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Le Bourg Baptiste |
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Ecologie trophique des étoiles de mer de l’Océan Austral : Influence des facteurs environnementaux sur la diversité trophique // Trophic ecology of Southern Ocean sea stars: Influence of environmental drivers on trophic diversity |
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2020 |
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Thèse de doctorat. université de liège |
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287 pp. |
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[fr] Comme le reste du monde, l’Océan Austral est impacté par le changement climatique. Néanmoins, ces changements varient en fonction des différentes régions de cet océan. Les modifications de la couverture par la glace de mer autour du continent Antarctique en sont une des manifestations les plus visibles. Ainsi, la couverture glaciaire a fortement diminué en Péninsule Antarctique Occidentale, tandis qu’elle tend à légèrement augmenter sur le reste de la façade du continent. Ces modifications des conditions environnementales influencent le fonctionnement des écosystèmes de cet océan. Notamment, la disparition ou la persistance de la couverture glaciaire dans une région où elle avait une présence saisonnière peut empêcher l’apparition des efflorescences estivales du phytoplancton, impactant ainsi la dynamique des populations de krill et de leurs prédateurs dans le milieu pélagique. Le milieu benthique sera lui aussi perturbé par les modifications environnementales résultant du changement climatique, puisque les flux de matière organique vers ce compartiment seront affectés. Les étoiles de mer (Echinodermata : Asteroidea) sont un élément clé du benthos de l’Océan Austral, avec 12% des espèces d’étoiles de mer connues y vivant. En milieu tempéré, ce taxon peut, au travers d’interactions trophiques, jouer un rôle important dans le fonctionnement des écosystèmes en contrôlant les populations d’autres organismes qui ont eux-mêmes un effet conséquent sur les écosystèmes. Ce groupe est considéré comme assez résistant aux changements de température de l’eau de mer en milieu Antarctique. Il est cependant susceptible d’être affecté par les modifications des conditions environnementales et leur influence sur le fonctionnement des réseaux trophiques. En effet, la réduction de l’abondance ou la disparition de proies communes peut entraîner un risque accru compétition dans les assemblages d’étoiles de mer. Afin de déterminer le rôle écologique des étoiles de mer dans l’Océan Austral, et de comprendre comment elles pourraient être impactées par le changement climatique, une étude de leur écologie trophique et des facteurs qui la contrôlent est nécessaire. Par conséquent, les objectifs de cette thèse de doctorat étaient de déterminer quelle est la place des étoiles de mer dans l’Océan Austral, et comment des facteurs environnementaux comme la turbidité, la profondeur et la glace de mer impactent leur diversité trophique. Pour ce faire, les rapports isotopiques du carbone (δ13C), de l’azote (δ15N) et du soufre (δ34S) ont été analysés dans des tissus d’étoiles de mer prélevées dans l’ensemble de l’Océan Austral (n = 2454 individus, auxquels s’ajoutent des données sur 204 individus issues de la littérature ou partagées par des collègues). Une proportion importante des échantillons d’étoiles de mer est issue de collections archivées dans des institutions et/ou des muséums, ce qui a permis de significativement augmenter la couverture spatiale et temporelle de l’échantillonnage. Cependant, ces échantillons ont pour la plupart été conservés dans de l’éthanol, et certains ont été préalablement fixés avec du formaldéhyde. Or, les liquides de préservation peuvent altérer les rapports isotopiques des tissus biologiques. Par conséquent, l’impact de la méthode de préservation sur les rapports isotopiques dans les tissus d’étoiles de mer a été étudié (chapitre 3) lors d’une expérience à long terme (deux ans). Celle-ci a montré qu’il était possible de corriger mathématiquement l’effet des liquides de préservation sur les rapports isotopiques des étoiles de mer, et donc d’utiliser des échantillons archivés pour des études d’écologie trophique. Afin de mieux connaître l’importance potentielle des étoiles de mer dans les écosystèmes de l’Océan Austral, le réseau trophique des forêts de Macrocystis pyrifera (Phaeophycées, Laminariales ; ou “kelp”) subantarctiques a été reconstitué grâce à l’analyse des rapports isotopiques dans des tissus d’invertébrés (19 taxa, dont 6 d’étoiles de mer) et de producteurs primaires échantillonnés dans les îles Kerguelen (chapitre 4). L’utilisation de modèles de mélange isotopique a démontré qu’il n’y a pas de consommateur majeur de Macrocystis pyrifera dans ces forêts de kelp, et que le réseau trophique est alimenté par la matière organique pélagique et du micro/macrophytobenthos vivant (à l’exception du kelp) ou sous forme de détritus (incluant probablement des détritus de kelp). Ceci suggère que les étoiles de mer n’ont pas les mêmes fonctions écologiques dans les forêts à Macrocystis pyrifera des régions subantarctiques que dans celles des régions plus tempérées où elles contrôlent partiellement les populations d’herbivores. Nos résultats démontrent également que les étoiles de mer subantarctiques ne se limitent pas à être des prédateurs au sommet des chaînes alimentaires. D’une part, certaines espèces occupent d’autres positions trophiques plus basales. D’autre part, leurs niches trophiques ne se recouvrent pas nécessairement : certaines exploitent plus la partie du réseau trophique soutenu par la production pélagique et d’autres la partie du réseau trophique soutenue par la production benthique (y compris détritique). Après cette analyse d’un réseau trophique entier, nous avons réalisé une comparaison des niches isotopiques d’étoiles de mer à une échelle locale, et étudié les liens possibles entre changements ontogénétiques, c’est-à-dire au cours de la vie et de la croissance, et écologie trophique (chapitre 5). L’analyse des rapports isotopiques dans des étoiles de mer prélevées dans l’anse d’Ezcurra (baie de l’Amirauté, île du Roi-George, îles Shetland du sud) a montré que la taille, et plus particulièrement le rayon du disque central, peut être reliée aux rapports isotopiques chez certaines espèces, indiquant des changements ontogénétiques de l’alimentation chez ces dernières. Cette observation n’est cependant valable que pour certaines espèces, en particulier des espèces omnivores, où on observe une élévation de la position trophique en fonction de la taille de l’étoile de mer. La relation entre le rayon du disque et l’alimentation peut notamment s’expliquer par le fait que les étoiles de mer les plus grandes peuvent dévaginer leur estomac sur de plus grandes surfaces et donc consommer des proies plus grandes et/ou de niveau trophique plus élevé. Chez les étoiles de mer consommant du sédiment ou potentiellement suspensivores, cette relation entre la taille et les rapports isotopiques n’est généralement pas observée, ce qui suggère qu’il y a probablement moins de variabilité du régime alimentaire en fonction de la taille des individus chez ces espèces. L’analyse des rapports isotopiques dans les étoiles de mer prélevées dans l’anse d’Ezcurra indique également que certaines espèces d’étoiles de mer présentent une variabilité de régime alimentaire importante qui détermine en partie leurs interactions trophiques interspécifiques. La turbidité générée par l’apport de matière terrigène par les eaux de ruissellement issues de la fonte des glaciers terrestres crée un gradient environnemental important de l’intérieur vers l’extérieur de l’anse d’Ezcurra, qui détermine les conditions d’habitat et la nature des ressources disponibles pour les étoiles de mer. Certaines espèces adaptent leurs régimes alimentaires en fonction de ces conditions variables. Les interactions entre espèces sont également affectées. A l’intérieur de l’anse, sujet à une importante turbidité, les espèces Diplasterias brandti et Odontaster validus ont des valeurs de δ13C plus différenciées (i.e. des sources de nourriture plus différentes) et des niches isotopiques se recouvrant moins, qu’à l’extérieur, sujet à une turbidité moindre. De même, la taille de la niche isotopique d’Odontaster validus est plus réduite vers l’intérieur de l’anse que vers l’extérieur. Ceci correspond probablement à une situation moins favorable en termes de disponibilité et de diversité des ressources à l’intérieur de l’anse. Ceci pourrait conduire à une constriction de la niche isotopique d’Odontaster validus ainsi qu’à une ségrégation des ressources accrue qui pourrait limiter la compétition interspécifique entre les quelques espèces capables de survivre à ces conditions défavorables. Dans le chapitre 6, nous avons étudié l’impact du groupe trophique, de la profondeur, de la concentration de glace et de la durée de la saison glaciaire sur l’alimentation des étoiles de mer par l’intermédiaire d’une analyse globale de notre jeu de données à l’échelle de l’Océan Austral. De plus, l’Océan Austral a été subdivisé en différentes écorégions benthiques d’après des données environnementales (température au fond, glace de mer, bathymétrie) et biologique (distribution des espèces) afin d’étudier les variations biogéographiques de l’alimentation des étoiles de mer. Cette subdivision a notamment mis en évidence la séparation des environnements antarctique et subantarctique, la différence des valeurs de δ13C de la matière organique dans les eaux de surface entre les écorégions subantarctiques et antarctiques étant reflétée dans les tissus d’étoiles de mer benthiques. Dans ce chapitre, la compilation des informations disponibles sur leur régime alimentaire a permis de procéder à une classification des taxa d’étoiles de mer de l’Océan Austral en groupes trophiques, allant des suspensivores aux prédateurs de proies mobiles. Les différences de rapports isotopiques entre ces groupes trophiques et leur variabilité dans certains d’entre eux suggèrent une diversité des sources de nourriture et/ou des stratégies d’alimentation entre et au sein des groupes trophiques. Ces résultats confirment bien que, contrairement à ce qui est parfois dit dans la littérature, les étoiles de mer de l’Océan Austral font preuve d’une grande diversité trophique. La profondeur est apparue comme un facteur influençant fortement l’écologie trophique des étoiles de mer, aussi bien à l’échelle de tout l’Océan Austral que des écorégions. En effet, les étoiles de mer côtières semblent exploiter des réseaux trophiques soutenus par une diversité de producteurs primaires pélagiques et benthiques tandis que les étoiles de mer plus profondes paraissent dépendre de la sédimentation de la production primaire de surface. Ainsi, les étoiles de mer côtières sont caractérisées par l’exploitation d’un plus grand nombre de sources de matière organique. Par contre, il est également apparu que les étoiles de mer plus profondes ont une plus grande diversité de positions trophiques que les côtières. La plus faible diversité et la plus faible disponibilité des ressources trophiques en milieu profond pourraient favoriser la diversification des comportements alimentaires (e.g. omnivorie, prédation, consommation de sédiment) des étoiles de mer, ce qui pourrait permettre de réduire la compétition entre espèces. L’impact de la glace de mer sur l’alimentation des étoiles de mer antarctiques a également été étudié. En cas de forte concentration en glace de mer, les communautés sympagiques semblent être utilisées comme ressources trophiques par les étoiles de mer. De même, pour certains groupes trophiques, une dépendance croissante à l’égard de phytodetritus dégradés semble exister dans les stations où la durée de la saison glaciaire est la plus élevée. Ce mécanisme pourrait permettre d’atténuer les impacts de la glace de mer sur la disponibilité des ressources trophiques pendant les plus longues périodes de couverture de glace de mer. Nos résultats suggèrent aussi que les liens entre glace de mer et alimentation des consommateurs benthiques sont multiples mais complexes à interpréter. En outre, ils ne sont pas tous consistants d’une écorégion à l’autre, vraisemblablement en lien avec les caractéristiques océanographiques très variées que l’on peut y rencontrer. Enfin, les impacts des différents paramètres environnementaux peuvent varier d’un groupe trophique à l’autre, illustrant la nécessité de prendre en compte la diversité trophique pour prédire la susceptibilité des étoiles de mer aux changements environnementaux futurs, qu’ils soient d’origine naturelle ou anthropique. En résumé, les travaux présentés dans cette thèse montrent que l’écologie trophique des étoiles de mer de l’Océan Austral est influencée par un ensemble de facteurs intrinsèques (taille, groupe trophique) et extrinsèques (turbidité, profondeur, glace de mer). Les informations sur l’influence des facteurs environnementaux permettent d’établir des hypothèses sur les possibles impacts du changement climatique sur les étoiles de mer et leur rôle dans les réseaux trophiques benthiques de l’Océan Austral. En effet, les paramètres environnementaux influencent, au travers de la disponibilité en ressources, l’écologie trophique des étoiles de mer, et notamment les interactions entre espèces. Ainsi, la plus faible diversité et la plus faible disponibilité des ressources trophiques en milieu turbide et en milieu profond ont été considérées comme pouvant favoriser la diversification des comportements alimentaires pour éviter la compétition entre espèces. Au contraire, leur plus grande disponibilité en milieu peu turbide et côtier permet la consommation de proies similaires avec un risque limité de compétition. La glace de mer a des impacts plus variés sur la disponibilité en ressources trophiques, servant d’habitat aux communautés sympagiques et favorisant les efflorescences de phytoplancton lors de sa débâcle, mais les inhibant en cas de persistance. Par conséquent, les modifications de la couverture glaciaire et de sa dynamique à cause du changement climatique vont induire des changements dans la disponibilité des ressources trophiques pour le benthos de l’Océan Austral. De même, des modifications de la turbidité en milieu côtier suite à la modification de la dynamique des glaciers terrestres pourraient avoir des conséquences sur la disponibilité en ressources dans ce type d’environnement. Ces changements vont probablement modifier la nature des interactions trophiques entre les taxa d’étoiles de mer, avec une hausse ou une baisse de l’importance de la compétition, ce qui pourrait entraîner des modifications dans la structure des assemblages d’étoiles de mer dans l’Océan Austral. [en] Like in the rest of the world, climate change impacts the Southern Ocean, but not in the same way in all regions of this ocean. Changes in sea ice cover around the Antarctic continent are one of the most visible manifestations. For example, sea ice cover has decreased significantly in the Western Antarctic Peninsula while it tends to increase slightly on the rest of the coastline of the continent. These changes in environmental conditions influence the functioning of the ecosystems of this ocean. In particular, the disappearance or persistence of sea ice in a region where it was seasonally present may prevent the appearance of summer phytoplankton blooms, thus impacting the dynamics of krill populations and their predators in the pelagic environment. The benthic environment will also be disturbed by environmental modifications resulting from climate change, as organic matter fluxes toward this compartment will be affected. Sea stars (Echinodermata: Asteroidea) are a key component of Southern Ocean benthos, with 12% of the known sea star species living there. In temperate environments, this taxon may play an important role in the functioning of ecosystems with its trophic interactions controlling the populations of other organisms, which themselves have a large effect on the ecosystems. This group is considered to be quite resistant to changes of seawater temperature in the Antarctic environment. However, it will likely be affected by changes of environmental conditions and functioning of food webs. Indeed, a reduction in the abundance or disappearance of common prey can lead to an increased competition in sea star assemblages. In order to determine the ecological role of sea stars in the Southern Ocean and to understand how they might be impacted by climate change, a study of their trophic ecology and of the factors controlling it is necessary. Therefore, the objectives of this PhD thesis were to determine the trophic role of sea stars in the Southern Ocean, and how environmental factors such as turbidity, depth and sea ice impact their trophic diversity. To do so, stable isotope values of carbon (δ13C), nitrogen (δ15N) and sulfur (δ34S) were analysed in tissues of sea star sampled across the Southern Ocean (n = 2454 individuals, plus data on 204 individuals from the literature or shared by colleagues). A significant proportion of the sea star samples came from collections archived in institutions and/or museums, which significantly increased the spatial and temporal coverage of the study. However, most of these samples were preserved in ethanol, and some have been previously fixed with formaldehyde. Preservative fluids may alter the stable isotope values of biological tissues. Therefore, the impact of the preservation method on the stable isotope values in sea star tissues was studied (chapter 3). A two year-long experiment showed that it was possible to mathematically correct the effect of preservative fluids on stable isotope values in sea stars, making it possible to use archived samples for trophic ecology studies. In order to better understand the potential importance of sea stars in the ecosystems of the Southern Ocean, the food web of subantarctic Macrocystis pyrifera (Phaeophyceae, Laminariales; or “kelp”) forests was reconstructed by analysing stable isotope values in invertebrate (19 taxa, including 6 sea star taxa) and primary producer tissues sampled in the Kerguelen Islands (chapter 4). Mixing models did not highlight major Macrocystis pyrifera consumers and showed that the food web is supported by pelagic organic matter and live (with the exception of kelp) or detrital micro/macrophytobenthos. This suggests that sea stars do not have the same ecological function in Macrocystis pyrifera forests from subantarctic regions than in those from more temperate regions, where they control grazer populations. The results also show that sea stars are not only top predators in the food chain. On the one hand, some species may occupy lower trophic positions. On the other hand, their trophic niches may not overlap: some species rely more on the food chain supported by pelagic production and others on the food chain supported by benthic (including detrital) production. Following this analysis of an entire food web, isotopic niches of sea stars were compared at a local scale, and the possible relationship between ontogenetic changes, i.e. changes during growth, and trophic ecology were studied (chapter 5). The analysis of stable isotope values in sea stars sampled in Ezcurra Inlet (Admiralty Bay, King George Island, South Shetland Islands, chapter 5) showed that the size, and especially the central disc radius, can be linked to stable isotope values in some species, indicating the occurrence of ontogenetic changes in the diet of these species. This was observed only in some species, and notably omnivore species, for which the trophic position increased with body size. The relationship between the disc radius and the trophic ecology may be explained by larger sea stars being able to evert their stomach over larger areas and thus consume larger prey and/or prey with higher trophic level. For sediment feeding and potentially suspension feeding species, the relationship between size and stable isotope values was usually not observed, suggesting lower variability of the trophic ecology between size classes in these species. The analysis of stable isotope values in sea stars sampled in Ezcurra Inlet also indicates that the diet variability of some sea star species may be important, which in part determines their interspecific trophic interactions. The turbidity generated by the terrestrial inputs provided by meltwater run-off from terrestrial glaciers results in an important environmental gradient from the inner to the outer Ezcurra Inlet, which determines the habitat conditions and the characteristics of the resources available for sea stars. Some species may adapt their diet depending on those variable conditions. Interspecific interactions are also impacted. Indeed, the Diplasterias brandti and Odontaster validus species have more different δ13C values (i.e. more different food sources) and lower isotopic niche overlap in the inner inlet, where high turbidity occurs, than in the outer, where turbidity is lower. Similarly, the size of the isotopic niche of Odontaster validus is smaller in the inner Ezcurra Inlet than in the outer. This is probably the result of a more limited availability and diversity of resources in the inner inlet. This may lead to the constriction of Odontaster validus isotopic niche and to resource segregation that may limit interspecific competition between the few species able to survive in these unfavourable conditions. In the chapter 6, the impact of trophic group, depth, sea ice concentration and sea ice season duration on the trophic ecology of sea stars was assessed thanks to a global analysis of the dataset at the scale of the whole Southern Ocean. The Southern Ocean was subdivided into different benthic ecoregions according to environmental (seabed temperature, sea ice, bathymetry) and biotic data (species distribution), to study biogeographic variations in the trophic ecology of sea stars. This subdivision notably highlighted the separation between Antarctic and Subantarctic environments, with the different δ13C values in organic matter from the surface in Subantarctic and Antarctic waters being reflected in tissues from benthic sea stars. In this chapter, the compilation of the available information on their diet allowed to make a classification of sea star taxa from the Southern Ocean into trophic groups, ranging from suspension feeders to predators of active prey. The differences of stable isotope values between trophic groups and their variability in some of them suggested a diversity of food sources and/or of feeding strategies between and within trophic groups. These results confirm that, contrary to what is sometimes stated in the literature, sea stars in the Southern Ocean show a great trophic diversity. Depth has important effects on the trophic ecology of sea stars, both across the entire Southern Ocean and within ecoregions. Indeed, coastal sea stars may exploit food webs supported by a variety of pelagic and benthic primary producers while deeper sea stars may depend on the sedimentation of the surface primary production. Coastal sea stars are then characterised by a high diversity of food sources, while deeper sea stars have a higher diversity of trophic positions than coastal ones. The lower diversity and availability of food sources in deep waters may induce the diversification of sea star feeding behaviours (e.g. omnivory, predation, sediment feeding), which would reduce competition between species. The impact of sea ice on the trophic ecology of sea stars was also investigated. The sympagic communities may be used as a food source by sea stars in case high sea ice concentrations. Furthermore, increasing reliance on degraded phytodetritus during longer periods of sea ice cover may occur in several trophic groups. This may dampen the impacts of sea ice presence on resource availability during long periods of sea ice cover. Our results also suggest that multiple relationships do exist between sea ice and the diet of benthic consumers, but they are not easy to interpret. Finally, not all of them are consistent across ecoregions, probably as a result of their contrasted oceanographic features. In addition, the impacts of the environmental parameters may differ between trophic groups, highlighting the importance of trophic diversity to predict the sensitivity of sea stars to future environmental changes, whether natural or anthropogenic. To summarise, the studies of this thesis show that the trophic ecology of sea stars from the Southern Ocean is impacted by a combination of intrinsic (body size, trophic group) and extrinsic features (turbidity, depth, sea ice). Information on the influence of environmental parameters may provide hypotheses regarding the possible impacts of climate change on sea stars and on their role in benthic food webs of the Southern Ocean. Indeed, the environmental parameters may influence the trophic ecology of sea stars and the trophic interactions between taxa thanks to their impact on resource availability. The lower diversity and availability of food sources in turbid and deep waters have been considered as potential sources of diversification of feeding behaviours to avoid competition between species. By contrast, their greater availability in less turbid and coastal environments allows the consumption of similar prey with limited risks of competition. Sea ice has more variable impacts on resource availability, being an habitat for sympagic communities and inducing phytoplankton blooms after its break up, but inhibiting them in case of persistence. Consequently, changes in the ice cover and its dynamics because of climate change will induce changes in the resource availability for the Southern Ocean benthos. Similarly, changes in turbidity in coastal areas as a result of modifications in the dynamics of terrestrial glaciers could have consequences on resource availability in this type of environment. These changes are likely to modify the trophic interactions between sea star taxa, with an increase or decrease of the importance of competition, which may result in modifications of the structure of sea star assemblages in the Southern Ocean. |
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8067 |
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Henri Weimerskirch, Julien Collet, Alexandre Corbeau, Adrien Pajot, Floran Hoarau, Cédric Marteau, Dominique Filippi, Samantha C. Patrick |
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Ocean sentinel albatrosses locate illegal vessels and provide the first estimate of the extent of nondeclared fishing |
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2020 |
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Proceedings of the National Academy of Sciences |
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117 |
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6 |
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3006-3014 |
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bio-logging conservation illegal fisheries seabird vessel attraction |
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With threats to nature becoming increasingly prominent, in order for biodiversity levels to persist, there is a critical need to improve implementation of conservation measures. In the oceans, the surveillance of fisheries is complex and inadequate, such that quantifying and locating nondeclared and illegal fisheries is persistently problematic. Given that these activities dramatically impact oceanic ecosystems, through overexploitation of fish stocks and bycatch of threatened species, innovative ways to monitor the oceans are urgently required. Here, we describe a concept of “Ocean Sentinel” using animals equipped with state-of-the-art loggers which monitor fisheries in remote areas. Albatrosses fitted with loggers detecting and locating the presence of vessels and transmitting the information immediately to authorities allowed an estimation of the proportion of nondeclared fishing vessels operating in national and international waters of the Southern Ocean. We found that in international waters, more than one-third of vessels had no Automatic Identification System operating; in national Exclusive Economic Zones (EEZs), this proportion was lower on average, but variable according to EEZ. Ocean Sentinel was also able to provide unpreceded information on the attraction of seabirds to vessels, giving access to crucial information for risk-assessment plans of threatened species. Attraction differed between species, age, and vessel activity. Fishing vessels attracted more birds than other vessels, and juveniles both encountered fewer vessels and showed a lower attraction to vessels than adults. This study shows that the development of technologies offers the potential of implementing conservation policies by using wide-ranging seabirds to patrol oceans. |
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0027-8424, 1091-6490 |
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7684 |
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L. G. Bouma, J. D. Hartman, R. Brahm, P. Evans, K. A. Collins, G. Zhou, P. Sarkis, S. N. Quinn, J. de Leon, J. Livingston, C. Bergmann, K. G. Stassun, W. Bhatti, J. N. Winn, G. Á Bakos, L. Abe, N. Crouzet, G. Dransfield, T. Guillot, W. Marie-Sainte, D. Mékarnia, A. H. M. J. Triaud, C. G. Tinney, T. Henning, N. Espinoza, A. Jordán, M. Barbieri, S. Nandakumar, T. Trifonov, J. I. Vines, M. Vuckovic, C. Ziegler, N. Law, A. W. Mann, G. R. Ricker, R. Vanderspek, S. Seager, J. M. Jenkins, C. J. Burke, D. Dragomir, A. M. Levine, E. V. Quintana, J. E. Rodriguez, J. C. Smith, B. Wohler |
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Cluster Difference Imaging Photometric Survey. II. TOI 837: A Young Validated Planet in IC 2602 |
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2020 |
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The Astronomical Journal |
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Volume |
160 |
Issue |
5 |
Pages |
239 |
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Keywords |
Astrophysics – Earth and Planetary Astrophysics Astrophysics – Solar and Stellar Astrophysics |
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Abstract |
We report the discovery of TOI 837b and its validation as a transiting planet. We characterize the system using data from the NASA TESS mission, the ESA Gaia mission, ground-based photometry from El Sauce and ASTEP400, and spectroscopy from CHIRON, FEROS, and Veloce. We find that TOI 837 is a $T=9.9$ mag G0/F9 dwarf in the southern open cluster IC 2602. The star and planet are therefore $35^{+11}{-5}$ million years old. Combining the transit photometry with a prior on the stellar parameters derived from the cluster color-magnitude diagram, we find that the planet has an orbital period of $8.3\,{\rm d}$ and is slightly smaller than Jupiter ($R{\rm p} = 0.77^{+0.09}{-0.07} \,R{\rm Jup}$). From radial velocity monitoring, we limit $M{\rm p}\sin i$ to less than 1.20 $M{\rm Jup}$ (3-$\sigma$). The transits either graze or nearly graze the stellar limb. Grazing transits are a cause for concern, as they are often indicative of astrophysical false positive scenarios. Our follow-up data show that such scenarios are unlikely. Our combined multi-color photometry, high-resolution imaging, and radial velocities rule out hierarchical eclipsing binary scenarios. Background eclipsing binary scenarios, though limited by speckle imaging, remain a 0.2% possibility. TOI 837b is therefore a validated adolescent exoplanet. The planetary nature of the system can be confirmed or refuted through observations of the stellar obliquity and the planetary mass. Such observations may also improve our understanding of how the physical and orbital properties of exoplanets change in time. |
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Programme |
1066 |
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Bachelor's thesis |
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ISSN |
1538-3881 |
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yes |
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Call Number |
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Serial |
7804 |
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Author |
Encelyn Voisine, Yann Rolland, Matthias Bernet, Julien Carcaillet, Guillaume Duclaux, Jérôme Bascou, Christian Sue, Mélanie Balvay, René-Pierre Ménot |
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Title |
Antarctic erosion history reconstructed by Terre Adélie moraine geochronology |
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Journal |
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Year |
2020 |
Publication |
Antarctic Science |
Abbreviated Journal |
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Volume |
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Pages |
1-14 |
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Keywords |
cosmogenic nuclide dating deglaciation exhumation fission-track dating Late Palaeozoic Ice Age subglacial incision |
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Abstract |
We report apatite fission-track and 10Be terrestrial cosmogenic nuclide (TCN) dating of 14 moraine boulders originating from inland Terre Adélie, East Antarctica. These data show cooling of the Proterozoic Terre Adélie craton at < ~120°C between 350 and 300 Ma, suggesting > 4 km temperate glacial erosion during the Late Palaeozoic Ice Age, followed by nearly null Mesozoic erosion and low glacial erosion (< 2 km) in the Cenozoic. Based on glacial flux maps, the origin of the boulders may be located ~400 km upstream. Preliminary TCN (10Be) datings of moraine boulders cluster within the last 30 ka. Cosmogenic ages from the Lacroix Nunatak suggest a main deglaciation after the Younger Dryas at c. 10 ka, while those of Cap Prud'homme mostly cluster at 0.6 ka, in agreement with an exhumation of boulders during the Little Ice Age. |
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Programme |
1003 |
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ISSN |
0954-1020, 1365-2079 |
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yes |
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Call Number |
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Serial |
7692 |
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Author |
Korbinian Sager, Christian Boehm, Laura Ermert, Lion Krischer, Andreas Fichtner |
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Title |
Global-Scale Full-Waveform Ambient Noise Inversion |
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Journal |
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Year |
2020 |
Publication |
Journal of Geophysical Research: Solid Earth |
Abbreviated Journal |
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Volume |
125 |
Issue |
4 |
Pages |
e2019JB018644 |
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Keywords |
computational seismology full-waveform inversion global tomography interferometry seismic noise |
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Abstract |
We present the first application of full-waveform ambient noise inversion to observed correlation functions that jointly constrains 3-D Earth structure and heterogeneous noise sources. For this, we model and interpret ambient noise correlations as recordings of correlation wavefields, which completely eliminates the limiting assumptions of Green's function retrieval, such as equipartitioning and homogeneous random noise sources. Our method accounts for seismic wave propagation physics in 3-D heterogeneous and attenuating media and also for the heterogeneous and nonstationary nature of the ambient noise field. Designed as a proof of concept, the study considers long periods from 100 to 300 s, thus focusing on the Earth's hum. Treating correlations as self-consistent observables allows us to make separate measurements on the causal and acausal branches of correlation functions, without any need to choose one of them or form the average. We validate our approach by assessing the quality of the obtained models and by comparing them to previous studies. This work is a step toward the establishment of full-waveform ambient noise inversion as a tomographic technique with the goal to go beyond ambient noise tomography based on Green's function retrieval. |
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Programme |
133 |
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ISSN |
2169-9356 |
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yes |
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Call Number |
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Serial |
7794 |
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Author |
Léo Bosse, Jean Lilensten, Nicolas Gillet, Sylvain Rochat, Alain Delboulbé, Stephane Curaba, Alain Roux, Yves Magnard, Magnar G. Johnsen, Unni-Pia Løvhaug, Pierre-Olivier Amblard, Nicolas Le Bihan, Maxime Nabon, Hanane Marif, Frédérique Auriol, Camille Noûs |
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Title |
On the nightglow polarisation for space weather exploration |
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Journal |
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Year |
2020 |
Publication |
Journal of Space Weather and Space Climate |
Abbreviated Journal |
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Volume |
10 |
Issue |
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Pages |
35 |
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Keywords |
polarisation / aurora |
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Abstract |
We present here observations of the polarisation of four auroral lines in the auroral oval and in the polar cusp using a new ground polarimeter called Petit Cru. Our results confirm the already known polarisation of the red line, and show for the first time that the three other lines observed here (namely 557.7 nm, 391.4 nm and 427.8 nm) are polarised as well up to a few percent. We show that in several circumstances, this polarisation is linked to the local magnetic activity and to the state of the ionosphere through the electron density measured with EISCAT. However, we also show that the contribution of light pollution from nearby cities via scattering can not be ignored and can play an important role in polarisation measurements. This series of observations questions the geophysical origin of the polarisation. It also leaves open its relation to the magnetic field orientation and to the state of both the upper atmosphere and the troposphere. |
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Programme |
1026 |
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ISSN |
2115-7251 |
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yes |
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Call Number |
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Serial |
7796 |
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Author |
F. Pitout, A. Marchaudon, K. J. Trattner, J. Berchem, H. Laakso, C. P. Escoubet |
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Title |
Simultaneous Polar and Cluster Observations in the Northern and Southern Middle-Altitude Polar Cusps Around Equinox |
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Journal |
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Year |
2020 |
Publication |
Journal of Geophysical Research: Space Physics |
Abbreviated Journal |
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Volume |
125 |
Issue |
12 |
Pages |
e2020JA028346 |
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Keywords |
dayside magnetosphere hemispheric asymmetry polar cusp |
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Abstract |
We present an event of simultaneous observations of the northern and southern middle-altitude polar cusps by the Polar spacecraft and Cluster fleet that occurred on 23 September 2004. We examine the possible asymmetries in the fields and plasma parameters, although the proximity of the equinox should limit these asymmetries. Ion sensors reveal two dispersions in both cusps, and data analysis leads to the conclusion that those dispersions are due to pulsed reconnection at a single X-line, which runs along the subsolar magnetopause. While the electromagnetic and particle energy fluxes injected in both cusp are globally very similar, we report significant differences in ion dispersions, width of the low-latitude boundary layer, and peak convection velocities. We ascribe these differences to the dipole tilt that introduces an asymmetry in the magnetosheath flow at the exterior cusps. |
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312 |
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2169-9402 |
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Approved |
yes |
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Call Number |
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Serial |
6363 |
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Author |
E. Aristidi, A. Agabi, L. Abe, E. Fossat, A. Ziad, D. Mékarnia |
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Title |
Dome C coherence time statistics from DIMM data |
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Journal |
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Year |
2020 |
Publication |
Monthly Notices of the Royal Astronomical Society |
Abbreviated Journal |
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Volume |
496 |
Issue |
4 |
Pages |
4822-4826 |
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Keywords |
Astrophysics – Instrumentation and Methods for Astrophysics |
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Abstract |
We present a reanalysis of several years of DIMM data at the site of Dome C, Antarctica, to provide measurements of the coherence time $\tau0$. Statistics and seasonal behaviour of $\tau0$ are given at two heights above the ground, 3m and 8m, for the wavelength $\lambda=500$nm. We found an annual median value of $2.9$ms at the height of 8m. A few measurements could also be obtained at the height of 20m and give a median value of 6ms during the period June--September. For the first time, we provide measurements of $\tau0$ in daytime during the summer, which appears to show the same time dependence as the seeing with a sharp maximum at 5pm local time. Exceptional values of $\tau0$ above 10ms are met at this particular moment. The continuous slow variations of turbulence conditions during the day offers a natural test bed for a solar adaptive optics system. |
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1066 |
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ISSN |
0035-8711, 1365-2966 |
ISBN |
0035-8711, 1365-2966 |
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yes |
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Call Number |
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Serial |
7825 |
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Author |
Catalina Morales-Yáñez, Zacharie Duputel, Luis Rivera |
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Title |
Impact of 3-D Earth structure on W-phase CMT parameters |
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Year |
2020 |
Publication |
Geophysical Journal International |
Abbreviated Journal |
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Volume |
223 |
Issue |
2 |
Pages |
1432-1445 |
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Keywords |
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Abstract |
We investigate the impact of unmodelled 3-D structural heterogeneity on inverted W-phase source parameters. We generate a large data set of synthetic seismograms accounting for the Earths 3-D structure for 250 earthquakes globally distributed. The W-phase algorithm is then used to invert for earthquake CMT parameters, assuming a spherical Earth model. The impact of lateral heterogeneity is assessed by comparing inverted source parameters with those used to compute the 3-D synthetics. Results show that the 3-D structure mainly affects centroid location while the effect on the other source parameters remains small. Centroid mislocations present clear geographical patterns. In particular, W-phase solutions for earthquakes in South America are on average biased 17 km to the east of the actual centroid locations. This effect is significantly reduced using an azimuthally well balanced distribution of seismological stations. Source parameters are generally more impacted by mantle heterogeneity while the scalar moment of shallow earthquakes seems to be mainly impacted by the crustal structure. Shallow earthquakes present a variability of Mrθ and Mrϕ moment tensor elements, resulting both from the small amplitude and a larger uncertainty of the associated Green’s functions. |
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133 |
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ISSN |
0956-540X |
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yes |
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Call Number |
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Serial |
7890 |
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Author |
Michael Lockwood, Mathew J. Owens, Carl Haines, Luke Barnard, Christopher John Scott, Aude Chambodut, Kathryn A. McWilliams, Alan W. P. Thomson |
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Title |
Universal Time Variations in Space Weather |
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Journal |
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Year |
2020 |
Publication |
Earth and space science open archive |
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Volume |
12 |
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We introduce the inductive effects of polar cap motions towards and way from the Sun into magnetospheric electrodynamics and show how this explains observed Universal Time variations in hemispheric geomagnetic indices. The large (and growing) hemispheric asymmetry in the offsets of the geomagnetic (dip or eccentric dipole) poles from Earth’s rotational axis means that the effect is not cancelled out in global indices. By adding this effect to that of the Russell-McPherron effect on solar wind-magnetosphere coupling, that of ionospheric conductivities, and that of the solar wind dynamic pressure and dipole tilt on the near-Earth tail lobe field and cross-tail current sheet, we are able to model the persistent “equinoctial” time-of-day/time-of-year pattern (with additional net Universal time variations) observed in the an, as and am geomagnetic indices since 1959. We discuss the implications for the longitudinal dependence of the effects of extreme space weather events |
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139 |
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yes |
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8464 |
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