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The gravitational sinking of organic particles is a vital component of the biological carbon pump. This sinking process is strongly modulated by the spatiotemporally varying eddy field, complicating the interpretation of particle flux measured by deep-moored sediment traps. By backtracking particles to 200 m depth based on the outputs of a realistic eddy-resolving simulation, we characterize the origins of particles collected at a long-term observatory site in the Northeast Atlantic and focus on the impact of mesoscale dynamics on particle transport. Our results show that mesoscale dynamics between 200 and 1,000 m control the statistical funnel. Over the long term, the horizontal sampling scales of traps are estimated as hundreds of kilometers, with containment radius ranging from 90 to 490 km, depending on sinking velocities. Particle travel time suggests that overall vertical flow acts to facilitate the export, with estimated deviations up to 1 ± 2 days for particles sinking at 50 m d−1 to 1,000 m. Statistical analyses of horizontal displacements reveal that mesoscale eddies at the site confine particle sources in a more local area. On average, particles in anticyclonic eddies sink faster to depth than expected from purely gravitational sinking, contrary to their counterparts in cyclonic eddies. The results highlight the critical role of mesoscale dynamics in determining particle transport in a typical open ocean region with moderate eddy kinetic energy. This study provides implications for the sampling design of particle flux measurements during cruises and the interpretation of deep-ocean mooring observations. Key Points The statistical funnel of deep-ocean sediment traps is mainly determined by mesoscale dynamics in the twilight zone On average the vertical flow enhances particle sinking, but with more variance in spring than in other seasons Coherent eddies reduce the particle dispersion within a local area, with vertical acceleration (deceleration) by anticyclones (cyclones) Plain Language Summary As plants in the ocean, phytoplankton organisms transform the atmospheric CO2 into organic carbon that forms particles of various sizes sinking to the deep ocean due to gravity. The falling particles can be collected by containers called sediment traps. However, particles may originate far from the surface ocean directly above the trap as ocean currents horizontally transport particles. Also, the time taken by particles to sink to the deep ocean varies due to vertical motions of seawater. To study the impact of ocean currents on sinking particles, we use an ocean model and virtual particles. We release particles at a fixed location, representing a sediment trap, and track particle trajectories back in time to identify their source regions. Our results show that the size of this source region is mainly determined by currents between 200 and 1,000 m. On average, particles tend to sink faster than expected from purely gravitational sinking. Large whirlpools of water above the trap lead to a local source region, which suggests that the particle flux can be better correlated to the surface production of organic carbon in this case. The finding has implications for the sampling strategy and the interpretation of particle export measurements in regional surveys.

Standard niche modeling is based on probabilistic inference from organismal occurrence data but does not benefit yet from genome-scale descriptions of these organisms. This study over-comes this shortcoming by proposing a new conceptual niche that encompasses the whole metabolic capabilities of an organism. The so-called metabolic niche resumes well-known traits such as nutrient needs and their dependencies for survival. Despite the computational challenge, its implementation allows the detection of traits and the formal comparison of niches of different organisms, emphasizing that the presence-absence of functional genes is not enough to approximate the phenotype. Further statistical exploration of an organism’s niche sheds light on genes essential for the metabolic niche and their role in understanding various biological experiments, such as transcriptomics, paving the way for incorporating better the genome-scale description in ecological studies.

Diel vertical migration (DVM) of zooplankton has been recognized to influence the biological pump by releasing carbon and nutrients at depth. However, uncertainties regarding the magnitude, spatial occurrence and variability of this “active transport”, as well as its impact on the carbon export, remain. To tackle these questions, a cost‐effective parameterization of the DVM is included into a regional biogeochemical model simulating the North Atlantic. In addition to a reference simulation in which no DVM is simulated, two relative biomasses of migrating zooplankton (30% and 60%) have been tested. It leads to an active to passive export ratio in agreement with published estimations and to an increase in the carbon export efficiency at 1000m between 20 and 40%. However, this effect is partially canceled out by a simulated primary production decrease. DVM differently impacts contrasted North Atlantic regions with the largest effect on export efficiency found in the subtropical area. Key Points Model evaluation of the zooplankton diurnal vertical migration (DVM) impacts in the North Atlantic. DVM increases the effectiveness of the carbon export. DVM, in model experiments, decrease primary production, which almost cancels out the above mentioned more efficient export.

32 pages, 14 figures, 3 tables, 2 appendixes, supporting Information https://doi.org/10.1029/2018JC014668.-- The source code of NEMO is available on the NEMO website (http://www. nemo-ocean.eu). The GlobColour data (http://globcolour.info) used in this study has been developed, validated, and distributed by ACRI-ST, France. The data used in this study, together with the studies they come from, are listed in the supporting information, Thorium‐234 (234Th), an insoluble radioisotope scavenged by marine particles, can be used as a proxy of the biological carbon pump. Thorium‐234 observations can constrain biogeochemical models, but a necessary first step is to estimate the poorly known partition coefficients between particulate and dissolved phases. In this study, the 234Th partition coefficients for five particle types, differing in size and chemical composition, are estimated by fitting a global 3‐D 234Th model based on the coupled ocean general circulation‐biogeochemistry model NEMO‐PISCES (at a resolution of 2°) to a global 234Th data set (including GEOTRACES data). Surface partition coefficients are estimated between 0.79 and 16.7×106. Biogenic silica has the smallest partition coefficients. Small particulate organic carbon and lithogenic dust have the largest. Thorium‐234 observations at depth cannot be recovered without allowing partition coefficients to increase by one order of magnitude from surface to 1,000 m deep. In our time‐dependent global 3‐D model, the biases introduced by three common assumptions made in biological carbon pump studies can be quantified. First, using the C:234Th ratio of large particles alone leads to an overestimation of carbon export at the base of the euphotic layer, by up to a factor 2. Furthermore, assuming steady state and neglecting transport by advection and diffusion can bias fluxes by as much as 50%, especially at high latitudes and in upwellings, with a sign and intensity depending on the season, This work is part of the first author's PhD, supported by the “Laboratoire d'Excellence” LabexMER (ANR-10-LABX-19) and cofunded by a grant from the French government under the program “Investissements d'Avenir”, by a grant from the Regional Council of Brittany and by project ANR CIGOEF

Radium 228 (228Ra), an almost conservative trace isotope of the ocean, supplied from the continental shelves and removed by a known radioactive decay (T1/2 = 5.75 yr), can be used as a proxy to constrain shelf fluxes of other trace elements, such as nutrients, iron, or rare earth elements. In this study, we perform inverse modeling of a global 228Ra dataset (including GEOSECS, TTO and GEOTRACES programs, and, for the first time, data from the Arctic and around the Kerguelen islands) to compute the total 228Ra fluxes toward the ocean, using the ocean circulation obtained from the NEMO 3.6 model with a 2° resolution. We optimized the inverse calculation (source regions, cost function) and find a global estimate of the 228Ra fluxes of 8.01–8.49 × 1023 atoms yr−1, lower and more precise than previous estimates. The largest fluxes are in the western North Atlantic, the western Pacific and the Indian Ocean, with roughly two thirds in the Indo-Pacific basin. A first estimate in the Arctic Ocean is assessed (0.20–0.50 × 1023 atoms yr−1). Local misfits between model and data in the Arctic, the Gulf Stream and the Kuroshio regions could result from flaws of the ocean circulation in these regions (resolution, atmospheric forcing). As radium is enriched in groundwater, a large part of the 228Ra shelf sources comes from submarine groundwater discharge (SGD), a major but poorly known pathway for terrestrial mineral elements, including nutrients, to the ocean. In contrast to the 228Ra budget, the global estimate of SGD is rather unconstrained, between 1.3 and 14.7 × 1013 m3 yr−1, due to high uncertainties on the other sources of 228Ra, especially diffusion from continental shelf sediments. Better precision on SGD cannot be reached by inverse modeling until a proper way to separate the contributions of SGD and diffusion at a global scale is found.

39 páginas, 6 figuras, 9 tablas, 1 apéndice, The variability of nitrate (N), phosphate (P), silicate (Si) and Apparent Oxygen Utilization (AOU) due to water mass mixing was objectively separated from the variability due to mineralization of biogenic materials in the western and eastern South Atlantic Ocean on basis of the constrained Optimum MultiParameter (OMP) analysis implemented in the companion manuscript. Using a consensus linear regression model, AOU/N/P/Si mineralization ratios and the corresponding oxygen utilisation rates (OURs) were obtained for the realm of each water mass defined after the OMP analysis. Combining these results with a stoichiometric model, the organic carbon to nitrogen (C/N) ratios and the biochemical composition (carbohydrates+lipids, proteins and phosphorus compounds) of the mineralized material, were derived. The vertical variability of the AOU/N, AOU/P and AOU/C mineralization ratios pointed to a significant fractionation during the mineralization of sinking organic matter. This fractionation was confirmed by preferential consumption of organic phosphorous compounds and proteins in shallower levels, which produced an increase of the C/N ratio of the mineralised materials of 0.5 ± 0.2 mol C mol N–1 every 1000 dbar. OURs in the twilight zone decreased quadratically with the C/N molar ratio of the mineralised material and exponentially with pressure (p, in 103 dbar) according to the following regression equation: Ln (OUR) = 6.2(±1.2) – 2.0 (±0.7) · Ln (C/N) – 0.6 (±0.2) · p (r2 = 0.87, p < 0.006, n = 8). This variability in the rates and stoichiometric ratios of the biogenic material mineralization compromises our capacity to predict the ocean biogeochemistry response to global change, including the CO2 uptake and storage and the corresponding feedback mechanisms, Spanish CICYT contracts ANT93-1156-E and ANT94-1168-E, IFREMER contract No 94 1430 087, Spanish Ministry of Science and Innovation (MALASPINA expedition, grant number CSD2008-00077), Spanish Ministry of Science and Technology, grant ORCASEX (RYC-2006- 001836), program Ramón y Cajal

International audience; It has long been recognised that there are strong interactions and feedbacks between climate, upper ocean biogeochemistry and marine food webs, and also that food web structure and phytoplankton community distribution are important determinants of variability in carbon production and export from the euphotic zone. Numerical models provide a vital tool to explore these interactions, given their capability to investigate multiple connected components of the system and the sensitivity to multiple drivers, including potential future conditions. A major driver for ecosystem model development is the demand for quantitative tools to support ecosystem-based management initiatives. The purpose of this paper is to review approaches to the modelling of marine ecosystems with a focus on the North Atlantic Ocean and its adjacent shelf seas, and to highlight the challenges they face and suggest ways forward. We consider the state of the art in simulating oceans and shelf sea physics, planktonic and higher trophic level ecosystems, and look towards building an integrative approach with these existing tools. We note how the different approaches have evolved historically and that many of the previous obstacles to harmonisation may no longer be present. We illustrate this with examples from the on-going and planned modelling effort in the Integrative Modelling Work Package of the EURO-BASIN programme.

30 pages, 1 table, 20 figures, A quasi-meridional hydrographic section located offshore from South America from 50°S to 10°N, and three shorter transverse lines to the continental slope, are used for a descriptive study of the water masses along the western boundary of the South and Equatorial Atlantic. At the upper and intermediate levels, the tracer analysis provides geographical limits of the wind-driven circulation regimes, and a comparison of the tracer values at the continental slope and along the meridional section shows where the boundary currents originate. At depths shallower than about 200 m, the subdivision of the subtropical gyre into two cells separated by the Subtropical Countercurrent near 28°S, that was pointed out in a previous study, is corroborated. South of this front, a warm variety (∼18°C) of Subtropical Mode Water in the inner recirculation of the Brazil Current appears, despite its limited extent, as a southern counterpart of the North Atlantic 18°C water. At the deep levels, the Upper Circumpolar Water and Upper North Atlantic Deep Water enter the South Atlantic in a significantly overlapping density range. The ensuing lateral encounter of both water masses occurs at 26°S near the western boundary, where most of the boundary flow of the latter water is stopped and deflected seaward by the base of the subtropical gyre. Other tracer anomalies signal significant eastward escapes of North Atlantic Deep Water: within two jets at about two degrees of latitude on either side of the equator, in another narrow current at 10°S, and at 34°S. The latter latitude marks the confluence, and eastward deflection, of the opposite boundary currents of Lower North Atlantic Deep Water and Lower Circumpolar Water. Near the bottom of the Argentine Basin, the Weddell Sea Deep Water that flows westward north of the Zapiola Ridge is more recently ventilated than the water carried by the boundary current near the Falkland Escarpment. While a part of it flows anticyclonically around the ridge, another part turns equatorward and enhances the southern property signatures of the water farther north, This investigation was supported by IFREMER (Grant 210161), INSU (Institut National des Sciences de l'Univers) and the CNRS (Centre de la Recherche Scientifique), in the framework of the Programme National d'Etude de la Dynamique du Climat (PNEDC) and its WOCE/France subprogramme. X.A. Alvarez-Salgado, C.G. Castro, and A.F. Rios acknowledge financial support from the Spanish CICYT (Contract No ANT93-1156-E)

We investigate the coupling between the physics and new production variability during the period April 1992 to June 1995 in the equatorial Pacific via two cruises and simulations. The simulations are provided by a high-resolution Ocean General Circulation Model forced with satellite-derived weekly winds and coupled to a nitrate transport model in which biology acts as a nitrate sink. The cruises took place in September-October 1994 and sampled the western Pacific warm pool and the upwelling region further east. The coupled model reproduces these contrasted regimes. In the oligotrophic warm pool the upper layer is fresh, and nitrate-depleted, and the new production is low. In contrast, the upwelling waters are colder, and saltier with higher nitrate concentrations, and the new production is higher. Along the equator the Eastern edge of the warm pool marked by a sharp salinity front, also coincides with a "new production front". Consistent with the persistent eastward surface currents during the second half of 1994, these fronts undergo huge eastward displacement at the time of the cruises. The warm/fresh poll and oligotrophic region has an average new production of 0.9 mmol NO3/m2/d, which is almost balanced by horizontal advection from the central Pacific and by vertical advection of richer water from the nitrate reservoir below. In contrast, the upwelling mesotrophic region shows average new production of 2.1 mmol NO3/m2/d and the strong vertical nitrate input by the equatorial upwelling is balanced by the losses, through westward advection and meridional divergence of nitrate rich waters, and by the biological sink. (Résumé d'auteur)

Chlorofluoromethanes were sampled along two zonal sections, at 4°30 S and 7°30 N between the African and American continents (A7 and A6 WOCE sections) and two meridional sections, at 35°W and 3°50W, during the CITHER 1 cruise (part of the French program CITHER (CIrculation THERmohaline) during January–March 1993. The results reported here deal primarily with the North Atlantic Deep Water, just ten years after the first CFM snapshot of the tropical Atlantic ocean obtained during the Transient Tracers in the Ocean Program (TTO) ( Weiss et al., 1985 . Nature 314, 608–610). The data provide evidence for the eastward bifurcation of the deep flow near the equator, on both UNADW and LNADW levels. The distributions clearly show the CFM signal corresponding to the UNADW penetrating into the eastern basin: at 3°50W the CFM core extends from 4°S to 3°N with a maximum around 2°S. On both UNADW and LNADW levels, the bifurcation does not occur exactly on the equator but a few degrees south and seems to be partly induced by topographic effects. Previously published circulation schemes for UNADW and LNADW levels are compared to the CITHER 1 CFM data. Great variability is revealed and new patterns from the data are highlighted. The “young” deep component of the AABW flow seems to be stopped by the topography just north of the equatorial channel. TTO and CITHER 1 data set comparison and “apparent” ages lead to minimal values of the propagation rate of the CFC signal at low latitudes.

International audience; In this study we first evaluate the small-scale spatial variability of particulate export, using a set of synoptic thorium-234 activity observations sampled within a one-degree radius. These data show significant variability of surface thorium activity on scales of the order of 100 km (∼270-550 dpm m−3). This patchiness of export potentially affects the robustness of point observations and our interpretation of them. Motivated by these observations we subsequently couple an explicit model of thorium-234 dynamics to a coupled physical-biogeochemical basin model capable of resolving these small-scales. The model supports the observations in displaying marked thorium variability on spatial scales of the order of 100 km and smaller, with highest values in the regions of large eddy kinetic energy and large primary productivity. The model is also used to quantify the impact of small-scale variability on export estimates. Our model shows that the primary source of error associated with the presence of small-scale spatial variability is related to the standard assumptions of steady state and non-steady state (>40% during bloom condition). The non-steady state method can misinterpret variations due to patchiness in thorium activity as temporal changes and lead to errors larger than those introduced by the simpler steady state approach. We show that the non-steady state approach could improve the flux estimates in some cases if the sampling was conducted in a Lagrangian framework. Undersampling the spatial variability results in further bias (>20%) that can be reduced when the sampling density is increased. Finally, errors due to the dynamical transport of thorium associated with small-scale structures are relatively low (

A theoretical approach to gas transfer by bubbles created by breaking waves at the air-waterinterface is undertaken. Based on a simple model, this study affords a basic understanding of thephysical mechanisms. The behaviour of a single bubble is examined. Similarities and differencesbetween the formulation of gas fluxes across a “flat” air-water interface and through bubbles areshown. The gas flux through bubbles is not strictly proportional to the solubility and to thedifference of concentrations between the bulk of the water and the surface. Nevertheless, fortrace gases, a linear relationship with this difference of concentrations, increasing with solubilityis predicted. Overpressure in the bubbles and dissolution favour invasion into the water andimply a water supersaturation at equilibrium. This supersaturation is more important when thegas solubility is low. The r61e of surfactants is studied and it is found that they can reduce the gasexchange by a factor of 5. The consequences of the model for the transfer velocity are brieflypresented. A better definition of the bubble lifetime, i t . , a better representation of waterdynamics under a breaking wave, is necessary to make more reliable model predictions, mostimportantly for gases with low solubility. Theoretical studies on the bubble source are necessaryin order to avoid, as far as possible, the use of the bubble distribution. DOI: 10.1111/j.1600-0889.1985.tb00075.x

Episodic events like hurricanes, storms and frontal- and eddy-driven upwelling can alter the partial pressure of CO 2 (pCO 2 ) at the sea surface by entraining subsurface waters into the surface mixed layer (ML) of the ocean. Since pCO 2 is a function of total dissolved inorganic carbon (DIC), temperature ( T ), salinity and alkalinity, it responds to the combined impacts of physical, chemical and biological changes. Here, we present an analytical framework for assessing the relative magnitude and sign in the short-term perturbation of surface pCO 2 arising from vertical mixing events. Using global, monthly, climatological datasets, we assess the individual, as well as integrated, contribution of various properties to surface pCO 2 in response to episodic mixing. The response depends on the relative vertical gradients of properties beneath the ML. Many areas of the ocean exhibit very little sensitivity to mixing owing to the compensatory effects of DIC and T on pCO 2 , whereas others, such as the eastern upwelling margins, have the potential to generate large positive/negative anomalies in surface pCO 2 . The response varies seasonally and spatially and becomes more intense in subtropical and subpolar regions during summer. Regions showing a greater pCO 2 response to vertical mixing are likely to exhibit higher spatial variability in surface pCO 2 on time scales of days.

International audience; Northeast Atlantic mode waters (NEAMW) are formed by subduction in a region known to be a strong sink of atmospheric CO2. This study investigates the biological and physical carbon pumps involved in this sink. For that purpose, we estimate the annual transfer of carbon, nitrogen, and oxygen from the surface into the ocean interior through NEAMW subduction. Our estimates are based on a model simulation constrained with data collected during the Programme Océan Multidisciplinaire Méso Echelle (POMME). We found that subduction accounts for a large proportion of carbon export below the mixed layer in the POMME region (38°N-45°N, 16°W-22°W). Fifty percent of labile organic carbon is exported by the biological pump and 50% by subduction; 98% of the total carbon flux below the mixed layer is due to carbon subduction, essentially in the form of dissolved inorganic carbon and of more refractory dissolved organic matter. This is because they are the dominant pools of carbon and are passing through the winter mixed-layer depth gradient that characterizes the region. Our results emphasize that the net export of carbon represents a small fraction of the fluxes of carbon across the mixed layer through the processes of subduction and obduction. Moreover, our results indicate that NEAMW subduction occurs before the spring bloom. This implies that NEAMW is mostly subducted with end-winter characteristics with, in particular, relatively high nutrient content. This reflects a relative inefficiency in nutrient utilization and contrasts with subtropical mode waters, which are subducted during the bloom and are thus relatively poor in nutrients when they are subducted.

The distribution of 3He across the Southern Ocean is depicted on the basis of a meridional section between Antarctica and South Africa measured during the INDIGO-3 survey (1988). A core of δ3He values above 10% is observed south of the Polar Front, associated with very low CFC concentrations. This 3He enriched layer is documented from the GEOSECS and INDIGO 3He data in the Southern Ocean. It is found at a density level around σθ=27.8 in all the waters close to Antarctica (i.e. south of 50°S ). Its zonal distribution suggests that it is likely that it originates from the central/eastern Pacific. Hence, it provides an indication of the deep Pacific waters in the Antarctic Circumpolar Current, which are not easily detectable from the standard hydrographie parameters.

International audience; A prognostic high-resolution model is established to provide an integrated view of the evolution of the spring bloom during the Programme Océan Multidisciplinaire Méso Echelle (POMME) experiments carried out at sea from February to May 2001 (16-22°W and 38°-45°N). Data collected during the first survey were used for model initialization, and data from three other cruises were used for model validation. The model successfully predicts the time evolution of the main reservoirs and fluxes, except for a storm event during postbloom conditions, for which the biological impact is underestimated. The bloom is long in duration (2 months), has low intensity (1 mg Chl m À3), and is characterized by a small f-ratio (0.45) and a small e-ratio (0.05). Furthermore, the model reveals much stronger space and time variability than sampled in the data. This large variability results both from the synoptic atmospheric variability and from the stirring induced by oceanic mesoscale eddies. In particular, the bloom starts in specific submesoscale features that correspond to filaments of minimum mixed layer depth. On short timescales (2-3 days), space and time variability have the same order of magnitude. On the seasonal timescale, time variability is larger than space variability. Considering the transient state of the system, this modeling exercise is also used to quantify the nonsynopticity of the observations, which occur mostly during bloom conditions, a crucial point for the data interpretation.

[1] A 5 year time series of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color images (SCHL) is compared with mixed layer depths (MLD) and atmospheric forcings from the Clipper model of the North Atlantic (1998–2002). This comparison is done over the region 16°–22°W, 30°–50°N, where subpolar mode waters are formed and which overlaps the region of the 2001 Programme Ocean Multidisciplinaire Meso Echelle (POMME) experiment at sea. Three production regimes are identified on the basis of the seasonal cycling of SCHL and MLD: the well-known subpolar and subtropical regimes and a midlatitude regime. The midlatitude regime is characterized by a single broad bloom weaker than the subpolar spring bloom and stronger than the subtropical fall bloom, which starts in fall as an entrainment bloom and peaks in spring as a restratification bloom. This specific regime is found between 35°N and 40°N (±2°) in the northeast Atlantic. It corresponds to winter MLDs between Ze (the depth of the euphotic layer) and 2Ze, i.e., it lays between the region where the winter MLD is greater than Sverdrup's critical depth (subpolar regime) and the region where the mixing is never deeper than the well-lit layer (subtropical regime). The very specific characteristics of the midlatitude regime strengthen the biological carbon pump since production is active in winter within the waters to be subducted. The midlatitude regime also may provide an explanation for the unexpectedly low f ratios sometimes observed during the bloom in the region (North Atlantic Bloom Experiment, POMME). A large interannual variability is observed for the three regimes in terms of the timing and the intensity of the blooms and of the geographical boundaries of the regimes. These variabilities appear to be mainly driven by the synoptic and the low-frequency atmospheric variabilities. It is also shown that in addition to the northward propagation of the subpolar spring bloom from 41°N (±1.3°) to 50°N, the (fall) entrainment bloom propagates southward over the whole latitudinal range (35°–50°N).

Conducted in the northeast Atlantic Ocean (15°20′–21°20′W, 38°N–45°N), the Programme Ocean Multidisciplinaire Meso Echelle (POMME) is a research project aimed at a better understanding of the biological production and the carbon budget of the region in relation to the formation mechanisms of the 11°–12°C mode water of the northeast Atlantic. With the help of two research vessels, several tens of floats and drifters, and nine moorings, the field experiment was carried out between autumn 2000 and autumn 2001, with a more intensive phase in the winter and early spring of 2001. The field experiment resolved small (several kilometers) to regional (several hundred kilometers) scales and daily to seasonal variability. A first analysis of the rich data set focused on the large-scale and the mesoscale variability. It shows that the distribution of water mass characteristics and biological activity is strongly influenced by the mesoscales in this supposedly quiet transition zone between the subtropical and subpolar gyres. The seasonal variability, however, presents an imprint of the large-scale structures with a clear north-south gradient in properties and budgets. This region is found on an annual average to be a sink of atmospheric CO2. Smaller scales, associated with fronts and filaments, were clearly observed in many fields (temperature, but also chlorophyll, oxygen, biogenic particles, etc.), with modeling studies suggesting that they play a significant role in subduction, ventilation, and transport of biogeochemical tracers in the POMME region.

[1] We examine the impact of mesoscale and submesoscale oceanic processes on the distribution of sea surface pCO2 to explain variability observed at length scales of order 10 km. We ask whether the large pCO2 excursions (50–150 μ-atm) that occur over 10–25 km of the sea surface could be induced by vertical advection associated with fronts and eddies in the pelagic ocean. A numerical model of a highly resolved, but idealized, mesoscale flow field is used to model the surface pCO2 response to submesoscale upwelling, taking into account the effect of wind, heat flux, and phytoplankton production. The effect of upwelled DIC on surface pCO2 is largely offset by the lower temperature of the upwelled water and the consumption of DIC by phytoplankton that respond to the simultaneously upwelled nitrate in a nutrient-limited setting. Since an upwelling-induced change in surface temperature, DIC, or nitrate is proportional to the vertical gradient of these properties beneath the mixed layer, the relative change in surface pCO2 is also dependent on the relative strengths of these gradients. We find that only small variations in surface pCO2 (∼10 μ-atm) are induced by submesoscale upwelling. The larger (50–150 μ atm) variations observed at small scales (∼10 km) are therefore not a direct consequence of submesoscale upwelling. Our results suggest that these surface pCO2 differences are likely to have been generated at larger scales (by differential properties and levels of biological productivity) and cascaded to smaller scales by horizontal advection at the sea surface.

42 pages, 10 figures, 4 tables.-- Pdf at Archimer http://www.ifremer.fr/docelec/ (Archive Institutionnelle de l’Ifremer), The physical and biogeochemical components of nutrients and inorganic carbon distributions along WOCE line A14 are objectively separated by means of a constrained least-squares regression analysis of the mixing of eastern South Atlantic water masses. Contrary to previous approaches, essentially devoted to the intricate South Atlantic circulation, this work is focused on the effects of circulation on nutrients and carbon biogeochemistry, with special emphasis on the stoichiometry and the rate of mineralization processes. Combination of nutrient and apparent CFC-age anomalies, derived from the mixing analysis, indicate faster mineralization rates in the equatorial (12 × 10−2 μmol P kg−1 yr−1) and subequatorial (5.3 × 10−2 μmol P kg−1 yr−1) than in the subtropical (4.3 × 10−2 μmol P kg−1 yr−1) regime at the South Atlantic Central Water (SACW) depth range. Lower rates are obtained in the Antarctic Intermediate Water (AAIW) domain (3.0 × 10−2 μmol P kg−1 yr−1). Significant variation with depth of O2/C/N/P anomalies indicates preferential mineralization of proteins in thermocline waters, as compared with the reference Redfield composition., Financial support from this work came from the Spanish ‘Comisión Interministerial de Ciencia y Tecnología (CICYT)’, contract No. ANT94–1168–E, and from the ‘Institut Français de Recherche pour l’exploitation de la Mer (IFREMER)’, contract No 94 1430 087.

International audience; One of the principal objectives of studying biogeochemical cycles is to obtain precise estimates of the main fluxes, such as total, new and export oceanic productions. Since models can incorporate the a priori knowledge of the most important processes, they are increasingly used for this purpose. However, biogeochemical models are characterized by a large number of poorly known parameters. Moreover, the available data are rather sparse in both time and space, and represent concentrations, not fluxes. Therefore, the major challenge is to constrain the relevant fluxes using information from a limited number of observations and from models incorporating poorly known internal parameters. The present study attempts to meet this challenge. In a 1D framework at the DYFAMED station (NW Mediterranean Sea), near-monthly nitrate and chlorophyll profiles and daily surface chlorophyll concentrations are assimilated in a coupled dynamical–biological model using the tangent linear and adjoint models. Following sensitivity analyses that show that some parameters cannot be recovered from the data set used, assimilation of observed 1997 data is performed. The first inversion considered clearly shows that, in agreement with previous studies, (1) the data impose a C/Chl ratio that varies with depth (i.e. light) and (2) the “initial” conditions (e.g. winter nitrate profile) strongly constrain the annual biogeochemical fluxes. After assimilation of the 1997 data, the agreement between the data and the model is quantitatively improved in 1995 and 1996, which can be considered a good validation of the methodology. However, the order of magnitude of the biogeochemical fluxes, and especially of the particulate export and regenerated production, are not correctly recovered. An analysis of the simulations shows that this result is associated with a strong decrease in zooplankton concentrations. An additional constraint of maintaining acceptable levels of zooplankton is therefore added. The results are improved, but remain unsatisfactory. A final inversion, which takes into account the a priori estimates of the major annual fluxes, is then performed. This shows that there is no inconsistency between the NO3 and chlorophyll data, the order of magnitude of the fluxes and the model. The work therefore demonstrates that recovering biogeochemical fluxes from available data of concentrations and stocks is not a straightforward exercise: the coverage and type of observations, and the nonlinearities of the biogeochemical model all contribute to this difficulty.

A 1D biogeochemical model simulating the nitrogen and carbon cycles is validated, using continuous observations obtained with the Carioca buoy (sea surface temperature (SST), pCO2; fluorescence) at the DYFAMED station (NW Mediterranean Sea) in 1995–1997, as well as other in situ data. Although the average surface pCO2 is generally slightly over-saturated (878matm), the NW Mediterranean Sea is a weak sink for atmospheric CO2 (0.1570.07 mol C m � 2 yr � 1 ), because the highest fluxes occur in winter and spring during the period of undersaturation when the winds are strong. The seasonal cycle is modulated by synoptic events (Mistral bursts), which can have a strong impact on the behaviour of the system and on the fluxes in winter and spring. While the atmospheric forcing constrains the annual balances and fluxes of tracers and CO2, the winter pre-conditioning of the bloom plays a major role in driving the interannual variability. Sensitivity analyses indicate that atmospheric forcing in this highly variable region should be averaged over periods of no longer than one week to simulate the biological and air–sea CO2 fluxes correctly. Also, because the model used is rather simple, this time scale is an upper limit. The sampling period must be no greater than a few days in order to estimate the CO2 fluxes with an error smaller than 20%. In the NW Mediterranean Sea, it is difficult to use ‘‘satellite proxies’’ (SST; sea colour) to estimate annual air–sea CO2 fluxes because SST does not vary much in winter and during the beginning of the bloom, while chlorophyll can either remain low (winter) or change rapidly (beginning of the bloom). At least several direct observations of pCO2 are needed during winter to define ‘‘initial conditions’’. r 2002 Published by Elsevier Science Ltd.

A number of processes are proposed to explain the time and space variability of the onset and decay of the spring phytoplankton bloom. This is done in the modeling framework of a case study most representative of the northwestern Mediterranean Sea (MEDOC area). The strategy followed is to isolate the different possible sources of variability (oceanic mesoscale dynamics, spring warming, wind bursts) in a series of process experiments (no flux, warming and wind experiments). The analysis of these experiments provides information for the analysis of a more realistic experiment, forced with daily atmospheric data (high-frequency experiment). On the basis of this study, we propose a categorization of the processes that control the spring bloom, in terms of their impact on the onset and decay of the bloom, and of the time and space scales on which they apply.

In the northwestern Mediterranean Sea, Coastal Zone Color Scanner images suggest that the eddies that participate in the restrati"cation following deep convection interact with the spring phytoplankton bloom. The mechanisms for this interaction are studied using a biogeochemical model embedded in an eddy-resolving primitive equation ocean model. The model is initialized with a patch of dense water surrounded by a strati"ed ocean, which is characteristic of the winter situation. The atmospheric forcing is arti"cially held constant, in order to focus solely on the mesoscale variability. After a few days, meanders develop at the periphery of the patch, inducing its sinking and spreading. Mesoscale upward motions are responsible for the shoaling of the mixing layer in the trough of the meanders. As sunlight is the main factor regulating primary production at this time of year, this shoaling increases the mean exposure time of the phytoplankton cells and thus enhances productivity. Consequently, the majority of phytoplankton production is obtained at the edge of the patch, in agreement with in situ data. Through advection, phytoplankton is then subducted from these sources towards the crest of the meanders. Our results suggest that this mesoscale transport is responsible for a decorrelation between phytoplankton biomass and primary production. ( 1999 Elsevier Science Ltd. All rights reserved.

International audience; Chlorofluoromethanes (CFMs) F-11 and F-12 were measured during August 1991 and November 1992 in the Romanche and Chain Fracture Zones in the equatorial Atlantic. The CFM distributions showed the two familiar signatures of the more recently ventilated North Atlantic Deep Water (NADW) seen in the Deep Western Boundary Current (DWBC). The upper maximum is centered around 1600 m at the level of the Upper North Atlantic Deep water (UNADW) and the deeper maximum around 3800 m at level of the Lower North Atlantic Deep Water (LNADW). These observations suggest a bifurcation at the western boundary, some of the NADW spreading eastward with the LNADW entering the Romanche and the Chain Fracture Zones. The upper core (σ1.5=34.70 kg m-3) was observed eastward as far as 5°W. The deep CFM maximum (σ4=45.87 kg m-3), associated with an oxygen maximum, decreased dramatically at the sills of the Romanche Fracture Zone: east of the sills, the shape of the CFM profiles reflects mixing and deepening of isopycnals. Mean apparent water "ages" computed from the F-11/F-12 ratio are estimated. Near the bottom, no enrichment in CFMs is detected at the entrance of the fracture zones in the cold water mass originating from the Antarctic Bottom Water flow.

We use a three-dimensional, off-line geochemical model to simulate the nitrate cycle in the equatorial Atlantic during the years 1983–1984 corresponding to the Francais Ocean et Climat dans l'Atlantique Equatorial (FOCAL) and Seasonal Response of the Equatorial Atlantic programs. After comparing our simulations with FOCAL data, we investigate interactions between equatorial circulation and biological activity on both seasonal and interannual timescales. Our results suggest that the upwelling of nitrate in the surface layer is strongly dependent on the behavior of both the nitracline and Equatorial Undercurrent (EUC). In the western basin, the equatorial upwelling partly feeds the EUC and has a low signature on surface nitrate. On the contrary, in the eastern basin, where the upwelling core and the nitracline are closer to the surface, vertical advection is the driving mechanism causing seasonal variations of nitrate concentration. Above the EUC, nitrate is transferred to the very surface by vertical diffusion, whereas the contribution by vertical advection is negligible. While slightly cold oceanic conditions prevailed in 1983, a warm anomaly produced by a decrease in trade winds and upwelling was observed in 1984. In our simulations, the significant changes in circulation do not notably alter the seasonal cycle of new production. Consequently, variations of annual primary production between 1983 and 1984 are small (9% decrease in the 2°S–2°N band) compared to the amplitude of the seasonal cycle (twofold variations). Contrary to the Pacific Ocean, where the interannual signal dominates, our results suggests that seasonal variability is the most significant large-scale signal on primary production in the equatorial Atlantic.

Skipjack tuna (#Katsuwonus pelamis$) contributes approximately to 70% of the total tuna catch in the Pacific Ocean. This species occurs in the upper mixed-layer throughout the equatorial region, but the largest catches are taken from the warmpool in the Western equatorial Pacific. Analysis of catch and effort data for US purse seine fisheries in the Western Pacific has demonstrated that one of the most successful fishing grounds is located in the vicinity of a convergence zone between the warm (greater than 28-29° C) low-salinity water of the warmpool and the cold saline water of equatorial upwelling in the central Pacific (Lehodey et al, 1997). This zone of convergence, identified by a well-marked salinity front and approximated by the 28.5° C isotherm, oscillates zonally over several thousands of km in correlation with the El Nino-Southern Oscillation. The present study focuses on the prediction of skipjack tuna forage that is expected to be a major factor in explaining the basin-scale distribution of the stock. It could also explain the close relation between displacements of skipjack tuna and the convergence zone on the eastern edge of the warmpool. A simple bio-geochemical model was coupled with a general circulation model, allowing reasonable predictions of new primary production in the equatorial Pacific from mid-1992 to mid-1995. The biological transfer of this production toward tuna forage was simply parameterized according to the food chain length and redistributed by the currents using the circulation model. Tuna forage accumulated in the convergence zone of the horizontal currents, which corresponds to the warmpool/equatorial upwelling boundary. Predicted forage maxima corresponded well with high catch rates. (Résumé d'auteur)

A biogeochemical model, BIOMELL (BIOgeochemical Model of the Euphotic Layer of Lodyc), has been developed to simulate the temporal evolution of the main nitrogen stocks and e uxes at the DyFAMed station (Dynamique des FluxAtmospheriques en Mediterranee), located in the Northwest- ern Mediterranean Sea. This mainly oligotrophic region is characterized by a strong seasonal cycle, and a signie cant export of dissolved organic matter compared to the particulate export measured by sediment traps. Validation of the model is made using temperature, nitrate and chlorophyll proe les acquired at DyFAMed approximately every month in 1991. Extended datasets from specie c years are also used to validate seasonal variations of other variables, for which the coverage in 1991 was rather poor (new and total production, particulate export, dissolved organic matter export, bacteria, zooplankton). Sensitivity studies on selected parameters are carried out in order to give an idea of the margin of error in the model predictions. The model is then used to analyze the behavior of the biogeochemical system during various production regimes (winter, spring bloom, oligotrophy, autumn bloom). It is shown that several processes, which are often neglected in biogeochemical models, must be taken into account: phytoplankton growth limitation by deep mixing, C:Chl ratio dependence on light, nitrie cation, and semi-refractory dissolved organic matter accumulation in the surface layer.

A simple, 3-D biogeochemical model is coupled to a dynamical model forced by weekly winds deduced from ERS1 scatterometer data, to simulate new production in the equation Pacific from April 1992 to June 1995. The biogeochemistry is modelled as a nitrate sink modulated by chlorophyll, using nitrate/chlorophyll regressions derived from field data. The first simulation was carried out assuming that remineralization below the euphotic layer is totally controlled by sinking particles. In the second simulation, it is shown that the simulation of nitrate and new production by the biogeochemical model is improved, in comparison with field data, by adding an explicit dissolved organic nitrogen compartment. In the equatorial band, the model simulates a nitrate-poor region (low new production) in the fresh warm pool separated from richer waters of the upwelling region by a salinity front. The zonal displacement of this salinity/nitrate front is associated with the El-Nino-Southern Oscillation (ENSO). The modeled new production and physics, both in good agreement with the field data, represent useful tools for the study of skipjack tuna (#Katsuwonus pelamis$) forage distribution in the Pacific. (Résumé d'auteur)

International audience; A quasi-meridional hydrographic section carried out between 60° and 20°N offshore from the European and African continental slopes is analyzed in terms of water masses and zonal transports in and out of the eastern boundary. Outstanding features of the meridional distribution of water masses are focused on, such as the transition between North Atlantic and South Atlantic Central Waters at 20°–25°N, the properties and anticyclonic circulation of the Rockall Channel mode water, and the northern boundary of the large-scale Mediterranean Water plume at about 50°N. An eastward transport of about 11×106 m3 s−1 is found to enter the eastern boundary layer at densities lower than 27.25 and feed southward alongshore currents. The relation of the incoming transport to the water mass distribution and its eventual splitting into several outflowing components are discussed. Apart from the downward entrainment of upper water known to occur in the Gulf of Cadiz, there is no sign of the so-called “eastern boundary ventilation” mechanism in the central water density range. Yet a significant transport is found to escape the winter mixed layer toward the interior of the subtropical gyre, suggesting the horizontal southward currents across the sloping bottom of the mixed layer to be the main cause of ventilation at the eastern boundary.

The North Atlantic circulation derived from an inverse calculation by singular-value decomposition is tested against the historical record of tritium. A forward calculation of the tritium transient is performed using the circulation model, published estimates of atmospheric injection rates, and plausible estimates of the tracer history at the open boundaries of the model. The results do not agree with observations of the interior distributions of tritium. Consideration is given to the possibility of improving the agreement by modifying the atmospheric injection rates and the initial estimates of open boundary time histories, treating the boundary conditions as control variables.

Gas exchange experiments have been performed in a wind-water tunnel filled with fresh water or seawater. Transfer velocities have been measured for nitrous oxide and argon in a range of wind speeds extending from 3 m/s to 14 m/s. The air-liquid interface was covered either with only normally developed wind waves or with mechanically generated waves in addition. For u 9 m/s), a jump of the values of kL up to a factor of 3 is observed first for Ar and then for N2O, the ratio kL Ar/kL being highly variable. This observation is interpreted as being the result of the onset of breaking waves which create bubbles through which mass transfer takes place. No difference for gas exchange either in fresh water or in seawater has been identified from the whole set of experiments. A theoretical model is developed to compute the contribution of bubbles to gas exchange. The transport equations, derived for a single bubble, take into account the diffusive mass flux exchanged between the bubble and the liquid, the hydrodynamic pressure, and the presence of two gaseous components inside the bubble. The equations used to represent the dynamics take care of the orbital motion of water associated with waves. A relation to link sources and spectra of bubble populations is derived. Numerical values for spectral parameters are taken from the literature. It is found that the results of experiments run at 9 m/s and 10 m/s are well explained for N2O and for Ar. A unique distribution of bubbles explains the excess of gas transfer associated with the presence of breaking waves. In a further step the theoretical treatment is extended to compute the transfer velocity kB for gas exchange through bubbles as a function of the concentration gradient in atmosphere and water for three gases, helium, argon, and nitrous oxide. It is shown that in wind tunnel conditions, kB is independent of the concentration gradient. This does not hold at the atmosphere-ocean interface. The transfer velocity kB is highly dependent on the gas concentrations in air and water when they are close to solubility equilibrium. It is demonstrated that equilibrium must occur at the ocean surface for supersaturation conditions. The higher saturation anomalies and the larger transfer velocities are computed for the less soluble gas. This conclusion is in good agreement with observations made at sea.

Eddy correlation measurements over the ocean give CO2 fluxes an order of magnitude or more larger than expected from mass balance measurements using radiocarbon and radon 222. In particular, Smith and Jones (1985) reported large upward and downward fluxes in a surf zone at supersaturations of 15% and attributed them to the equilibration of bubbles at elevated pressures. They argue that even on the open ocean such bubble injection may create steady state CO2 supersaturations and that inferences of fluxes based on air-sea pCO2 differences and radon exchange velocities must be made with caution. We defend the global average CO2 exchange rate determined by three independent radioisotopic means: prebomb radiocarbon inventories; global surveys of mixed layer radon deficits; and oceanic uptake of bomb-produced radiocarbon. We argue that laboratory and lake data do not lead one to expect fluxes as large as reported from the eddy correlation technique; that the radon method of determining exchange velocities is indeed useful for estimating CO2 fluxes; that supersaturations of CO2 due to bubble injection on the open ocean are negligible; that the hypothesis that Smith and Jones advance cannot account for the fluxes that they report; and that the pCO2 values reported by Smith and Jones are likely to be systematically much too high. The CO2 fluxes for the ocean measured to date by the micrometeorological method can be reconciled with neither the observed concentrations of radioisotopes of radon and carbon in the oceans nor the tracer experiments carried out in lakes and in wind/wave tunnels.

16 páginas, 12 figuras, 2 tablas, The meridional WOCE line A14, just east of the South Atlantic Mid-Atlantic Ridge, was surveyed during the austral summer of 1995 from 4°N to 45°S. Full-depth profiles of pH, total alkalinity (TA), and total inorganic carbon (CT) were measured, allowing a test of the internal consistency of the CO2 system parameters. The correlation between CT measured and calculated from pH and TA was very good (r2 = 0.998), with an insignificant average difference of 0.1 ± 3.0 μmol kg−1 (n = 964 data). CO2 certified reference materials (CRMs) and a collection of selected samples subsequently analyzed at the Scripps Institution of Oceanography were used to assess the accuracy of our measurements at sea with satisfactory results. The three measured CO2 system variables were then used to identify the characteristic array of zonal flows throughout the South Atlantic intersected by A14. Equatorial, subequatorial, subtropical, and subantarctic domains were identified at the depth range of the surface water, South Atlantic Central Water (SACW), Antarctic Intermediate Water (AAIW), Upper Circumpolar Water (UCPW), North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW). The nonconservative CO2 system parameters (pH, TA, CT) have been useful in identifying the transition from aged subequatorial to ventilated subtropical surface, central and intermediate waters. They have been identified as good tracers of the zonal circulation of NADW, with marked flows at the equator, 13°S, and 22°S (the “Namib Col Current”) and the sharp transition from UNADW to UCPW at 23°S. The anthropogenic CO2 inventory (CANT) was estimated and compared with CFC-derived apparent ages for different water masses along A14. The anthropogenic entry reached maximum in the relatively young and ventilated subantarctic and subtropical domains where AAIW was the most efficient CO2 trap. The calculated annual rate of CANT entry by AAIW was 0.82 μmol kg−1 y−1, in agreement with the annual rate estimated from the equilibrium between the atmospheric pCO2 increase and the upper mixed layer., Special thanks to M. Arhan (coordinator of the WOCE-France program CITHER) and H. Mercier (chief scientist of cruise CITHER 3). This work was supported by the Spanish CICYT (grant ANT94-1168-E) and IFREMER (University contract 94 1430 087).

In a collaboration between the Laboratoire de Geochimie Isotopique (Centre d'Etudes Nucleaires, Saclay), the Institut de Mecanique Statistique de la Turbulence (IMST, Marseille), and the Institut fur Umweltphysik (Heidelberg), for the first time gas exchange experiments have been carried out in the large IMST wind-wave facility. The experiments included simultaneous measurements of Rn and He gas exchange rates, wave slope measurements at four fetches, and bubble measurements. Compared with transfer velocities measured previously in smaller tunnels, our results are considerably lower. This effect can be explained qualitatively by differences in the wave field, which must be taken into account as an important parameter for gas exchange. Wave breaking, starting at 12 m/s wind, was not intense. Consequently, only low bubble densities are obtained, not significantly enhancing gas exchange.

The most recent models dealing with CO2 uptake by the ocean divide the ocean by latitude, but they do not take into account the influence of the different wind regimes on the CO2 fluxes across the air-Water interface. However, new gas exchange experiments run in wind tunnels have shown a great enhancement of the transfer velocity at high wind speeds as a result of the development of breaking waves. A study of sensitivity of two different models (Siegenthaler outcrop box diffusion model and Takahashi-Azevedo inclined box diffusion model) to an increase of the CO2 transfer velocity at high latitudes (outcrop regions of the deep ocean) is undertaken. It is shown that the variations in the transfer velocity with latitude must be taken into account.

The oceanic cycles of carbon and the main nutrients are poorly known since they are affected by many physical, chemical or biological sources and sinks that are difficult to estimate by direct measurements.One way to better constrain these important processes is to use the information contained in more simple tracers called "proxies". As radium 228 (228Ra) flows from the continental shelves, it is used as a proxy of water and mineral elements fluxes from the coast to the open ocean. In particular, it is often used to estimate the SGD (Submarine Groundwater Discharge). For its part, thorium 234 (234Th), an insoluble radio-isotope, is used to constrain the dynamics of the solid particles onto which it is adsorbed. The carbon flux from the surface to the deep ocean, called "biological carbon pump" (BCP), is often estimated by a 234Th-based method.During this PhD, a numerical model with a resolution of 2°, based on the circulation of the NEMO-OPA model and the particle fields of the PISCES model, was built for each of the two radioisotopes.Several unknown model parameters were constrained by observations using an inverse technique.The inverse modeling of 228Ra was used to constrain 228Ra fluxes from 38 coastal regions.However, the SGD fluxes are poorly constrained by this method, because SGD can be confused with another source of 228Ra: diffusion from sediments.The inverse modeling of 234Th produced estimates of partition coefficients, representing the affinity of different particle types for this isotope. It was also used to estimate the errors associated with some common simplifications made in 234Th-based BCP studies., Les cycles océaniques du carbone et des principaux nutriments sont mal connus car ils sont affectés par de nombreux puits et sources physiques, chimiques ou biologiques difficiles à estimer par des mesures directes. Une manière de mieux contraindre ces processus importants est d’utiliser l’information contenue dans des traceurs plus simples : les proxies. Le radium 228 (228Ra), émis par les plateaux continentaux, est utilisé comme proxy des flux d’eau et d’éléments minéraux de la côte vers l’océan ouvert. Il permet en particulier d’estimer les flux d’eau souterraine ou SGDs (Submarine Groundwater Discharge). Le thorium 234(234Th), insoluble, permet quant à lui de contraindre la dynamique des particules par lesquelles il est adsorbé. Il est régulièrement utilisé pour estimer la pompe biologique du carbone (PBC), c’est-à-dire le flux de carbone de la surface vers l’océan profond.Au cours de cette thèse, un modèle numérique à une résolution de 2° a été construit pour chacun de ces deux radio-isotopes, en s’appuyant sur la circulation du modèle NEMO-OPA et les champs de particules du modèle PISCES. Plusieurs paramètres inconnus des modèles ont été contraints par des observations dans le cadre d’une méthode inverse.La modélisation inverse du 228Ra a permis d’estimer les flux de 228Ra venant de 38 régions côtières. En revanche, l’estimation des SGDs est imprécise, car les SGDs sont difficiles à distinguer d’une autre source de 228Ra: la diffusion par les sédiments.La modélisation inverse du 234Th a permis d’estimer les coefficients de partage du 234Th, qui représentent l’affinité de différents types de particules pour cet isotope. Elle a aussi permis d’estimer les erreurs associées à quelques simplifications courantes dans les études de la PBC fondées sur le 234Th.

Phytoplankton diversity depends on physical, chemical and biological local conditions. Moreover, physical transport could also impact the distribution of autotrophic organisms/species within phytoplankton communities. Therefore, phytoplanktonic assemblages exhibit significant spatial (bioregionalization) and temporal (successions) variability in terms of species diversity as well as productivity. Coastal regions are particularly contrasted area with sharp environmental gradients underlying strong heterogeneity in phytoplankton communities' composition. In this context, the Iroise Sea presents a seasonal, highly productive, tidal front, which separates two distinctregimes. Eastside, continental shelf waters are regularly mixed by tidal currents while offshore waters remain stratified throughout the whole summer period.Thus, the Iroise Sea may be an opportune region to study the processes linking the frontal structure dynamic and its impact on phytoplanktonic diversity.This thesis aims, more specifically, at characterizing phytoplankton communities in the Iroise Sea interms of both functional and phenotypic diversity over a seasonal cycle in general and during the summer at the frontal interface in particular. This work is based on a 3D numerical modeling approach using a physical/biogeochemical coupled model. As a first part of this thesis, the implementation of a regional configuration for the Iroise Sea has been conducted, including the technical coupling between the hydrodynamical model (ROMS-AGRIF) and the phytoplankton diversity model (DARWIN). This work was the first necessary step to simulate and study the Iroise sea phytoplanktonic diversity.Our results show a pronounced seasonal cycle of the phytoplankton functional composition, driven by the surface mixed layer depth that influenced light and nutrients' availabilty. Indeed, during winter, the biomass is dominated by picoplankton in the Iroise Sea. Then, as water column becomes stratified offshore, in April, a characteristic phytoplankton bloom occurs with a larger contribution of microphytoplancton (mainly diatoms). During summer, the nutrient-replete coastal well-mixed area remains highly productive and dominated by diatoms while surface stratified offshore waters (where phytoplankton growth is nutrient-limited) show higher coexistence between phytoplankton functional types.We also examined the phytoplankton phenotypic diversity simulated within the frontal region in relation with the surrounding areas in September (summer conditions). The results highlight a diversity maximum located at the surface slightly westward from the biomass maximum of the front. This diversity maximum is suggested to be the result of the upward transport of typical phenotypes from the offshore Deep Chlorophyll Maximum (DCM) toward the Surface at the west warm side of the front. Indeed, picoplanktonic phenotypes growing in the DCM coexist, in this diversity maximum, with those from the surface oligotrophic waters and ubiquitous ones growing in the eastside mixed coastal waters.Finally, the effect of tidal spring/neap tide cycle has been investigated and shows, for the first time, how this cycle impacts the phytoplankton biomass and the phenotypic community composition within the coastal well-mixed, homogeneous system. Indeed, the neap-tide conditions of reduced vertical mixing and stabilization lead to an increase in total phytoplankton biomass associated with the rapid development of fast-growing, opportunistic, diatoms species and a decrease in phenotypic diversity., Les facteurs environnementaux (physiques, chimiques et biologiques) influencent la composition des communautés de phytoplancton marin. D'autre part, le transport dynamique peut lui aussi impacter l'abondance relative des organismes au sein de ces communautés. Par conséquent, les biomasses phytoplanctoniques, ainsi que la nature des organismes qui les composent, présentent une importante variabilité à la fois sur le plan spatial (biorégionalisation) et sur le plan temporel (successions). Les régions côtières sont des zones particulièrement contrastées dans lesquelles les gradients environnementaux sont généralement marqués. Ainsi, la mer d'Iroise se caractérise par la présence d'un front de marée saisonnier (front d'Ouessant), particulièrement productif, qui sépare deux régimes distincts. A l'Est du front, les eaux du plateau continental sont régulièrement rendues homogènes par la présence de forts courants de marée alors que la zone du large est soumise à un cycle saisonnier marqué par une stratification verticale estivale. Il s'agit donc d'une région propice à l'étude plus générale des mécanismes d'interaction entre les structures frontales et la diversité phytoplanctonique.L'objet plus spécifique de cette thèse est de caractériser, à l'aide de la modélisation 3D, la nature et la diversité du phytoplancton en mer d'Iroise, à la fois en termes de groupes fonctionnels puis de diversité phénotypique, au cours d'un cycle saisonnier en général et plus particulièrement lors de la période estivale au niveau de la zone frontale.Les premiers résultats obtenus ont montré que la composition en groupes fonctionnels du phytoplancton présente un cycle saisonnier marqué, principalement influencé par la profondeur de la couche de mélange. Durant l'hiver, le picoplancton domine partout dans la zone d'étude. La stratification, qui s'installe à partir du mois d'Avril entraîne par la suite un bloom phytoplanctonique dominé par le microphytoplancton (principalement des diatomées). La période estivale correspond ensuite à la mise en place d'une bio-régionalisation des conditions environnementales en mer d'Iroise avec (i) la zone côtière mélangée qui reste fortement productive et dominée par les diatomées et (ii) la zone offshore, dans laquelle la croissance autotrophe est limitée par les nutriments en surface, ce qui favorise la coexistence entre microphytoplancton et picophytoplancton.Une seconde étude a porté sur la composition de la communauté phytoplanctonique en termes de diversité phénotypique pendant le mois de septembre. Les résultats mettent en évidence une zone de forte diversité en surface, légèrement décalée vers l'Ouest par rapport à la zone frontale (dans laquelle la biomasse est maximale). Au niveau de ce maximum de diversité, l'importance des échanges verticaux (upwelling et mélange) du côté chaud (stratifié) du front a été mise en évidence. Ainsi, un mélange entre des phénotypes ubiquistes présents dans la zone mélangée à l'Est du front et du picoplancton, issu à la fois du maximum de chlorophylle de subsurface et de la surface oligotrophe à l'ouest, est observé dans le maximum de diversité.Finalement, une dernière étude portant sur l'effet du cycle de marée vives eaux/ mortes eauxa permis de comprendre, et ce pour la première fois, les processus qui expliquent l'impact de ce cycle sur la modification de la biomasse phytoplanctonique et sur la composition de la communautéen terme de diversité phénotypique dans le système côtier homogène. Les résultats montrent une augmentation de la biomasse totale ainsi que de la proportion de diatomées et une diminution de la diversité lors des périodes de stratification associées aux mortes eaux.

The objective of this thesis is to study the physical and biogeochemical characteristics of the pelagic habitat (0-200m), its spatio-temporal variability and the processes that govern them in the Gulf of Tadjourah (Djibouti).Singular spectrum analysis (SSA) and empirical orthogonal function (EOF) were applied to two data sets derived from AVHRR_MetopA radiometry and sea colour sensors (Modis and Meris). These statistical analysis shows that the variability of sea surface temperature (SST) and chlorophyll a concentration (CHLa) are mainly explained by annual and semi-annual cycles. The annual cycle of SST consists of an alternation between warmer (April to October) and cooler (November to March) surface water. The semi-annual cycle shows a slight drop of SST between July and August, particularly in the west of the gulf. For the CHLa, the variability is fully dominated by the annual cycle indicating summer enrichment of seashore water (July-November) with a strong seaward gradient which is reversed from October to May. Multichannel spectrum analysis (M-SSA) and cross correlation function analysis applied to the oceanic (SST, CHL-a) and atmospheric parameters (wind speed, air temperature and humidity) showed that the annual cycle of SST is connected to heat flux at the air-sea interface, while the summer drop of SST and its associated CHL-a increase were attributed to upwelling. In the second part, in order to elucidate this oceanic influence on SST and CHL-a, the vertical thermohaline and biogeochemical structures of the upper layer (0-200 m) were studied using high-resolution hydrographic data collected in July-August 2013, September 2013 and February 2014. During summer, the superficial layer consisted of the mixed layer (ML) extending to a depth of about 20-30 m, followed by the thermocline located between 30 and 50 m depth. The ML was thicker in the west and the southeast where the thermal gradient and chlorophyll a concentrations were particularly high. During September, this stratification persisted but the ML became warmer and saltier and the thermocline moved slightly deeper. In February, the ML extended to about 120 m, and the thermocline was less pronounced. A comparison of the directly measured currents to the wind induced Ekman currents and to geostrophic current profiles revealed that the thermohaline and the biogeochemical features were related to the southeast and northeast monsoon winds (SWM & NEM). Between June and August, the SWM drives surface water from the Gulf of Tadjourah to the Gulf of Aden and thus induces the westward intrusion of high salinity thermocline water from the Gulf of Aden. This near surface flow mixes surface waters in the extreme west of the Gulf of Tadjourah. In contrast, the NEM which blow from September to May, bring cold water toward the Gulf of Tadjourah and thickens the ML through convective mixing. Our thesis shows that the monsoon winds and their seasonal reversal play a crucial role in the stratification of the water column, but that their effects are additionally influenced by basin topography. During summer the bowl-shape of the basin and its elongated slope in the west enhance the upwelling in this area where negative sea surface temperature anomalies and high chlorophyll a concentrations were observed., L’objectif de cette thèse est d’étudier les caractéristiques physiques et biogéochimiques de l’habitat épipélagique (0-200 m), ses variabilités spatio-temporelles et les processus qui les gouvernent dans le Golfe de Tadjourah (Djibouti). L’analyse spectrale singulière (SSA) et la fonction empirique orthogonale (EOF) sont appliquées à deux jeux des données satellitales dérivées du radiomètre AVHRR_MetopA et des capteurs de la couleur de l’océan (Modis et Meris). Cette analyse statistique montre que les variabilités de la température de la surface de la mer (SST) et de la concentration de la chlorophylle a (CHLa) sont essentiellement expliquées par les cycles annuels et semi-annuels. Le cycle annuel de la SST montre l’alternance des eaux chaudes d’avril à octobre et des eaux froides de novembre à mars. Le cycle semi-annuel indique une légère baisse de la SST entre juillet et aout, particulièrement à l’ouest du golfe. Pour la CHLa, la variabilité est entièrement représentée par le cycle annuel. Celui-ci indique l’enrichissement des eaux du large avec un fort gradient côte-large de juillet à novembre et une tendance inverse de décembre à juin. En outre, l’analyse spectrale singulière multi-canal (M-SSA) et la fonction de corrélation croisée avec fenêtre de 120 jours, appliquées à l’ensemble des paramètres océanique (SST et CHLa) et atmosphériques (Vent, Température et humidité spécifique de l’air) révèlent que le cycle annuel de la SST est relié aux flux de chaleur à l’interface air-mer. En revanche, le refroidissement des eaux durant juillet-aout associé au pic de la CHLa, a été attribué au phénomène d’upwelling. Dans la seconde partie, afin d’élucider l’influence océanique sur la SST et la CHLa, les structures thermohalines and biogéochemiques de la couche supérieure (0-200 m) sont étudiées à l’aide des données collectées durant juillet-aout 2013, septembre 2013 et Février 2014. Les résultats montrent qu’en juillet-aout, la couche superficielle se composait d’une couche de mélange (CM) s’étendant sur environ 20-30 m de profondeur, suivie d’une thermocline localisée entre 30 and 50 m. La CM était réduite à l’ouest et au sud-est du golfe où le gradient thermique et la CHLa étaient plus élevées proche de la surface. En septembre, cette stratification persistait mais la CM était plus chaude et salée. En Février, la CM s’étendait sur environ 120 m de profondeur et la thermocline était moins prononcée. La comparaison des courants mesurés avec les courants de dérive d’ekman et les courants géostrophiques ont révélé que les structures thermohalines et biogéochimiques sont influencées par les vents de moussons du Sud-Ouest (MSO) et du Nord-Est (MNE). Les MSO qui soufflent de juin à aout, déplacent les eaux de surface du Golfe de Tadjourah vers le Golfe d’Aden et induisent la remontée des eaux profondes à l’ouest et l’intrusion par le nord-est des eaux salée de la thermocline. Celle-ci se rapproche de la surface particulièrement à l’ouest où elle se mélange avec les eaux de surface. En revanche, les vents de moussons du nord-est (MNE) prédominants de novembre à mai, emmènent les eaux froides vers le Golfe de Tadjourah. Le mélange convectif profond épaissit la CM. Cette thèse montre que les vents de moussons et leur renverse saisonnière jouent un rôle crucial dans la stratification de la colonne d’eau et que la topographie du bassin influence et module leurs effets. Durant l’été, la forme en cuvette du bassin et la pente continentale plus allongée à l’ouest favorise l’upwelling à l’ouest du golfe où les anomalies de la SST et de fortes concentrations de la CHLa ont été observées.

This thesis falls in a realistic ocean modelling framework, focused on the study of the Iroise Sea. The first two parts deals with the numerical set up and the validation compared with observations. The emphasis is then on the analysis of the main physical structures and process at work in Iroise Sea: first the Ushant tidal front and mixed layer connexion process (surface and bottom) associated, then we analysed the mean eulerian residual circulation, for which we tried to isolate the relative contributions of the different physical process acting on it. Then, we mapped the different dynamical regime at work: we observe cyclostrophic regime near islands and in shallow water area and a strong geostrophic at the tidal front, especially in summer. Nevertheless, in the main part of the study domain, the different dynamical terms are acting of an equivalent. To a thermal point of view, the analysis of heat budget shows that the air-sea flux dominate largely the advective flux, although these ones induce a high frequency modulation in the net heat gain. Then, a diagnostic parameter has been established and the role of different external forcings (air/sea heat fluxes, wind stress, spring/neap tidal cycles) upon the Ushant front variability has been highlighted through sensitivity experiments. Finally, the study of the cross-frontal mass transport, based on a lagrangian particles tracking technique, has been established. The methodology is reliable and the first results reveal that these exports take place through specific latitude "channel"., Cette thèse s'insère dans un cadre de modélisation réaliste pour l'étude de la mer d'Iroise. Les deux premières parties concernent la mise en œuvre numérique et la validation par rapport aux observations. L'accent est ensuite mis sur l'analyse des principales structures et processus physiques à l'œuvre en mer l'Iroise : d'abord le front de marée d'Ouessant et les processus associés de connexion de couches de mélange de surface et de fond puis la circulation résiduelle eulérienne moyenne pour laquelle nous avons cherché à quantifier la part relative des différents processus la régissant. Nous avons ensuite cartographié les différents régimes dynamiques présents : on observe ainsi des régimes cyclostrophiques autour des îles et en zone peu profonde et un régime géostrophique apparaît nettement au niveau du front, en particulier en été. Néanmoins, dans la plus grande partie du domaine d'étude, les différents termes dynamiques interviennent de manière équivalente. Du point de vue thermique, l'analyse du bilan de chaleur indique que les flux air-mer dominent nettement les flux advectifs bien qu'ils induisent une modulation haute fréquence marquée du gain net de chaleur. Ensuite, un paramètre diagnostic du front a ensuite été établi et le rôle de différents forçages externes (flux de chaleur air/mer, tension de vent, cycle de marée mortes-eaux/vives-eaux) sur la variabilité du front d'Ouessant est mis en relief grâce à des expériences de sensibilité. Enfin, l'étude des échanges de masse cross-frontaux, basée sur une technique de suivi de particules lagrangiennes, a été mise en place. La méthodologie apparaît robuste et les premiers résultats indiquent que ces exports s'effectuent selon des 'canaux' privilégiés de latitude.

The present modeling study aims to estimate the impact of dynamic variability, from large to sub-mesoscale, on planktonic ecosystem structure and functioning in the North-East Atlantic Ocean. In a first part, we investigate the ecosystem seasonal cycle using 1D configuration. On average, the model shows a strong annual cycle, with important regeneration fluxes, forced by the evolution of the mixed layer depth (MLD). Considering the MLD distribution, the model reproduces meridian variations of the seasonal cycle and of the flux amplitudes. In a second part, a 3D configuration shows contrasting ecosystem behavior between eddies structures and surrounding waters. At sub-mesoscale, the model indicates important MLD gradients and vertical velocities which modify the input of light and nutrients, and hence the primary production. The impact of the three considered scales on the plankton production varies according to the seasonal cycle.; Cette étude vise à estimer l'impact des échelles dynamiques, allant de la grande à la sub-mésoéchelle, sur la structure et le fonctionnement de l'écosystème planctonique, en s'appuyant sur la modélisation. Elle se situe en Atlantique Nord-Est dans le cadre du programme POMME. Une première partie étudie le cycle saisonnier de l'écosystème à partir d'une configuration 1D. En moyenne, le modèle montre un cycle annuel marqué, avec des flux de régénération importants, contraint par l'évolution de la profondeur de couche de mélange (MLD). Le modèle reproduit les variations méridiennes du cycle saisonnier et de l'amplitude des flux liées à la variabilité spatiale de la MLD. Une seconde partie, basée sur une configuration 3D, montre un écosystème contrasté entre les tourbillons à mésoéchelle et les eaux ambiantes. A sub-mésoéchelle, de forts gradients de MLD et de vitesses verticales modifient les apports de nutriments ainsi que la disponibilité en lumière, et donc la production primaire. Les trois échelles considérées ont une influence significative sur la distribution et la production planctonique, influence qui varie au cours du cycle saisonnier.