The trophodynamics of the numerically dominant euphausiid species within a region of high mesoscale oceanographic variability in the southwest Indian sector of the Antarctic Polar Frontal Zone (PFZ) were investigated during the austral autumns April/May) of 2004 and 2005. During the 2004 survey, sub-surface (200 m) temperature profiles indicated that an intense frontal feature, formed by the convergence of the Sub-Antarctic Front (SAF) and the Antarctic Polar Front (APF) bisected the survey area into two distinct zones, the Sub- Antarctic Zone (SAZ) and the Antarctic Zone (AAZ). Total integrated chlorophyll-a (chl-a) biomass was typical for the region (< 25 mg chl-a m⁻²), and was dominated by picophytoplankton. Total euphausiid abundance and biomass ranged from 0.1 to 3.1 ind m⁻³ and 0.1 to 8.1 mg dwt m⁻³, respectively, and did not differ significantly between the stations occupied in the SAZ and AAZ (p > 0.05). The multivariate analysis identified two interacting mechanisms controlling the distribution patterns, abundance and biomass of the various euphausiid species, namely (1) diel changes in abundance and biomass, (2) and restricted distribution patterns associated with the different water masses. Ingestion rates were determined for five euphausiid species. E. triacantha was found to have the highest daily ingestion rate ranging from 1 226.1 to 6 029.1 ng pigm ind⁻¹d⁻¹, while the lowest daily ingestion rates were observed in the juvenile Thysanoessa species (6.4 to 943.0 ng pigm ind⁻¹ d⁻¹). The total grazing impact of the selected euphausiids ranged from < 0.1 to 20.1 μg pigm m⁻²d⁻¹, corresponding to < 0.15 % of the areal chl-a biomass. The daily ration estimates of autotrophic carbon for the euphausiids suggested that phytoplankton represented a minor component in their diets, with only the sub-adult E. vallentini consuming sufficient phytoplankton to meet their daily carbon requirements. A cyclonic cold-core eddy spawned from the region of the APF located in the southwest Indian sector of the PFZ was the dominant feature during the 2005 survey. The total areal chl-a biomass throughout the region was low, ranging between 5.6 and 11.4 mg chl-a m⁻², and was significantly higher within the core of the eddy compared to the surrounding waters (p < 0.05). RMT-8 and WP-2 total euphausiid abundance and biomass estimates were high, and ranged from 0.004 to 0.36 ind m⁻³ and 0.065 to 1.21 mg dwt m⁻³, and from 0.01 to 18.2 ind m⁻³ and 0.01 to 15.7 mg dwt m⁻³, respectively. A distinct spatial pattern in the euphausiid community was evident with the Antarctic species, Euphausia frigida, E. triacantha and E. superba predominating within the core of the eddy, while the PFZ waters were characterized by the sub-Antarctic species, E. longirostris, Stylocheiron maximum, Nematoscelis megalops and Thysanoessa gregaria. The eddy edge acted as a transition zone where species from both regions co-occurred. Within the survey area the combined ingestion rate of the six numerically dominant euphausiid species ranged between 0.02 and 5.31 μg pigm m⁻²d¹, which corresponded to a loss of between < 0.001 and 0.11 % of the available chl-a biomass. E. triacantha and juvenile T. macura were identified as the dominant grazers. There was no apparent spatial pattern in the grazing activity of the euphausiids within the region of investigation. The average daily rations of the euphausiids examined were < 2 % of their body carbon. The low daily ration of the euphausiids could be ascribed to the predominance of small picophytoplankton in the region of investigation, which are too small to be grazed efficiently by larger zooplankton. The marked spatial patterns in species composition and the elevated abundance and biomass of euphausiids, suggest that the mesoscale eddies contribute to the spatial and temporal heterogeneity of the planktonic community of the PFZ and may represent important foraging regions for many of the apex predators within the region.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Meteorology, 1980. / Microfiche copy available in Archives and Science. / Vita. / Bibliography: leaves 315-325. / by John Merrill Toole. / Sc.D.

Three-dimensional gravity analysis is the process of removing the predictable components from the free-air gravity anomalies and has proven to be useful for interpreting the subsurface structures and active processes at mid-ocean ridges. The three-dimensional effects of the seafloor and Moho topography, assuming a constant crustal thickness and constant crust and upper mantle densities, are subtracted from the free-air anomalies, yielding the mantle Bouguer anomalies. Mantle Bouguer anomalies at mid-ocean ridges are believed to be largely due to the three-dimensional thermal structure, which can be predicted using a simple passive flow model. When the gravity contribution from the predicted thermal structure is removed from the mantle Bouguer anomalies, the residual mantle Bouguer anomalies are created, which represent lateral variations in the crustal thickness and/or density variations from the assumed model. Three-dimensional gravity analysis has been carried out over three areas along the Pacific-Antarctic East Pacific Rise (EPR): (1) the eastern intersection of the Menard transform with the EPR, (2) the overlapping spreading center (OSC) at 36.5°S and, (3) the western intersection of the Raitt transform with the EPR. This geophysical analysis provides an essential tool for understanding the subsurface crustal/upper mantle structure of the fast spreading EPR, and more specifically at transform and nontransform offsets along the EPR. Several interesting features were observed at the eastern intersection between the Menard transform and the EPR. The continuous nature of the residual mantle Bouguer anomalies along the ridge axis suggests that the 60 km of ridge axis surveyed here has a fairly uniform crustal/upper mantle structure. Significant features are not observed in the residual mantle Bouguer anomalies at the ridge-transform intersection or along the eastern 75 km of the Menard transform. At the ridge-transform intersection, fresh lavas from the observed overshot ridge have filled in the transform valley and have subsequently thickened the crust, eliminating any crustal thinning that is occurring there. The large OSC at 36.5°S has a left-stepping offset of approximately 34 km. The most significant feature in the gravity data from this study area is the observed low in the mantle Bouguer anomalies which extends from the northern ridge segment, eastward to the "inactive" rift and continued along the southern ridge segment. This gravity low suggests that this region is underlain by thicker crust and/or hotter, less dense material. No significant features are observed in the residual mantle Bouguer anomalies associated with the overlap basin or the two smaller basins that border the "inactive" rift. The western ridge-transform intersection (RTI) between the Raitt transform and the EPR significantly differs from the Menard transform study area. A transform valley is not observed at this RYE and neither is an overshot ridge. Instead, a transformparallel median ridge is observed east of the RTI, and a fossil transform valley is observed north of the RTI. A low in the residual mantle Bouguer anomalies is associated with the fossil transform valley and the median ridge, suggesting that these areas are underlain by thicker crust and/or less dense material. Positive residual mantle Bouguer anomalies observed at the inside corner of the RYE suggest that this area is underlain by thinner crust and/or colder, more dense material; while at the outside corner of the ridge-transform intersection, a residual anomaly low is observed which suggests that the outside corner is underlain with thicker crust and/or hotter, less dense material. / Graduation date: 1995 / Figures in original document are black and white photocopies. Best scan available.

When light intensities become supersaturating for photosynthesis, phytoplankton must be able to protect the photosynthetic machinery from potential damage by excess energy absorption. One of the most important photoprotective mechanisms involves the nonradiative dissipation of excess light energy by the interconversion of the carotenoid pigments of the so-called xanthophyll cycle. Very little is known about how the xanthophyll cycle of natural communities of phytoplankton responds to high light conditions and the relationship of this photoprotective mechanism to the surrounding physical environment. The purpose of this thesis was to examine the functioning, activation and relationship to the physical environment of the xanthophyll cycle in phytoplankton from the Antarctic ecosystem and the Southern Ocean. Experiments in Antarctica were conducted in austral spring under various natural and artificial light regimes including the use of a newly developed light mixing simulator (LMS). Photoprotective carotenoid pigment concentrations were determined using a carotenoid specific protocol for High Performance Liquid Chromatography (HPLC). The photoprotective xanthophyll cycle was not active in Antarctic sea ice algae under the low light conditions under the annual sea ice. When sea ice algae are exposed to high irradiance, there was an initial rapid deepoxidation of the xanthophyll pigment diadinoxanthin (DD) to diatoxanthin (DT). With on-going irradiance exposure, slower deepoxidation of DD continued. The recovery of DD in the dark or under low light was found to be significantly faster than in temperate algal communities, and is likely a particular adaptation to the unique light environment in Antarctica. The temporal accumulation of pigments of the violaxanthin (VX) xanthophyll cycle was observed for the first time in a natural phytoplankton population. It is hypothesized that the VX cycle may function as a pathway to maintain the pool of DD cycle pigments rather than as a separate photoprotective pathway as observed in higher plants. The high irradiances of ultraviolet - B (290 - 320 nm) radiation (UVB) as a result of stratospheric ozone depletion over Antarctica in spring was found to significantly impact on the DD cycle. Exposure to high levels of both ultraviolet-A (320- 400 nm) radiation (UVA) and UVB reduced the photoprotective xanthophyll pigment pool with the greatest reduction occurring after exposure to high levels of UVB. The reduction in the amount of cellular DD after exposure to high levels of UVB was greater than can be explained by deepoxidation activity, which implies that high UVB exposure can lead to a loss of DD from the community. The first-order kinetic rates of the DD cycle were found to be similar to other studies and did not vary with light intensity. Simulations under natural light using the LMS demonstrated that the response of the DD cycle to static in situ incubations and when subject to vertical mixing was not similar, and that static incubations overestimate DD-cycle activity Over the long term, algae in a simulated vertically mixed environment were able to increase the pool of xanthophyll pigments compared to static conditions where the pool remained the same or decreased. Oceanographic observations from the subantarctic waters south-east of New Zealand in austral autumn provided the physical background for new insights into the xanthophyll cycle of Southern Ocean phytoplankton. The circulation flow and water masses between the Bounty Plateau and Bollons Seamount was resolved and shown to differ from numerical models. Relatively little of the warm and salty Subantarctic Mode Water (SAMW) from the Tasman Sea is carried in the flow of the Subantarctic Front (SAF). The spatial distribution of photoprotective xanthophyll pigments showed higher than expected concentrations in the surface mixed layer of the region. The high concentration of photoprotective pigments is considered to be a consequence of the low iron concentrations in southern waters and the highly variable light and vertical mixing environment. The high cellular concentrations of photoprotective pigments constrains photosynthetic activity implying that the photoprotective pigments may play a more significant role in controlling phytoplankton production in the Southern Ocean than previously thought. Analysis of the xanthophyll pigments and physical oceanography with a Self-Organising map (SOM) Artificial Neural Network (ANN) showed that the photophysiological index DT/ (DD+DT) can be used to resolve a change in water type properties. A simple numerical model was developed which can be used to provide a quantitative index of the relative magnitudes of vertical mixing and phytoplankton photoprotection in the water column. This approach may be useful to identify the effects of physical changes in the surface mixed layer of the Southern Ocean as predicted by climate change modelling.

Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Trophodynamics of carnivorous zooplankton in the region of the Subtropical Convergence (STC) in the Indian sector of the Southern Ocean was investigated during austral autumn (April 2007) as part of the first cruise of the Southern Ocean Ecosystem Variability Study. Within the region of the study, the STC was well defined by the 14°C surface isotherm which separated the Agulhas Return Current and Subtropical water in the north from Sub-Antarctic waters to the south. Total average abundance (3.89 ± 5.46ind 100m-3) and biomass (0.14 ± 0.27mg Dwt 100m-3) of carnivorous zooplankton south of the front were significantly higher than the total average abundance (1.33 ± 1.81ind 100m-3) and biomass (0.03 ± 0.05mg Dwt 100m-3) north of the front (p<0.001). There were no significant correlations between the selected physico-chemical (temperature and salinity) and the biological (mesozooplankton abundance and biomass) variables and the total abundance and biomass of the carnivorous zooplankton during the investigation (p>0.05 in all cases). There was no evidence of enhanced biomass and abundance values at stations occupied in the immediate vicinity of the front. Total average carnivorous zooplankton abundance was dominated by chaetognaths (Eukrohnia hamata Möbius 1875, Sagitta gazellae Ritler-Záhony 1909 and S. zetesios Fowler 1905) and euphausiids (Nematoscelis megalops Sars 1883, Euphausia longirostris Hansen 1908 and E. spinifera Sars 1883), which contributed up to 86.58 ± 32.91% of the total counts. The total average biomass was dominated by euphausiids and amphipods (Themisto gaudichaudii Guérin-Méneville 1825, Phronima sedentaria Forsskål 1775 and Vibilia armata Bovallius 1887) which contributed up to 71.45 ± 34.85% of the total counts. In general the populations of both the euphausiids and amphipods were dominated by females while the chaetognaths were dominated by juveniles. Numerical analysis identified two major zooplankton groupings within the survey area which did not coincide with the water masses within the survey area. The SIMPER procedure of the PRIMER package indicated differences between the groups were mainly attributed to changes in the abundance of the numerically dominant species rather than the presence or absence of individual species. The absence of any significant spatial patterns in the distribution of the carnivorous zooplankton suggests that the STC did not act as a biogeographical barrier during the present study. The mean feeding rates of the chaetognaths E. hamata, S. gazellae and S. zetesios were 1.82 ± 0.85prey d-1, 3.63 ± 2.08prey d-1 and 2.18 ± 0.59prey d-1, respectively. These rates correspond to a combined predation impact equivalent to <5% of the mesozooplankton standing stock or <10% of the mesozooplankton secondary production. Mesozooplankton, comprising mainly copepods was the dominant prey in the guts of the three chaetognath species. Total predation impact of the euphausiids, chaetognaths and amphipods, estimated using published daily ration data, on the mesozooplankton standing stock and secondary production ranged from 0.01% to 1.53% and from 0.03% to 30.54%, respectively. Among the carnivorous zooplankton, chaetognaths were generally identified as the dominant predators of mesozooplankton. Low predation impact of selected carnivorous zooplankton suggested that these organisms contributed little to the vertical carbon flux within the region of investigation during the study.

<p align="justify">The Southern Ocean (ca. 20% of the world ocean surface) is a key place for the regulation of Earth climate thanks to its capacity to absorb atmospheric carbon dioxide (CO2) by physico-chemical and biological mechanisms. The biological carbon pump is a major pathway of absorption of CO2 through which the CO2 incorporated into autotrophic microorganisms in surface waters is transferred to deep waters. This process is influenced by the extent of the primary production and by the intensity of the remineralization of organic matter along the water column. So, the annual cycle of sea ice, through its in situ production and remineralization processes but also, through the release of microorganisms, organic and inorganic nutrients (in particular iron)into the ocean has an impact on the carbon cycle of the Southern Ocean, notably by promoting the initiation of phytoplanktonic blooms at time of ice melting.</p><p><p align="justify">The present work focussed on the distribution of organic matter (OM) and its interactions with the microbial network (algae, bacteria and protozoa) in sea ice and ocean, with a special attention to the factors which regulate the biological carbon pump of the Southern Ocean. This thesis gathers data collected from a) late winter to summer in the Western Pacific sector, Western Weddell Sea and Bellingshausen Sea during three sea ice cruises ARISE, ISPOL-drifting station and SIMBA-drifting station and b) summer in the Sub-Antarctic and Polar Front Zone during the oceanographic cruise SAZ-Sense.</p><p><p align="justify">The sea ice covers were typical of first-year pack ice with thickness ranging between 0.3 and 1.2 m, and composed of granular and columnar ice. Sea ice temperature ranging between -8.9°C and -0.4°C, brines volume ranging between 2.9 to 28.2% and brines salinity from 10 to >100 were observed. These extreme physicochemical factors experienced by the microorganisms trapped into the semi-solid sea ice matrix therefore constitute an extreme change as compared to the open ocean. Sea ice algae were mainly composed of diatoms but autotrophic flagellates (such as dinoflagellates or Phaeocystis sp.) were also typically found in surface ice layers. Maximal algal biomass was usually observed in the bottom ice layers except during SIMBA where the maxima was localised in the top ice layers likely because of the snow and ice thickness which limit the light available in the ice cover. During early spring, the algal growth was controlled by the space availability (i.e. brine volume) while in spring/summer (ISPOL, SIMBA) the major nutrients availability inside sea ice may have controlled algal growth. At all seasons, high concentrations of dissolved and particulate organic matter were measured in sea ice as compared to the water column. Dissolved monomers (saccharides and amino acids) were accumulated in sea ice, in particular in winter. During spring and summer, polysaccharides constitute the main fraction of the dissolved saccharides pool. High concentrations of transparent exopolymeric particles (TEP), mainly constituted with saccharides, were present and their gel properties greatly influence the internal habitat of sea ice, by retaining the nutrients and by preventing the protozoa grazing pressure, inducing therefore an algal accumulation. The composition as well as the vertical distribution of OM in sea ice was linked to sea ice algae.</p><p><p align="justify">Besides, the distribution of microorganisms and organic compounds in the sea ice was also greatly influenced by the thermodynamics of the sea ice cover, as evidenced during a melting period for ISPOL and during a floodfreeze cycle for SIMBA. The bacteria distribution in the sea ice was not correlated with those of algae and organic matter. Indeed, the utilization of the accumulated organic matter by bacteria seemed to be limited by an external factor such as temperature, salinity or toxins rather than by the nature of the organic substrates, which are partly composed of labile monomeric saccharides. Thus the disconnection of the microbial loop leading to the OM accumulation was highlighted in sea ice.</p><p><p align="justify">In addition the biofilm formed by TEP was also involved in the retention of cells and other compounds(DOM, POM, and inorganic nutrients such as phosphate and iron) to the brine channels walls and thus in the timing of release of ice constituents when ice melts. The sequence of release in marginal ice zone, as studied in a microcosm experiments realized in controlled and trace-metal clean conditions, was likely favourable to the development of blooms in the marginal ice zone. Moreover microorganisms derived from sea ice (mainly <10 µm) seems able to thrive and grow in the water column as also the supply of organic nutrients and Fe seems to benefit to the pelagic microbial community.</p><p><p align="justify">Finally, the influence of the remineralization of organic matter by heterotrophic bacterioplankton on carbon export and biological carbon pump efficiency was investigated in the epipelagic (0-100 m) and mesopelagic(100-700 m) zones during the summer in the sub-Antarctic and Polar Front zones (SAZ and PFZ) of the Australian sector (Southern Ocean). Opposite to sea ice, bacterial biomass and activities followed Chl a and organic matter distributions. Bacterial abundance, biomass and activities drastically decreased below depths of 100-200 m. Nevertheless, depth-integrated rates through the thickness of the different water masses showed that the mesopelagic contribution of bacteria represents a non-negligible fraction, in particular in a diatom-dominated system./</p><p><br><p><p align="justify">L’océan Antarctique (± 20% de la surface totale des océans) est un endroit essentiel pour la régulation du climat de notre planète grâce à sa capacité d’absorber le dioxyde de carbone (CO2) atmosphérique par des mécanismes physico-chimique et biologique. La pompe biologique à carbone est un processus majeur de fixation de CO2 par les organismes autotrophes à la surface de l’océan et de transfert de carbone organique vers le fond de l’océan. Ce processus est influencé par l’importance de la production primaire ainsi que par l’intensité de la reminéralisation de la matière organique dans la colonne d’eau. Ainsi, le cycle annuel de la glace via sa production/reminéralisation in situ mais aussi via l’ensemencement de l’océan avec des microorganismes et des nutriments organiques et inorganiques (en particulier le fer) a un impact sur le cycle du carbone dans l’Océan Antarctique, notamment en favorisant l’initiation d’efflorescences phytoplanctoniques dans la zone marginale de glace.</p><p><p align="justify">Plus précisément, nous avons étudié les interactions entre le réseau microbien (algues, bactéries et protozoaires) et la matière organique dans le but d’évaluer leurs impacts potentiels sur la pompe biologique de carbone dans l’Océan Austral. Deux écosystèmes différents ont été étudiés :la glace de mer et le milieu océanique grâce à des échantillons prélevés lors des campagnes de glace ARISE, ISPOL et SIMBA et lors de la campagne océanographique SAZ-Sense, couvrant une période allant de la fin de l’hiver à l’été.</p><p><p align="justify">La glace de mer est un environnement très particulier dans lequel les microorganismes planctoniques se trouvent piégés lors de la formation de la banquise et dans lesquels ils subissent des conditions extrêmes de température et de salinité, notamment. Les banquises en océan ouvert étudiées (0,3 à 1,2 m d’épaisseur, températures de -8.9°C à -0.4°C, volumes relatifs de saumure de 2.9 à 28.2% et salinités de saumures entre 10 et jusque >100) étaient composées de glace columnaire et granulaire. Les algues de glace étaient principalement des diatomées mais des flagellés autotrophes (tels que des dinoflagellés ou Phaeocystis sp.) ont été typiquement observés dans les couches de glace de surface. Les biomasses algales maximales se trouvaient généralement dans la couche de glace de fond sauf à SIMBA où les maxima se trouvaient en surface, probablement en raison de l’épaisseur des couches de neige et de glace, limitant la lumière disponible dans la colonne de glace. Au début du printemps, la croissance algale était contrôlée par l’espace disponible (càd le volume des saumures) tandis qu’au printemps/été, la disponibilité en nutriments majeurs a pu la contrôler. A toutes les saisons, des concentrations élevées en matière organique (MO) dissoute et particulaire on été mesurées dans la glace de mer par rapport à l’océan. Des monomères dissous (sucres et acides aminés) étaient accumulés dans la glace, surtout en hiver. Au printemps et été, les polysaccharides dissous dominaient le réservoir de sucres. La MO était présente sous forme de TEP qui par leurs propriétés de gel modifie l’habitat interne de la glace. Ce biofilm retient les nutriments et gêne le mouvement des microorganismes. La composition et la distribution de la MO dans la glace étaient en partie reliées aux algues de glace. De plus, la thermodynamique de la couverture de glace peut contrôler la distribution des microorganismes et de la MO, comme observé lors de la fonte de la glace à ISPOL et lors du refroidissement de la banquise à SIMBA. La distribution des bactéries n’est pas corrélée avec celle des algues et de la MO dans la glace. En effet, la consommation de la MO par les bactéries semble être limitée non pas par la nature chimique des substrats mais par un facteur extérieur affectant le métabolisme bactérien tel que la température, la salinité ou une toxine. Le dysfonctionnement de la boucle microbienne menant à l’accumulation de la MO dans la glace a donc été mis en évidence dans nos échantillons.</p><p><p align="justify">De plus, le biofilm formé par les TEP est aussi impliquée dans l’attachement des cellules et autres composés aux parois des canaux de saumure et donc dans la séquence de largage lors de la fonte. Cette séquence semble propice au développement d’efflorescences phytoplanctoniques dans la zone marginale de glace. Les microorganismes originaires de la glace (surtout ceux de taille < 10 μm) semblent capables de croître dans la colonne d’eau et l’apport en nutriments organiques et inorganiques apparaît favorable à la croissance des microorganismes pélagiques.</p><p><p align="justify">Enfin, l’influence des activités hétérotrophes sur l’export de carbone et l’efficacité de la pompe biologique à carbone a été évaluée dans la couche de surface (0-100 m) et mésopélagique (100-700 m) de l’océan. Au contraire de la glace, les biomasses et activités bactériennes suivaient les distributions de la chlorophyll a et de la MO. Elles diminuent fortement en dessous de 100-200 m, néanmoins les valeurs intégrées sur la hauteur de la colonne d’eau indiquent que la reminéralisation de la MO par les bactéries dans la zone mésopélagique est loin d’être négligeable, spécialement dans une région dominée par les diatomées.</p> / Doctorat en Sciences agronomiques et ingénierie biologique / info:eu-repo/semantics/nonPublished

The biological process of the carbon cycle in the Antarctic Ocean is controlled by the photosynthetic activity of the primary producers. The amount of fixed carbon does not only depend on the photosynthetic activity but also on the carbon losses due to respiration. Thus, the ratio photosynthesis to respiration (rP/R) is an important parameter to predict the effect of climate change on the Antarctic ecosystem. Indeed, the ongoing changes in climate change are influencing the dynamics of environmental conditions, which has tremendous effects on the phytoplankton community. Therefore, two ecologically relevant species from the Southern Ocean were here investigated: the diatom Chaetoceros sp. and the prymnesiophyte Phaeocystis antarctica, studying the changes in the rP/R under global climate change conditions. Three main parameters were examined i.e temperature, salinity and iron limitation. The P/R ratio was significantly affected by temperature, while salinity had only a secondary importance, although with species-specific differences. More specifically, the values were ranging from 12.3 to 7.5 for Chaetoceros sp. and from 12.4 to 2.5 for P. antarctica. The changes in this ratio were principally due to variations in respiration, rather than in photosynthesis. Chaetoceros sp. appears to be less flexible in the regulation of the extent of photoprotective mechanisms (non-photochemical quenching and alternative electrons), but its photoprotective level was generally higher than in P. antarctica. Regarding iron limitation, data were successfully collected only for Chaetoceros sp.. The P/R ratio, equal to 2.8, did not change under iron limitation, with iron limited cells showing a very efficient acclimation to the lowered assimilatory metabolism by decreasing their respiratory losses.

We investigate the silicon (Si) cycle in the Southern Ocean through two isotopic approaches: (1) 30Si-incubation experiments and (2) natural silicon isotopic composition (ä30Si). 30Si-spiked incubation allows to discriminate the short-term (~ 1 day) net Si-uptake flux in bSiO2 production and dissolution. ä30Si of both biogenic silica and dissolved silicon integrates at seasonal/annual scale bSiO2 production or dissolution and mixing.<p>(1) A new mass spectrometer method (HR-SF-ICPMS) has been developed for 30Si-isotopic abundance measurements. This methodology is faster and easier than the previous available methodologies and has the same precision. A complete set of incubation was coupled with parallel 32Si-incubations and the two methodologies give not significantly different bSiO2 production rates. In the Southern Ocean, especially in the southern Antarctic Circumpolar Current, the large silicic acid concentration degrades the sensitivity of the method with Si dissolution fluxes staying generally below the detection limit. In contrast, the 28Si-isotopic dilution was sensitive enough to assess low biogenic silica dissolution rates in silicic acid poor waters of the northern ACC. We show that large accumulation of detrital dissolving biogenic silica after productive period implies really efficient silicon loop with integrated (euphotic layer) dissolution:production ratio equal or larger than 1.<p> (2) We largely expand the silicic acid isotopic data in the open ocean. Relatively simple mass and isotopic balances have been performed in the Antarctic Zone and have allowed to apply for the first time ä30Si in a quantitative way to estimate regional net silica production and quantify source waters fueling bSiO2 productivity. We observe that at the end of the productive period as suggested with 30Si-incubation, large accumulation of detrital biogenic silica in the surface waters increase the D:P ratio and subsequently dampens the bSiO2 production mediated isotopic fractionation with residual biogenic silica carrying heavier ä30Si than expected. Seasonal isotopic evolution is simulated and seems in agreement with our observations. These simulations strongly suggest working with non-zero order equations to fully assess the seasonal expression of the different processes involved: mixing, uptake, dissolution. Si-isotopes are also tracking the origin and fates of the different ACC pools across the Southern Ocean meridional circulation. Moreover during the circumpolar eastward pathway, the bSiO2 dissolution in deep water decreases the corresponding ä30Si values and this imprint is further transmitted via the upper limb of the meridional circulation in the intermediate water masses.<p> / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Thesis (MComm)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The Southern Ocean is a complex system, where the relationship between CO2 concentrations and its drivers varies intra- and inter-annually. Due to the lack of readily available in situ data in the Southern Ocean, a model approach was required which could predict the CO2 concentration proxy variable, fCO2. This must be done using predictor variables available via remote measurements to ensure the usefulness of the model in the future. These predictor variables were sea surface temperature, log transformed chlorophyll-a concentration, mixed layer depth and at a later stage altimetry. Initial exploratory analysis indicated that a non-parametric approach to the model should be taken. A parametric multiple linear regression model was developed to use as a comparison to previous studies in the North Atlantic Ocean as well as to compare with the results of the non-parametric approach. A non-parametric kernel regression model was then used to predict fCO2 and nally a combination of the parametric and non-parametric regression models was developed, referred to as the mixed regression model. The results indicated, as expected from exploratory analyses, that the non-parametric approach produced more accurate estimates based on an independent test data set. These more accurate estimates, however, were coupled with zero estimates, caused by the curse of dimensionality. It was also found that the inclusion of salinity (not available remotely) improved the model and therefore altimetry was chosen to attempt to capture this e ect in the model. The mixed model displayed reduced errors as well as removing the zero estimates and hence reducing the variance of the error rates. The results indicated that the mixed model is the best approach to use to predict fCO2 in the Southern Ocean and that altimetry's inclusion did improve the prediction accuracy. / AFRIKAANSE OPSOMMING: Die Suidelike Oseaan is 'n komplekse sisteem waar die verhouding tussen CO2 konsentrasies en die drywers daarvoor intra- en interjaarliks varieer. 'n Tekort aan maklik verkrygbare in situ data van die Suidelike Oseaan het daartoe gelei dat 'n model benadering nodig was wat die CO2 konsentrasie plaasvervangerveranderlike, fCO2, kon voorspel. Dié moet gedoen word deur om gebruik te maak van voorspellende veranderlikes, beskikbaar deur middel van afgeleë metings, om die bruikbaarheid van die model in die toekoms te verseker. Hierdie voorspellende veranderlikes het ingesluit see-oppervlaktetemperatuur, log getransformeerde chloro l-a konsentrasie, gemengde laag diepte en op 'n latere stadium, hoogtemeting. 'n Aanvanklike, ondersoekende analise het aangedui dat 'n nie-parametriese benadering tot die data geneem moet word. 'n Parametriese meerfoudige lineêre regressie model is ontwikkel om met die vorige studies in die Noord-Atlantiese Oseaan asook met die resultate van die nieparametriese benadering te vergelyk. 'n Nie-parametriese kern regressie model is toe ingespan om die fCO2 te voorspel en uiteindelik is 'n kombinasie van die parametriese en nie-parametriese regressie modelle ontwikkel vir dieselfde doel, wat na verwys word as die gemengde regressie model. Die resultate het aangetoon, soos verwag uit die ondersoekende analise, dat die nie-parametriese benadering meer akkurate beramings lewer, gebaseer op 'n onafhanklike toets datastel. Dié meer akkurate beramings het egter met "nul"beramings gepaartgegaan wat veroorsaak word deur die vloek van dimensionaliteit. Daar is ook gevind dat die insluiting van soutgehalte (nie beskikbaar oor via sateliet nie) die model verbeter en juis daarom is hoogtemeting gekies om te poog om hierdie e ek in die model vas te vang. Die gemengde model het kleiner foute getoon asook die "nul"beramings verwyder en sodoende die variasie van die foutkoerse verminder. Die resultate het dus aangetoon dat dat die gemengde model die beste benadering is om te gebruik om die fCO2 in die Suidelike Oseaan te beraam en dat die insluiting van altimetry die akkuraatheid van hierdie beraming verbeter.