Contrasting responses of the ocean’s oxygen minimum zones to artificial re-oxygenation

Studies assessing potential measures to counteract the marine deoxygenation attributed to anthropogenic activities have been conducted in a few coastal environments and at regional scale, but not yet on a global scale. One way toward global scale artificial oxygenation would be to use oxygen produced as a by-product from hydrogen-production through electrolysis. The low-carbon footprint renewable production of hydrogen from offshore wind energy offers such a possibility. Here, we assessed the potential of this artificial oxygenation method on a global scale using a coupled physical-biogeochemical numerical model. The anthropogenic oxygen source scenario assumes worldwide adoption of hydrogen, considering demographic changes and the feasibility of offshore wind turbine deployment. Following this scenario, artificial oxygenation had a negligible effect on the overall oxygen inventory (an increase of 0.07%) but showed a reduction in the overall volume of Oxygen Minimum Zones (OMZs) between 1.1% and 2.4%. Despite the decrease in the mean OMZ volume globally, OMZs display distinct and contrasting regional patterns notably due to the oxygen impacts on the nitrogen cycle. Artificial oxygenation can inhibit denitrification resulting in a net gain of nitrate that promotes locally and remotely increased biological productivity and consequent respiration. Increased respiration could ultimately lead to an oxygen loss at and beyond injection sites as in the Tropical Pacific and Indian Ocean and particularly expand the Bay of Bengal OMZ. In contrast, the tropical OMZ shrinkage in the Atlantic Ocean is attributed to oxygen enrichment induced by advective transport into the OMZ, while the absence of denitrification in this area precludes any biochemical feedback effect on oxygen levels. These results suggest that the impacts of artificial oxygenation on oxygen concentrations and ecosystems are highly non-linear. It can produce unexpected regional responses that can occur beyond the injection sites which make them difficult to forecast.publishedVersio

Linking Spatial and Temporal Dynamic of Bacterioplankton Communities With Ecological Strategies Across a Coastal Frontal Area

Ocean frontal systems are widespread hydrological features defining the transition zone between distinct water masses. They are generally of high biological importance as they are often associated with locally enhanced primary production by phytoplankton. However, the composition of bacterial communities in the frontal zone remains poorly understood. In this study, we investigate how a coastal tidal front in Brittany (France) structures the free-living bacterioplankton communities in a spatio-temporal survey across four cruises, five stations and three depths. We used 16S rRNA gene surveys to compare bacterial community structures across 134 seawater samples and defined groups of co-varying taxa (modules) exhibiting coherent ecological patterns across space and time. We found that bacterial communities composition was strongly associated with the biogeochemical characteristics of the different water masses and that the front act as an ecological boundary for free-living bacteria. Seasonal variations in primary producers and their distribution in the water column appeared as the most salient parameters controlling heterotrophic bacteria which dominated the free-living community. Different dynamics of modules observed in this environment were strongly consistent with a partitioning of heterotrophic bacterioplankton in oligotroph and copiotroph ecological strategies. Oligotroph taxa, dominated by SAR11 Clade members, were relatively more abundant in low phytoplankton, high inorganic nutrients water masses, while copiotrophs and particularly opportunist taxa such as Tenacibaculum sp. or Pseudoalteromonas sp. reached their highest abundances during the more productive period. Overall, this study shows a remarkable coupling between bacterioplankton communities dynamics, trophic strategies, and seasonal cycles in a complex coastal environment

Challenges in integrative approaches to modelling the marine ecosystems of the North Atlantic: Physics to Fish and Coasts to Ocean

It has long been recognized 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

Cycle benthique du silicium dans les estuaires (observations et modélisation à différentes échelles spatio-temporelles)

Les estuaires sont des zones complexes et hétérogènes, soumises à de fortes pressions anthropiques, qui peuvent avoir un rôle déterminant de filtre lors du transport et de la transformation de la matière vers les zones côtières. Le cycle du silicium (Si), essentiel à la croissance des diatomées qui constituent la base des réseaux trophiques sains, est encore mal défini au sein de ces interfaces. Cette thèse a pour objectif d étudier le cycle benthique du Si dans les deux principaux estuaires de la Rade de Brest. Afin d aborder les interactions d échelles propres aux estuaires, et de les intégrer aux estimations de flux, les hétérogénéités spatiales et les variations tidales du cycle benthique du Si ont été quantifiées et comparées aux variations saisonnières le long des estuaires. Dans le but d étudier les interactions entre les cycles de la matière (N, P, C, Si) exerçant un contrôle sur le fonctionnement des écosystèmes côtiers, les interactions entre Si et Pont été explorées et semblent favoriser la rétention de P. Enfin, le cycle benthique du Si a été étudié à l aide d u outil de modélisation de la diagénèse précoce, afin de palier à la difficile estimation directe des flux de dépôt dans les estuaires, et évaluer le rôle des estuaires dans la rétention et le recyclage de Si. Cette première contribution à l étude du fonctionnement du cycle du Si dans les estuaires de la Rade de Brest ouvre de nombreuses perspectives en termes d études de processus et de modélisation, que ce soit dans la perspective d un modèle intégré du continuum terre-mer de la Rade de Brest, ou d un modèle générique du cycle du Si ai sein des marges continentales.Estuaries are complex and heterogeneous areas, subject to many anthropogenic activities, which can have strong filtering capacities during transport and processing of terrestrial matter to coastal zones. The cycling of silicon (Si), which is essential for the growth of diatoms constituting the basis of healthy food webs, is coarsely defined at these interfaces (Dürr et al., 2011). This thesis aims to study the benthic Si cycle in the two main estuaries of the Bay of Brest. To address the interactions of scales and to integrate them in flux estimates spatial heterogeneity and tidal variations of benthic Si cycle were quantified and compared with seasonal variations along estuaries. In order to study the interactions between the different cycles of matter (N, P, C, Si) controlling the functioning of coastal ecosystems, the interactions between Si and P have been explored and appear to favor the retention of P. Finally, the benthic cycle of Si was studied using a diagenetic model to estimate deposition fluxes, that are difficult to estimate directly in estuaries, and to evaluate the retention and recycling of Si at a seasonal scale. This first contribution to the study of the Si cycle in the estuaries of the Bay of Brest offers many opportunities in terms of experimental studies and modelling, whether from the perspective of an integrative model of the land-sea interface in the Bay of Brest, or from the implementation of a generic model of the Si cycle in continental margins.BREST-BU Droit-Sciences-Sports (290192103) / SudocSudocFranceF

Étude numérique de la mer d'Iroise (dynamique, variabilité du front d'Ouessant et évaluation des échanges cross-frontaux)

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 oeuvre numérique et la validation par rapport aux observations. L accent est ensuite mis sur l analyse des principales structures et processus physiques à l oeuvre en mer d Iroise d abord le front de marée d Ouessant et les processus de stratification/déstratification associés, puis la circulation résiduelle eulérienne moyenne afin d isoler et d évaluer ses différentes composantes. La cartographie des différents régimes dynamiques présents a permis l observation de régimes cyclostrophiques autour des îles et en zone peu profonde et fait clairement apparaître un régime géostrophique 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 été les premiers résultats indiquent que ces exports s effectuent selon des canaux privilégiés de latitude.This thesis work falls within a realistic ocean modelling framework focused on the study of the Iroise Sea. The first two parts deal with the numerical implementation and validation through comparison with observations. Then, emphasis is put on the analysis of the main physical structures and processes at work in the Iroise Sea At first, the ushant tidal front along with the associated stratification/destratification processes, then the mean eulerian residual circulation in order to isolate and assess its different components. Then, the mapping of the different dynamical regimes at work allowed us to observe cyclostrophic regimes around the islands and in shallow water area and made clearly appear a strong geostrophic regime at the tidal front, especially in summer. Nevertheless, in the main part of the domain under study, the contributions by the different dynamical terms are quite alike. From a thermal point of view, the analysis of heat budget showed a high domination of the advective fluxes by the air-sea ones despite the induction by the latter of a marked high-frequency modulation of the net heat gain. Then, a diagnostic parameter was established, and the role of different external forcings (air/sea heat fluxes, wind stress, springjneap tidal cycles) upon the ushant front variability was highlighted through sensitivity experiments. Finally, a study of the cross-frontal mass transport by a lagrangian particles tracking technique was launched. The methodology proved to be reliable, and according to the first results, these exports are developing through specific latitude channels .BREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocSudocFranceF

Modélisation des processus biogéochimiques à petite et moyenne échelles en Atlantique Nord Est

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Statistical Analysis of Sea Surface Temperature and Chlorophyll-a Concentration Patterns in the Gulf of Tadjourah (Djibouti)

The sea surface temperature (SST) and chlorophyll-a concentration (CHL-a) were analysed in the Gulf of Tadjourah from two set of 8-day composite satellite data, respectively from 2008 to 2012 and from 2005 to 2011. A singular spectrum analysis (SSA) shows that the annual cycle of SST is strong (74.3% of variance) and consists of warming (April-October) and cooling (November-March) of about 2.5C than the long-term average. The semi-annual cycle captures only 14.6% of temperature variance and emphasises the drop of SST during July-August. Similarly, the annual cycle of CHL-a (29.7% of variance) depicts high CHL-a from June to October and low concentration from November to May. In addition, the first spatial empirical orthogonal function (EOF) of SST (93% of variance) shows that the seasonal warming/cooling is in phase across the whole study area but the southeastern part always remaining warmer or cooler. In contrast to the SST, the first EOF of CHL-a (54.1% of variance) indicates the continental shelf in phase opposition with the offshore area in winter during which the CHL-a remains sequestrated in the coastal area particularly in the south-east and in the Ghoubet Al-Kharab Bay. Inversely during summer, higher CHL-a quantities appear in the offshore waters. In order to investigate processes generating these patterns, a multichannel spectrum analysis was applied to a set of oceanic (SST, CHL-a) and atmospheric parameters (wind speed, air temperature and air specific humidity). This analysis shows that the SST is well correlated to the atmospheric parameters at an annual scale. The windowed cross correlation indicates that this correlation is significant only from October to May. During this period, the warming was related to the solar heating of the surface water when the wind is low (April-May and October) while the cooling (November-March) was linked to the strong and cold North-East winds and to convective mixing. The summer drop in SST followed by a peak of CHL-a, seems strongly correlated to the upwelling. The second EOF modes of SST and CHL-a explain respectively 1.3% and 5% of the variance and show an east-west gradient during winter that is reversed during summer. This work showed that the seasonal signals have a wide spatial influence and dominate the variability of the SST and CHL-a while the east-west gradient are specific for the Gulf of Tadjourah and seem induced by the local wind modulated by the topography

Effects of Monsoon Winds and Topographical Features on the Vertical Thermohaline and Biogeochemical Structure in the Gulf of Tadjourah (Djibouti)

The vertical thermohaline and biogeochemical structures of the upper layer (0 - 200 m) were studied in the Gulf of Tadjourah 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 current and to geostrophic velocity profiles revealed that the thermohaline and the biogeochemical features in summer were related to the southwest monsoon (SWM). The SWM drives surface water from the Gulf of Tadjourah to the Gulf of Aden and thus induces westward intrusion of the high salinity thermocline water from the Gulf of Aden; this near surface flow mixes surface waters in the extreme west of the Gulf of Tajourah. In contrast, the northeast monsoon (NEM), predominant in winter, brings cold water toward the Gulf of Tadjourah and thickens the ML through convective mixing. Our study shows that the SWM plays a crucial role in the stratification of the water column during summer but bathymetry influences its effects. 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

Assessing spatial and temporal variability of phytoplankton communities' composition in the Iroise Sea ecosystem (Brittany, France): A 3D modeling approach: Part 2: Linking summer mesoscale distribution of phenotypic diversity to hydrodynamism

Tidal front ecosystems are especially dynamic environments usually characterized by high phytoplankton biomass and high primary production. However, the description of functional microbial diversity occurring in these regions remains only partially documented. In this article, we use a numerical model, simulating a large number of phytoplankton phenotypes to explore the three-dimensional spatial patterns of phytoplankton abundance and diversity in the Iroise Sea (western Brittany). Our results suggest that, in boreal summer, a seasonally marked tidal front shapes the phytoplankton species richness. A diversity maximum is found in the surface mixed layer located slightly west of the tidal front (i.e., not strictly co-localized with high biomass concentrations) which separates tidally mixed from stratified waters. Differences in phenotypic composition between sub-regions with distinct hydrodynamic regimes (defined by vertical mixing, nutrients gradients and light penetration) are discussed. Local growth and/or physical transport of phytoplankton phenotypes are shown to explain our simulated diversity distribution. We find that a large fraction (64%) of phenotypes present during the considered period of September are ubiquitous, found in the frontal area and on both sides of the front (i.e., over the full simulated domain). The frontal area does not exhibit significant differences between its community composition and that of either the well-mixed region or an offshore Deep Chlorophyll Maximum (DCM). Only three phenotypes (out of 77) specifically grow locally and are found at substantial concentration only in the surface diversity maximum. Thus, this diversity maximum is composed of a combination of ubiquitous phenotypes with specific picoplankton deriving from offshore, stratified waters (including specific phenotypes from both the surface and the DCM) and imported through physical transport, completed by a few local phenotypes. These results are discussed in light of the three-dimensional general circulation at frontal interfaces. Processes identified by this study are likely to be common in tidal front environments and may be generalized to other shallow, tidally mixed environments worldwide

Contrasting responses of the ocean’s oxygen minimum zones to artificial re-oxygenation

Studies assessing potential measures to counteract the marine deoxygenation attributed to anthropogenic activities have been conducted in a few coastal environments and at regional scale, but not yet on a global scale. One way toward global scale artificial oxygenation would be to use oxygen produced as a by-product from hydrogen-production through electrolysis. The low-carbon footprint renewable production of hydrogen from offshore wind energy offers such a possibility. Here, we assessed the potential of this artificial oxygenation method on a global scale using a coupled physical-biogeochemical numerical model. The anthropogenic oxygen source scenario assumes worldwide adoption of hydrogen, considering demographic changes and the feasibility of offshore wind turbine deployment. Following this scenario, artificial oxygenation had a negligible effect on the overall oxygen inventory (an increase of 0.07%) but showed a reduction in the overall volume of Oxygen Minimum Zones (OMZs) between 1.1% and 2.4%. Despite the decrease in the mean OMZ volume globally, OMZs display distinct and contrasting regional patterns notably due to the oxygen impacts on the nitrogen cycle. Artificial oxygenation can inhibit denitrification resulting in a net gain of nitrate that promotes locally and remotely increased biological productivity and consequent respiration. Increased respiration could ultimately lead to an oxygen loss at and beyond injection sites as in the Tropical Pacific and Indian Ocean and particularly expand the Bay of Bengal OMZ. In contrast, the tropical OMZ shrinkage in the Atlantic Ocean is attributed to oxygen enrichment induced by advective transport into the OMZ, while the absence of denitrification in this area precludes any biochemical feedback effect on oxygen levels. These results suggest that the impacts of artificial oxygenation on oxygen concentrations and ecosystems are highly non-linear. It can produce unexpected regional responses that can occur beyond the injection sites which make them difficult to forecast