, 10, 71–88, 1996.

Kajihara, M.: Settling velocity and porosity of large suspended particle, Journal of the Oceano- graphical Society of Japan, 27, 158–162, doi:10.1007/BF02109135, 1971.

Khatiwala, S., Primeau, F., and Hall, T.: Reconstruction of the history of anthropogenic CO2 concentrations in the ocean, Nature, 462, 346–349, doi:10.1038/nature08526, 2009.

Klaas, C. and Archer, D. E.: Association of sinking organic matter with various types of mineral ballast in the deep sea: Implications for the rain ratio, Global Biogeochemical Cycles, 16, 1116, doi:10.1029/2001GB001765, 2002.

Lam, P. J. and Bishop, J. K. B.: High biomass, low export regimes in the Southern Ocean, Deep Sea Research Part II: Topical Studies in Oceanography, 54, 601–638, doi:10.1016/j.dsr2.2007. 01.013, 2007.

Le Qu´er´e, C., Moriarty, R., Andrew, R. M., Peters, G. P., Ciais, P., Friedlingstein, P., Jones, S. D., Sitch, S., Tans, P., Arneth, A., Boden, T. A., Bopp, L., Bozec, Y., Canadell, J. G., Chevallier, F., Cosca, C. E., Harris, I., Hoppema, M., Houghton, R. A., House, J. I., Jain, A., Johannessen, T., Kato, E., Keeling, R. F., Kitidis, V., Klein Goldewijk, K., Koven, C., Landa, C. S., Landsch¨utzer, P., Lenton, A., Lima, I. D., Marland, G., Mathis, J. T., Metzl, N., Nojiri, Y., Olsen, A., Ono, T., Peters, W., Pfeil, B., Poulter, B., Raupach, M. R., Regnier, P., R¨odenbeck, C., Saito, S., Salisbury, J. E., Schuster, U., Schwinger, J., S´ef´erian, R., Segschneider, J., Steinhoff, T., Stocker, B. D., Sutton, A. J., Takahashi, T., Tilbrook, B., van der Werf, G. R., Viovy, N., Wang, Y.-P., Wanninkhof, R., Wiltshire, A., and Zeng, N.: Global carbon budget 2014, Earth System Science Data Discussions, 7, 521–610, doi: 10.5194/essdd-7-521-2014, 2014.

Legendre, L., Rivkin, R. B., Weinbauer, M. G., Guidi, L., and Uitz, J.: The microbial carbon pump concept: Potential biogeochemical significance in the globally changing ocean, Progress in Oceanography, pp. –, doi:10.1016/j.pocean.2015.01.008, 2015.

Lundsgaard, C.: Use of a high viscosity medium in studies of aggregates, in: Sediment trap studies in the Nordic countries. 3. Proceeding of the Symposium on Seasonal Dynamics of Planktonic Ecosystems and Sedimentation in Coastal Nordic Waters. Finnish Environment Agency, edited by Floderus, S., Heisakanen, A. S., Oleson, M., and Wassmann, P., pp. 141– 152, Numi Print, Oy, Germany, 1995.

Martin, J. H.: Glacial-interglacial CO2 change: The iron hypothesis, Paleoceanography, 5, 1–13,

doi:10.1029/PA005i001p00001, 1990.

Martin, J. H., Knauer, G. A., Karl, D. M., and Broenkow, W. W.: VERTEX: carbon cycling in the northeast Pacific, Deep Sea Research Part A. Oceanographic Research Papers, 34, 267–285, doi:10.1016/0198-0149(87)90086-0, 1987.

McCave, I. N.: Size spectra and aggregation of suspended particles in the deep ocean, Deep Sea Research Part A. Oceanographic Research Papers, 31, 329–352, doi:10.1016/0198-0149(84) 90088-8, 1984.

Michaels, A. F. and Silver, M. W.: Primary production, sinking fluxes and the microbial food web, Deep Sea Research Part A. Oceanographic Research Papers, 35, 473–490, doi:10.1016/ 0198-0149(88)90126-4, 1988.

Møller, E. F.: Sloppy feeding in marine copepods: prey-size-dependent production of dissolved organic carbon, Journal of Plankton Research, 27, 27–35, doi:10.1093/plankt/fbh147, 2005. Neuer, S., Davenport, R., Freudenthal, T., Wefer, G., Llin´as, O., Rueda, M.-J., Steinberg, D. K.,

and Karl, D. M.: Differences in the biological carbon pump at three subtropical ocean sites, Geophysical Research Letters, 29, 32–1–32–4, doi:10.1029/2002GL015393, 2002.

Park, Y.-H., Durand, I., Kestenare, E., Rougier, G., Zhou, M., d’Ovidio, F., Cott´e, C., and Lee, J.-H.: Polar Front around the Kerguelen Islands: An up-to-date determination and associated circulation of surface/subsurface waters, Journal of Geophysical Research: Oceans, 119, 6575– 6592, doi:10.1002/2014JC010061, 2014.

Passow, U. and Carlson, C.: The biological pump in a high CO2 world, Marine Ecology Progress