Plankton community structure in response to hydrothermal iron inputs along the Tonga-Kermadec arc

The Western Tropical South Pacific (WTSP) basin has been identified as a hotspot of atmospheric dinitrogen fixation due to the high dissolved iron ([DFe]) concentrations (up to 66 nM) in the photic layer linked with the release of shallow hydrothermal fluids along the Tonga-Kermadec arc. Yet, the effect of such hydrothermal fluids in structuring the plankton community remains poorly studied. During the TONGA cruise (November-December 2019), we collected micro- (20-200 μm) and meso-plankton (>200 μm) samples in the photic layer (0-200 m) along a west to east zonal transect crossing the Tonga volcanic arc, in particular two volcanoes associated with shallow hydrothermal vents (< 500 m) in the Lau Basin, and both sides of the arc represented by Melanesian waters and the South Pacific Gyre. Samples were analyzed by quantitative imaging (FlowCam and ZooScan) and then coupled with acoustic observations, allowing us to study the potential transfer of phytoplankton blooms to higher planktonic trophic levels. We show that micro- and meso-plankton exhibit high abundances and biomasses in the Lau Basin and, to some extent, in Melanesian waters, suggesting that shallow hydrothermal inputs sustain the planktonic food web, creating productive waters in this otherwise oligotrophic region. In terms of planktonic community structure, we identified major changes with high [DFe] inputs, promoting the development of a low diversity planktonic community dominated by diazotrophic cyanobacteria. Furthermore, in order to quantify the effect of the shallow hydrothermal vents on chlorophyll a concentrations, we used Lagrangian dispersal models. We show that chlorophyll a concentrations were significantly higher inside the Lagrangian plume, which came into contact with the two hydrothermal sites, confirming the profound impact of shallow hydrothermal vents on plankton production

Comment les niveaux mi-trophiques marins répondent-ils aux processus à petite échelle ?

La compréhension du couplage entre les processus biologiques et physiques est un élément fondamental pour évaluer l’état des océans afin de protéger les écosystèmes marins des effets du changement climatique, de l’exploitation humaine, de la pollution, ainsi que pour comprendre le rôle des océans dans le système climatique. En effet, comme les organismes marins vivent dans un environnement fluide et sont continument transportés par les courants, les phénomènes physiques et biologiques sont intimement liés. Ainsi, au contraire de ce qui se passe sur terre, où la topographie du paysage change sur des échelles de temps évolutionnaires (dans l’ordre de centaines ou millions d’années), dans l’océan le paysage évolue sur les mêmes échelles de temps que celles des processus écologiques. Dans cette thèse j’analyse, en particulier, le rôle des structures à fine échelle (désignées dans la suite de ce document comme les « fine échelles »), qui présentent un pic dans le spectre d’énergie océanique, et dont les échelles de temps (jours à semaines) se superposent avec d’importants processus écologiques tels que le développement des blooms phytoplanctoniques ou la durée des périodes de nourrissage des prédateurs supérieurs. Il a été déjà démontré que les fines échelles jouent un rôle central dans le conditionnement de la production primaire, dans l’abondance et la composition des niveaux trophiques inférieurs, et dans le comportement des prédateurs supérieurs. Cependant, leur influence sur les niveaux trophiques intermédiaires est moins connue, alors que ces échelons constituent une partie essentielle de la chaine trophique et sont sous une pression sans précédents de la part activités humaines. Ceci est principalement causé par la disponibilité limitée des données à grande échelle, et à des difficultés de mesures depuis les navires océanographiques. Cette thèse traite ce manque de connaissances sur deux problématiques. La première question concerne l’impossibilité de détecter les niveaux trophiques intermédiaires par satellite, ce qui nécessite le développement des nouvelles stratégies de surveillance ad hoc. La deuxième question irrésolue concerne l’interaction entre la capacité de nage du necton avec les dynamiques de fine échelle. Pour essayer de répondre à ces questions, dans ce travail j’adopte une approche Lagrangienne, en me focalisant donc sur les trajectoires des particules d’eau, et je l’intègre à des nouvelles méthodologies appliquées aux données acoustiques, à l’analyse des systèmes complexes ainsi qu’à la théorie des réseaux. Je me focalise sur la région de Kerguelen, à cause de son importance écologique et de la grande disponibilité d’information qui a permis de caractériser ses dynamiques écologiques relativement simples, basées principalement sur la limitation de la production primaire par le fer qui est fourni par le plateau. Je considère les myctophidés comme le poisson de référence de cette étude par leur abondance dans tous les bassins océaniques, et par l’intérêt qu’ils pourraient recevoir prochainement de la part de la pêche commerciale. (...)The comprehension of the coupling between physical and biological dynamics is a pivotal step to assess the health of the oceans, in order to protect the ecosystems therein from the effects of global change, human exploitation and pollution as well as for understanding the role of the ocean in the climate system. Indeed, in the oceans, physical phenomena and biological processes are intimately linked, since marine organisms live in a fluid environment, continuously under the effect of the currents. Thus, contrary to what happens on land, where the landscape topography changes over evolutionary timescales (periods in the order of hundreds to millions of year) in the ocean the landscape ("seascape") evolves on the same timescales of ecological processes. In the present thesis I analyse in particular the role of the fine scales, which present a peak in the ocean energy spectrum, and whose time scales (of days to weeks) overlap important marine ecological processes like the development of planktonic blooms and the duration of foraging trips for top predators. The fine scale features have been already shown to play a central role into conditioning primary production, lower trophic levels abundance and composition, and apex predators behaviors. However, less is known on their influence on intermediate trophic levels, i.e. swimming organisms (such as fish), which however constitute an essential part of the trophic chain, and which are under unprecedented pressure by human activities. This is mainly due to the scarce availability of data on them at large scales, and to problems of ship-based measurements. Two knowledge gaps are addressed in this thesis. The first is the fact that intermediate trophic levels distributions cannot be detected by remote sensing, and thus require the development of novel, ad hoc sampling strategies. The second open challenge addressed by this thesis is how the swimming ability of the nekton can interact with the fine scale physical dynamics. In order to address the aforementioned questions, in this work I adopt a Lagrangian approach, therefore focusing on water parcel trajectories, and I integrate it with novel methodologies applied to acoustic data, complex system analysis and network theory. I focus on the Kerguelen region, because of its ecological importance and the large availability of informations, which permitted to characterize its relatively simple ecological dynamics, mainly based on iron limitation which is furnished by the plateau. I consider the myctophids as reference fish of the present study, for their worldwide abundance and for their importance for the ecology of the area, and because they may constitute a future target by commercial fishing. (...

Frontal systems as mechanisms of fish aggregation

International audienceIn contrast to terrestrial environments, the open ocean has a dynamics whose timescales overlap with the demography of the organisms it hosts. In particular, so called meso-and submeso-scale processes (1-100 km, days to weeks) have been shown to play a key role in structur-ing the distribution of phytoplankton, which form the large majority of the base of the trophic chain [2]. However, how the (sub)mesoscale turbulence affect higher trophic levels, which have typically swimming capabilities , is largely not known. Here we explore the capability of frontal system to aggregate swimming organisms (fish) by analyzing an idealised model of the stretching region which is often found in between mesoscale vortices. The rationale behind this approach is that an optimal niche for fish, defined in terms of physical properties or prey availability, may shrink with time under the coupled effect of stretching and diffusion. If the shrinking speed is less than the fish swimming capability, fish schools originally dispersed over a wide region may move inside a smaller area, and therefore increase their local density. The model is parameterised for one of the most abundant mesopelagic fish, the myctophid [4], particularly important in the Southern Ocean, using physical conditions representative of their environment

Low-Density Plastic Debris Dispersion beneath the Mediterranean Sea Surface

International audiencePlastic is a widespread marine pollutant, with most studies focusing on the distribution of floating plastic debris at the sea surface. Recent evidence, however, indicates a significant presence of such low density plastic in the water column and at the seafloor, but information on its origin and dispersion is lacking. Here, we studied the pathways and fate of sinking plastic debris in the Mediterranean Sea, one of the most polluted world seas. We used a recent Lagrangian plastic-tracking model, forced with realistic parameters, including a maximum estimated sinking speed of 7.8 m/d. Our simulations showed that the locations where particles left the surface differed significantly from those where they reached the seafloor, with lateral transport distances between 119 and 282 km. Furthermore, 60% of particles deposited on the bottom coastal strip (20 km wide) were released from vessels, 20% from the facing country, and 20% from other countries. Theoretical considerations furthermore suggested that biological activities potentially responsible for the sinking of low density plastic occur throughout the water column. Our findings indicate that the responsibility for seafloor plastic pollution is shared among Mediterranean countries, with potential impact on pelagic and benthic biota

Interaction of the Antarctic Circumpolar Current With Seamounts Fuels Moderate Blooms but Vast Foraging Grounds for Multiple Marine Predators

International audienceIn the Antarctic Circumpolar Current region of the Southern Ocean, the massive phytoplankton blooms stemming from islands support large trophic chains. Contrary to islands, open ocean seamounts appear to sustain blooms of lesser intensity and, consequently, are expected to play a negligible role in the productivity of this area. Here we revisit this assumption by focusing on a region of the Antarctic Circumpolar Current zone which is massively targeted by marine predators, even if no island fertilizes this area. By combining high resolution bathymetric data, Lagrangian analyses of altimetry-derived velocities and chlorophyll a observations derived from BGC-Argo floats and ocean color images, we reveal that the oligotrophic nature of the study region considered in low chlorophyll a climatological maps hides in reality a much more complex environment. Significant (chlorophyll a in excess of 0.6 mg/m 3) phytoplankton blooms spread over thousands of kilometers and have bio-optical signatures similar to the ones stemming from island systems. By adopting a Lagrangian approach, we demonstrate that these moderate blooms (i) originate at specific sites where the Antarctic Circumpolar Current interacts with seamounts, and (ii) coincide with foraging areas of five megafauna species. These findings underline the ecological importance of the open ocean subantarctic waters and advocate for a connected vision of future conservation actions along the Antarctic Circumpolar Current

Massive and localized export of selected marine snow types at eddy edges in the South Atlantic Ocean

The open ocean plays a critical role in mitigating climate change by sequestering carbon dioxide (CO2) from the atmosphere for long periods of time. This carbon storage occurs over decades to millennia and relies on the physical pump that transports cold, dense, and DIC-rich waters to the deep ocean, as part of the ocean’s overturning circulation, and the biological carbon pump (BCP). The BCP encompasses a wide range of processes, from the fixation of atmospheric CO2 by phytoplankton activity to carbon sequestration in the deep ocean. Atmospheric CO2 concentrations would be about 200 ppm higher than in a world without biology, and the global climate would be much warmer by default. This study highlights the idea that BCP efficiency is enhanced by the ocean dynamics at mesoscale and submesoscale. In fact, our results suggest that frontal regions, such as those between mesoscale eddies, could lead to an important accumulation and transport of particulate organic matter (POM) from the mixed layer depth (MLD) down to depths of about 600 meters. To reach these conclusions, a multifaceted approach was applied. It included in-situ measurements and marine snow images from a BGC Argo float equipped with an Underwater Vision Profiler (UVP6), satellite altimetry data, and Lagrangian physics diagnostics. We focused our study on three intense features in marine snow distribution observed during the 17-month long float mission in the Cape Basin, southwest of Africa. These features were located in the frontal region between mesoscale eddies. Our study suggests that a particle injection pump induced by a frontogenesis-driven mechanism has the potential to enhance the effectiveness of the biological pump by increasing the depth at which carbon is injected into the water column. This work also emphasizes the importance of establishing repeated sampling campaigns targeting the interface zones between eddies. This could improve our understanding of the mechanisms involved in the deep accumulation of marine snow observed at eddy interfaces

Optimal monitoring of the ocean surface by observing the transport crossroads

Trabajo presentado en la European Geosciences Union General Assembly (EGU General Assembly 2021), celebrada online del 19 al 30 de abril de 2021.In the context of tracer transport in the ocean, we introduce a quantity, the crossroadness [1], which allows identifying the optimal disposition of a set of locations in order to monitor a given ocean surface region. The optimization is performed so that these sites observe the largest amount of water coming from the region and, at the same time, monitor waters coming from separate parts of the ocean. These are key criteria when deploying a marine observing network. Considering surface circulation, crossroadness measures at any location the extent of the ocean surface which transits in its neighborhood in a given time window. When the analysis is performed backward in time, this method allows us to identify the major sources which feed a target region. The method is first applied to a minimalistic model of a mesoscale eddy field, and then to realistic satellite-derived ocean currents in the Kerguelen area. In this region, we identify the optimal location of fixed stations capable of intercepting the trajectories of 43 surface drifters. We then illustrate the temporal persistence of the stations determined in this way. Finally, we identify possible hotspots of micro-nutrient enrichment for the recurrent spring phytoplanktonic bloom occurring there. Promising applications to other fields, such as larval connectivity or contaminant detection are discussed

Crossroads of the mesoscale circulation

Quantifying the mechanisms of tracer dispersion in the ocean remains a central question in oceanography, for problems ranging from nutrient delivery to phytoplankton, to the early detection of contaminants. Until now, most of the analysis has been based on Lagrangian concepts of transport, often focusing on the identification of features that minimize fluid exchange among regions, or more recently, on network tools which focus instead on connectivity and transport pathways. Neither of these approaches, however, allows us to rank the geographical sites of major water passage, and at the same time, to select them so that they monitor waters coming from separate parts of the ocean. These are instead key criteria when deploying an observing network. Here, we address this issue by estimating at any point the extent of the ocean surface which transits through it in a given time window. With such information we are able to rank the sites with major fluxes that intercept waters originating from different regions. We show that this allows us to optimize an observing network, where a set of sampling sites can be chosen for monitoring the largest flux of water dispersing out of a given region. When the analysis is performed backward in time, this method allows us to identify the major sources which feed a target region. The method is first applied to a minimalistic model of a mesoscale eddy field, and then to realistic satellite-derived ocean currents in the Kerguelen area. In this region, we identify the optimal location of fixed stations capable of intercepting the trajectories of 43 surface drifters, along with statistics on the temporal persistence of the stations determined in this way. We then identify possible hotspots of micro-nutrient enrichment for the recurrent spring phytoplanktonic bloom occurring here. Promising applications to other fields, such as larval connectivity, marine spatial planning or contaminant detection, are then discussed

Swirling in the ocean: Immature loggerhead turtles seasonally target old anticyclonic eddies at the fringe of the North Atlantic gyre

In a highly heterogeneous open ocean, swirling oceanographic structures such as eddies drive ocean productivity and aggregate many predators, including oceanic sea turtles. During early life, juvenile loggerhead turtles can spend more than a decade feeding on gelatinous zooplankton in the open ocean, but the way they use mesoscale eddies is still poorly understood. Here, we investigated the relationships between (1) the distribution and (2) the diving behaviour of immature loggerhead turtles of the North-East Atlantic and mesoscale eddies. For this purpose, 28 turtles were satellite tracked from the Azores archipelago. Using the Residence Time (RT) analysis as a proxy to identify high-use areas, the tracks and dive data of the turtles as well as drifter trajectories were analysed in relation to eddy characteristics, which include eddy radius, amplitude, type (cyclonic vs. anticyclonic), lifetime and region (inner core, outer core, periphery). The turtles dispersed widely using many distinct high-use areas. Although there were always more cyclones than anticyclones over the study region, the individuals seasonally associated more with the inner cores of old anticyclonic eddies, likely due to the higher productivity of decaying anticyclones. The comparison between passive drifters and turtles’ movements showed an active swimming behaviour from the turtles rather than a passive advection through currents. Three dive types were identified, and the one associated with the highest RT was characterized by dives of medium duration and depths in the inner cores of eddies. This study is the first to highlight strong affinities of oceanic loggerhead turtles for old anticyclonic eddies around the Azores, suggesting a greater complexity of these warm-core eddies that appear to be much more productive than cyclones

From network theory to dynamical systems and back: Lagrangian Betweenness reveals bottlenecks in geophysical flows

Transport phenomena, including diffusion, mixing, spreading, and mobility, are crucial to understand and model dynamical features of complex systems. In particular, the study of geophysical flows attracted a lot of interest in the last decades as fluid transport has proven to play a fundamental role in climatic and environmental research across a wide range of scales. Two theoretical frameworks have been effectively used to investigate transport phenomena in complex systems: Dynamical Systems Theory (DST) and Network Theory (NT). However, few explicit connections between these two different views have been established. Here, we focus on the betweenness centrality, a widely used local measure which characterizes transport and connectivity in NT. By linking analytically DST and NT we provide a novel, continuous-in-time formulation of betweenness, called Lagrangian Betweenness, as a function of Lyapunov exponents. This permits to quantitatively relate hyperbolic points and heteroclinic connections in a given dynamical system to the main transport bottlenecks of its associated network. Moreover, using modeled and observational velocity fields, we show that such bottlenecks are present and surprisingly persistent in the oceanic circulation illustrating their importance in organizing fluid motion. The link between DST and NT rooted in the definition of the Lagrangian Betweenness has the potential to promote further theoretical developments and applications at the interface between these two fields. Finally, the identification of such circulation hotspots provides new crucial information about transport processes in geophysical flows and how they control the redistribution of various tracers of climatic (e.g. heat, carbon, moisture), biological (e.g. larvae, pathogens) and human (e.g. pollutants, plastics) interests.N