Дисертації з теми "Hypoxie marine"

L’oxygène dissous dans l’eau de mer a régulièrement baissé dans de nombreux écosystèmes marins côtiers ces 50 dernières années du fait du réchauffement climatique et des activités humaines côtières croissantes. Cette diminution de l’oxygène présent dans l’eau de mer provoque des épisodes d’hypoxie de plus en plus fréquents et sévères dans les zones côtières et estuariennes. Certains organismes peuvent échapper à ces contraintes environnementales par migration ou dispersion. D’autres, présentant des capacités de nages plus réduites, doivent mettre en oeuvre des régulations physiologiques (plasticité phénotypique) pour leur permettre de s’adapter et survivre. C’est notamment le cas des larves de poissons marins qui sont susceptibles d’être présentes dans les nourriceries côtières à la fin de leur développement. Il est aujourd’hui bien établi que les régulations mises en oeuvre par les organismes pour faire face à leur environnement durant les jeunes stades de vie peuvent imprégner leur fonctionnement physiologique sur le long-terme et modifier leur trajectoire de vie. Si de nombreuses informations relatives aux conditionnements précoces sont disponibles dans le domaine médical, les connaissances de ces effets chez les organismes marins dans un contexte écologique sont encore très fragmentaires. L’objectif principal de ce travail de thèse a donc été d’évaluer les conséquences physiologiques, non seulement à court-terme mais surtout à long-terme, d’une exposition de poissons marins en fin de développement larvaire à une hypoxie modérée (8 jours, 40% de saturation à l’air). L’autre originalité de ce travail reposait sur la comparaison des réponses adaptatives mises en oeuvre chez deux espèces, l’une pélagique ─ le bar Européen (Dicentrarchus labrax) ─ et l’autre benthique ─ la sole commune (Solea solea). Nos résultats nous ont permis de montrer un effet significatif à long-terme de la contrainte hypoxique précoce sur les performances physiologiques des juvéniles et notamment sur leur croissance. Les juvéniles de bars et de soles présentent respectivement de plus faibles et de plus fortes croissances après avoir été exposés à un environnement hypoxique au stade larvaire. L’effet négatif sur la croissance observé chez le bar s’explique, au moins en partie, par une moins bonne assimilation énergétique due à un impact sur la fonction digestive. Les analyses moléculaires réalisées au niveau hépatique chez le juvénile de bar révèlent, par ailleurs, une imprégnation au niveau transcriptionnel de l’expression de gènes impliqués dans la réponse à l’hypoxie. L’effet positif observé sur la croissance des juvéniles de sole est associé à une meilleure tolérance aux stress thermiques et hypoxiques et à une diminution du registre métabolique. Les résultats obtenus chez le bar et la sole suggèrent que des modifications d’allocations énergétiques sous-tendent les effets observés à long-terme sur les performances physiologiques. Plus globalement, l’ensemble des données obtenues au cours de cette thèse soutiennent l’importance que revêt la nature de l’environnement rencontré par le poisson au cours de ses jeunes stades de vie sur ses futures performances physiologiques. Nos résultats indiquent par ailleurs que l’imprégnation physiologique par l’environnement est espèce-spécifique révélant des probables stratégies d’adaptation différentes suivant les espèces
The dissolved oxygen in seawater has steadily declined in many coastal marine ecosystems in the last 50 years because of global warming and increasing coastal human activities. This decrease in oxygen in seawater causes episodes of hypoxia increasingly frequent and severe in coastal and estuarine areas. Some organisms may escape these environmental constraints by migration or dispersion. Others, with limited swimming capabilities must implement physiological regulation (phenotypic plasticity) to enable them to adapt and survive. This is particularly the case of marine fish larvae that are likely to be present in coastal nursery at the end of their development. It is now well established that the regulations implemented by organisms to cope with their environment during the early stages of life can imprint their physiological function in the long-term and change their life trajectory. While much information on early conditioning is available in the field of medicine, knowledge of these effects in marine organisms in an ecological context are still very fragmentary. Therefore, the main objective of this thesis was to evaluate the physiological consequences, not only in the short-term but especially in the long-term, of an exposition to a moderate hypoxia (8 days, 40% air saturation) of marine fish at the end of larval development. Another originality of this work was based on the comparison of adaptive responses implemented in two species, one pelagic ─ European sea bass (Dicentrarchus labrax) ─ and one benthic ─ the common sole (Solea solea). Our results have allowed us to show a significant long-term effect of early hypoxic stress on the physiological performance of juvenile and especially their growth. Juveniles of European sea bass and juveniles of common sole have, respectively, lower and higher growth after being exposed to a hypoxic environment in the larval stage. The negative effect on growth observed in sea bass was due, in part, to lower energy assimilation as a consequence of an impact on the digestive function. Moreover, molecular analyzes in the liver of sea bass juveniles indicate an impregnation at the transcriptional level of the expression of genes involved in the response to hypoxia. The positive effect observed on the growth of sole juveniles is associated with better tolerance to thermal and hypoxic stress and a decrease in metabolic scope. The results obtained in European sea bass and common sole suggest that changes in energy allocations could explain the physiological performances observed in the long-term. Globally, all the data obtained in this thesis underline the importance of the environmental parameters encountered by fish during their young life stages on their future physiological performances. Such physiological impregnation by the early environment is species-specific, revealing potential different adaptation strategies

Because of the eutrophication resulting from increasing anthropogenic activities, hypoxia (i.e. dissolved oxygen < 2.8 mg O2 L-1) is on the rise globally. The objective of this research was to understand more about the effects of hypoxia on the marine benthic communities. Particularly, it focused on the latent effects and indirect effects of hypoxia by investigating how early exposure to hypoxia affect the later life stage of a marine gastropod Crepidula onyx, and how hypoxia alter the bacterial composition of biofilms and the subsequent larval settlement of marine invertebrates. In the first study, the larvae of C. onyx were exposed to 2, 3, and 6 mg O2 l-1. Under low food concentration (Isochrysis galbana at 1 × 105 cells l-1), larvae in both hypoxic treatments (2 and 3 mg O2 l-1) required a longer time to become competent to metamorphose. But when they did, they had a similar size and total lipid content to the control larvae. Moreover, the latent effects of early hypoxic exposure on the juvenile growth were evident. After 2 weeks development in field, the growth rate, mean dry weight and filtration rate of juveniles were significantly reduced in the hypoxic treatments. However, there was no discernible effect on larvae or juveniles when the food concentration during the larval stage was doubled (I. galbana at 2 × 105 cells l-1), suggesting that the latent effects of hypoxia can be offset by larval access to high algal concentration. In the second study, the biofilms were exposed to hypoxia and normoxia in microcosms for up to 7 days, and their bacterial community composition was analysed by terminal-restriction fragment length polymorphism (T-RFLP). The results suggested that hypoxia altered the bacterial community structure within biofilms, and the difference between the hypoxia and normoxia treatments increased through the length of exposure period. The resulting changes in biofilms did not alter the larval settlement response of a model species (i.e. C. onyx) in laboratory assays. Nevertheless, when the biofilms were deployed in the field to allow natural larval settlement and recruitment, biofilms that had been exposed to hypoxia altered the overall larval settlement pattern of different marine invertebrates, potentially leading to a shift in the benthic invertebrate community. This research suggested that periodic hypoxic events and the resulting exposure of organisms to hypoxia during their early development might have effects that persist across the life history. Moreover, it highlighted the possibility that the effects of hypoxia on species composition and structure of benthic invertebrate communities might be mediated through changes in biofilms and subsequently larval settlement and recruitment. To conclude, this research demonstrated that hypoxia could affect the growth in the later life stages of marine invertebrates and the recruitment of the benthic communities.
published_or_final_version
Biological Sciences
Master
Master of Philosophy

Hypoxia and anoxia have significant deleterious ecological effects on living resources throughout many estuarine and marine ecosystems worldwide. Brief periods of low oxygen facilitate transfer of benthic production to higher trophic levels as many benthic infaunal species have shallower sediment depth distributions during hypoxic events. A baited time-lapse camera equipped with a water quality datalogger was used to document in situ exploitation of oxygen-stressed benthic invertebrate prey organisms by mobile fish and crustacean predators during alternating normoxia-hypoxia cycles in the York River. Based on photographic and diver observations, this hypoxiainduced benthic-pelagic transfer of production is more likely to occur when environmental dissolved oxygen concentrations rise above an apparent threshold between 1 and 2 ml/1. When oxygen concentrations decline below 2 ml/1, the functional response of the predator to increased prey availability is interrupted. There is no energy gain by the predator until oxygen concentrations rise above this critical level when predators return to affected areas and resume feeding activity.

Les écosystèmes côtiers sont parmi les plus vulnérables aux changements globaux actuels, qui entraînent notamment une augmentation de la température de l'eau, ainsi que de la fréquence des épisodes hypoxiques. La coquille Saint-Jacques, Pecten maximus, est une espèce subtidale évoluant à des profondeurs de 2 à 210 m. Malgré son intérêt commercial et un intérêt écologique majeur, cette espèce n'a fait l'objet que de peu d'études au niveau moléculaire. L'objectif de cette thèse était de caractériser les mécanismes moléculaires régissant l'acclimatation de cette espèce aux contraintes thermique et hypoxique. Nous avons dans un premier temps caractérisé les modifications d'expression des gènes/protéines, par des approches transcriptomiques (RNAseq) et protéomiques (2-DE), dans un tissu, le manteau, d'animaux exposés à une contrainte thermique prolongée (56 jours). Nous avons ainsi pu identifier les voies majeures de régulation (eg., AP-1), les grandes fonctions (eg., cytosquelette) et processus (eg., apoptose) impliqués dans la réponse, mais également d'observer les grandes orientations du métabolisme (eg., dégradation des lipides de réserve). La réponse des organismes à l'hypoxie dépend de leur manière de gérer les faibles teneurs en oxygène. Nous avons d'abord, par une approche comparative avec une espèce intertidale, la moule (Mytilus spp.), caractérisée la réponse physiologique de la coquille Saint-Jacques à l'hypoxie. Nous avons pu ainsi déterminer ses paramètres d'oxyregulation, plus particulièrement son Point critique en 02 (Pc02). Le développement d'une approche protéomique, couplant l'effet de la température et de l'hypoxie, nous a ensuite permis d'identifier plusieurs protéines (CK2, GLN, etc.) potentiellement impliquées dans la réponse au niveau moléculaire. Enfin, dans l'optique de mieux comprendre la physiologie particulière de ces mollusques dans leur environnement naturel, nous avons comparé les signatures protéomiques de deux populations de P. maximus évoluant dans des écosystèmes contrastés, i.e. en limite nord- (Norvège) et au centre- (Brest) de l'aire de répartition de l'espèce. Les résultats suggèrent des différences majeures entre les deux populations au niveau du cytosquelette. En conclusion, ce travail ouvre des perspectives nouvelles pour la compréhension des mécanismes moléculaires régissant l'adaptation des mollusques aux contraintes thermiques et hypoxiques, deux stress particulièrement importants pour les organismes marins dans le contexte du changement global
Coasts are among the most vulnerable ecosystems to the ongoing global changes, which result in increased water temperatures and frequencies of hypoxic episodes. The great scallop, Pecten maximus, is a subtidal species living at depths of 2-210 m. In spite of its commercial and major ecological values, only few studies at the molecular level were performed on this species. This thesis aimed at characterizing the molecular mechanisms implied in acclimation of this species to thermal and hypoxia stresses. We first characterized the changes of expression of the genes / proteins in response to a long-term thermal stress (56 days), by using both a transcriptomic- (RNAseq) and a proteomic- (2-DE based) approaches, in the mantle tissue of scallops. This allowed us to identify key regulatory pathways (eg., AP-1), the major functions (eg., cytoskeleton) and processes (eg., apoptosis) involved in the response, but also to observe the main orientations of metabolism (eg., degradation of lipid reserves). The response of organisms to hypoxia depends on how they cope with low oxygen availability. Therefore, we first carried out a comparative approach with an intertidal species, the mussel (Mytilus spp.) to characterize the physiological response of P. maximus to hypoxia. Of note, we could determine its oxyregulatory parameters, particularly its critical point in 02 (Pc02). Then, coupling the effects of temperature and of hypoxia, we developed a proteomic approach that allowed us to identify several proteins (CK2, GLN, etc.) potentially involved in the response at the molecular level. Finally, in an effort to better understand the particular physiology of these mollusks in their natural environment, we compared the proteomic signatures of two populations of P. maximus living in highly contrasted ecosystems, ie in the northern limit- (Norway) and the center- (Brest) of the biogeographical distribution of this species. The results suggest major differences between the two populations, especially at the cytoskeleton level. In all, this work opens new avenues for understanding the molecular mechanisms governing the adaptation of mollusks to heat and hypoxia, two stresses that will most probably greatly influence the lifestyle of marine organisms and populations in future years

Although rivers are the primary source of dissolved inorganic nitrogen (DIN) inputs to the Chesapeake Bay, direct atmospheric DIN deposition and DIN fluxes from the continental shelf can also significantly impact Chesapeake Bay hypoxia. The relative role of these additional sources of DIN has not previously been thoroughly quantified. In this study, the three-dimensional Estuarine-Carbon-Biogeochemistry model embedded in the Regional Ocean Modeling System (ChesROMS-ECB) is used to examine the relative impact of these three DIN sources. Model simulations highlight that DIN inputs from the atmosphere have roughly the same impact on hypoxia as the same gram for gram change in riverine DIN loading. DIN inputs from the shelf have a similar overall impact on hypoxia as those from the atmosphere (~0.2 mg L-1), however the mechanisms driving these impacts are different. While atmospheric DIN impacts dissolved oxygen (DO) primarily via the decomposition of autochthonous organic matter, coastal DIN also impacts DO via the decomposition of allochthonous organic matter entering the Bay from the continental shelf. The impacts of coastal and atmospheric DIN on estuarine hypoxia are greatest in the summer, and occur farther downstream (lower mesohaline) in wet years than in dry years (upper mesohaline). Integrated analyses of the relative contributions of all three DIN sources on summer bottom DO concentrations indicate that impacts of atmospheric deposition are largest in shallow near-shore regions, riverine DIN has dominant impacts in the largest tributaries and the oligohaline Bay, while coastal DIN fluxes are most influential in the polyhaline region. During the winter when estuarine circulation is strong and shelf DIN concentrations are relatively high, coastal DIN impacts bottom DO throughout the Bay.

Kyoto University (京都大学)
0048
新制・課程博士
博士(農学)
甲第21830号
農博第2343号
新制||農||1068(附属図書館)
学位論文||H31||N5202(農学部図書室)
京都大学大学院農学研究科応用生物科学専攻
(主査)教授 佐藤 健司, 教授 澤山 茂樹, 准教授 豊原 治彦
学位規則第4条第1項該当

Les inclusions des cellules gliales et des neurones des ganglions cérébroïdes de crépidule et cérépleuraux de la moule ont été examinés. Une première approche a permis de distinguer les granules de neurosécretion de type neuroendocrine des inclusions pigmentées. L'ultrastructure de ces inclusions permet de les caracteriser avec précision. L'étude biochimique révèle la présence de carotènes et de différentes xanthophylles. On précise enfin le rôle de ces inclusions dans des conditions d'hypoxie et d'anoxie du milieu

Les scénarios de changements globaux sont précieux pour guider les stratégies de gestion et de gouvernance, inciter à la prise de décision et augmenter la prise de conscience collective des tendances futures de la biodiversité. Le degré de réalisme et d'intégration des modèles écosystémiques utilisés à cet effet est en constante progression, mais ils négligent encore souvent l'évolution des populations marines dans les projections futures. Or, celles-ci s'adaptent aux changements globaux, que ce soit par la plasticité phénotypique ou l'évolution, au travers de modifications de leurs caractéristiques biologiques telles que les traits d'histoire de vie, physiologiques et bioénergétiques. L'enjeu de cette thèse est de développer un modèle écosystémique qui permette d'explorer des scénarios de biodiversité aux échelles intra- et inter-spécifiques en représentant explicitement la plasticité phénotypique des traits d'histoire de vie, leur variabilité génétique, leur sélection et leur évolution sous l'influence combinée de la pêche et du changement climatique, ainsi que la dérive génétique et la perte de diversité génétique qui en résultent. Appliqué à la mer du Nord, ce nouveau modèle est utilisé pour comprendre les processus responsables des changements de traits d'histoire de vie qu'ils soient d'origine plastique ou d'origine évolutive. D'une part, les processus bioénergétiques sous-jacents aux changements plastiques sont étudiés par une approche originale comparant les différences entre les courbes de réponses thermiques fondamentales et réalisées pour différentes espèces et stades du cycle de vie. D'autre part, les changements des traits d'histoire de vie sont explorés à travers le prisme de l'évolution grâce à la prise en compte de pressions de sélection multiples telles que la pêche, les interactions proies-prédateurs et le changement climatique.L'intégration des processus plastiques et évolutifs dans les modèles écosystémiques permet de décrire la variabilité interindividuelle des traits biologiques et de comprendre leurs tendances temporelles observées dans le milieu marin. En cela, elle répond à l'enjeu crucial de crédibilité des projections de la biodiversité intra- et inter-spécifique sous scénarios combinant climat et pêche. L'intégration de ces processus permettra également de quantifier plus précisément les effets synergiques et antagonistes de ces deux pressions et de prendre en compte la capacité d'adaptation des populations aux changements globaux pour estimer de manière plus fiable leur résilience
Global change scenarios are valuable for guiding management and governance strategies, stimulating decision making, and increasing collective awareness of future biodiversity trends. The degree of realism and integration of ecosystem models used for this purpose is constantly improving, but they still often neglect the evolution of marine populations in future projections. However, marine populations adapt to global changes, either through phenotypic plasticity or evolution, through modifications of their biological characteristics such as life history traits, physiological and bioenergetic traits. The challenge of this thesis is to develop an ecosystem model that allows the exploration of biodiversity scenarios at intra- and inter-specific scales by explicitly representing the phenotypic plasticity of life history traits, their genetic variability, selection and evolution under the combined influence of fisheries and climate change, and the resulting genetic drift and loss of genetic diversity. Applied to the North Sea, this new model is used to understand the processes responsible for changes in life history traits, whether they are of plastic or evolutionary origin. On the one hand, the bioenergetic processes underlying plastic changes are studied by an original approach comparing the differences between the fundamental and realized thermal response curves for different species and life history stages. On the other hand, changes in life history traits are explored through an evolutionary lens by taking into account multiple selection pressures such as fishing, prey-predator interactions and climate change.The integration of plastic and evolutionary processes in ecosystem models allows to describe the inter-individual variability of biological traits and to understand their temporal trends observed in the marine environment. In this way, it responds to the crucial issue of credibility of intra- and inter-specific biodiversity projections under scenarios combining climate and fisheries. The integration of these processes will also allow to quantify more precisely the synergistic and antagonistic effects of these two pressures and to take into account the capacity of populations to adapt to global changes in order to estimate more reliably their resilience

The northwestern (NW) continental shelf of the Black Sea undergoes seasonal hypoxia. The benthic environment, the exchanges at the water-sediment interface and the diagenetic reactions are influenced by this phenomenon. In the framework of the BENTHOX project, two field cruises were conducted on the shelf in spring 2016 and in summer 2017.The first part of this investigation concerned the study of the impact of low oxygen levels in bottom waters on the diagenetic reactions. The microprofilings of geochemical parameters and the flux measurements showed both spatial and temporal variabilities in the benthic compartment of the NW continental shelf for the two seasons studied. The areas closest to the coasts exhibited the most important fluxes of oxygen consumption and of sulfate. These regions were strongly influenced by riverine inputs inducing a higher productivity and in turn resulting in an increase in the fluxes of organic matter deposited on the seabed.The diagenetic reactions were impacted by seasonal deoxygenation in bottom waters. The oxic respiration was less important in the summer as reflected by the shallower penetration depth of dissolved oxygen. Since 1995, the diffusive oxygen uptake (DOU) reported during hypoxic period indicated that the concentration of dissolved oxygen played an important role in the benthic exchange fluxes. Furthermore, a shallower reduction of sulphate and of Mn- and Fe-oxides observed in the sediments evidenced the impact of low oxygen levels on the diagenetic cascade. As a consequence, the benthic mineralization of organic matter was affected. During summer 2017, the oxic mineralization of organic carbon was less important and the contribution of the reducing species to oxygen consumption increased. The anaerobic mineralization of organic matter became thus the dominant process during the period of deoxygenation.The second part concerned the study of the sulfur and iron cycling in the shelf sediments during the low oxygen event of summer 2017. The sediments from the station close to the mouth of the Danube and that near the Dnieper exhibited a signature of detrital material different from that of the upper crust. The inputs of organic matter, in particular transported by the rivers, could influence the signal. The sediments of the NW shelf investigated were however of marine origin.Following the sulfate reduction already observed and mentioned above, the hydrogen sulfides produced were rapidly consumed as suggested by the low concentrations of acid volatile sulfide (AVS) and of the dissolved sulfide. The presence of pyrite in the upper layers of the sediments close to the water-sediment interface was plausible because the formation of pyrite in the water column had been reported due to the higher resistance of pyrite to oxidation compared to AVS (Wijsman et al. 2001). A non steady-state deposition was observed as suggested by Wijsman et al. (1999). It was characterized by an abrupt drop in the pyrite content caused possibly by fluctuations in salinity, dissolved O2 concentrations and organic matter fluxes.The non-reactive iron constituted the principal fraction of the total iron present in the sediments. Its contribution fell in the range reported for the marine sediments of the continental margins. A spatial variation of the concentration of highly reactive iron was nevertheless observed. Near the Dnieper mouth, the contents of reactive iron were lower and could be attributed to the less important fluvial inputs, to the sorption and/or precipitation processes or yet to the reallocation of the reactive iron of the shelf deposits towards the sediments in deep and euxinic waters.The last part of this thesis concerned the biogeochemistry of metals (Fe, Mn, Zn, Cu, Ni, Hg, Co and Cd) during the early diagenesis and the assessment of enrichment and pollution of these metals in the sediments of the shelf. The results showed that trace metals are probably linked to iron and manganese oxides as well as to sulphides. Ni and Zn could be involved in adsorption onto and co-precipitation with Fe- and Mn-oxides. Cu seemed to be associated preferentially to sulphides but no clear correlation was found. The enrichment and pollution of these metals in the shelf sediments showed spatial variability. The Danube delta area was enriched in Ni, Cu and Zn while the Odessa region was enriched in Co and Cd. The pollution of shelf sediments ranged from “unpolluted” to “very highly polluted”. Since the late 20th century, the heavy metal contents in shelf sediments remained stable in the Odessa region but increased in the Danube delta area.
Le plateau continental du Nord-Ouest de la Mer Noire subit une hypoxie saisonnière. L'environnement benthique, l'échange à l'interface eau-sédiment et les réactions diagénétiques sont influencés par ce phénomène. Dans le cadre du projet BENTHOX, deux campagnes de prélèvement ont été menées sur le plateau au printemps 2016 et à l'été 2017 dans le but de comprendre ces changements.La première partie de cette recherche concerne l’impact des faibles concentrations en oxygène dans les eaux de fond sur les réactions diagénétiques. Les micro-profils de paramètres géochimiques et les mesures de flux ont montré des variabilités spatiales et temporelles dans le compartiment benthique pour les deux saisons étudiées. Les zones les plus proches des côtes ont présenté les flux les plus importants de consommation d'oxygène et de sulfate. Ces régions ont été fortement influencées par les apports fluviaux induisant une productivité plus élevée et entraînant à son tour une augmentation des flux de matière organique déposée sur le fond marin. Les réactions diagénétiques sont affectées par la désoxygénation saisonnière des eaux de fond. La respiration oxique etait moins importante en été comme en témoigne la profondeur de pénétration moins profonde de l’oxygène. Depuis 1995, l’absorption d’oxygène dissous (DOU) rapportée pendant la période limitée en oxygène a indiqué que la concentration d'oxygène dissous jouait un rôle important dans les flux d'échanges benthiques. De plus, une diminution de la profondeur à laquelle la réduction des sulfates et la réduction des oxydes métalliques témoigne de l'impact d'une faible concentration en oxygène sur la cascade diagénétique. En conséquence, la minéralisation benthique de la matière organique a été affectée. Au cours de l'été 2017, la minéralisation oxique du carbone a été moins importante et la contribution des espèces réductrices à la consommation d’oxygène a augmenté. La respiration anaérobie de la matière organique est ainsi devenue le processus dominant pendant la période d'hypoxie.La deuxième partie portait sur l'étude du cycle du soufre et du fer dans les sédiments du plateau lors de l'événement de faible teneur en oxygène de l'été 2017. Les sédiments de la station proche de l'embouchure du Danube et de celle du Dniepr présentaient une signature de matière détritique différente de celle de la croûte supérieure. Les apports de matière organique, notamment transportés par les rivières, pourraient influencer le signal. Les sédiments du plateau nord-ouest étudiés étaient cependant d'origine marine. Suite à la réduction du sulfate déjà observée et mentionnée ci-dessus, les sulfures d'hydrogène produits ont été rapidement consommés comme le suggèrent les faibles concentrations en sulfure acide volatil (AVS) et en sulfure dissous. La présence de pyrite dans les couches supérieures des sédiments près de l'interface eau-sédiment était plausible car la formation de pyrite dans la colonne d'eau avait été signalée en raison de la résistance plus élevée de la pyrite à l'oxydation par rapport à l'AVS (Wijsman et al. 2001). Un dépôt à l'état non stationnaire a été observé comme le suggèrent Wijsman et al. (1999). Elle a été caractérisée par une chute brutale de la teneur en pyrite causée peut-être par des fluctuations de salinité, des concentrations d'O2 dissous et des flux de matière organiqueLe fer non réactif constituait la fraction principale du fer total présent dans les sédiments. Sa contribution est tombée dans la gamme signalée pour les sédiments marins des marges continentales. Une variation spatiale de la concentration en fer hautement réactif a néanmoins été observée. Près de l'embouchure du Dniepr, les teneurs en fer réactif étaient plus faibles et pouvaient être attribuées aux apports fluviaux moins importants, aux processus de sorption et / ou de précipitation ou encore à la réallocation du fer réactif des dépôts du plateau vers les sédiments en profondeur et eaux euxiniques.La dernière partie de cette thèse concerne la biogéochimie des métaux (Fe, Mn, Zn, Cu, Ni, Hg, Co et Cd) lors de la diagenèse précoce et l'évaluation de l'enrichissement et de la pollution de ces métaux dans les sédiments du plateau. Les résultats ont montré que les métaux traces sont probablement liés aux oxydes de fer et de manganèse ainsi qu'aux sulfures. Ni et Zn pourraient être impliqués dans l'adsorption et la coprécipitation avec les oxydes de Fe et de Mn. Le Cu semble être associé préférentiellement aux sulfures mais aucune corrélation claire n'a été trouvée. L'enrichissement et la pollution de ces métaux dans les sédiments du plateau ont montré une variabilité spatiale. La zone du delta du Danube a été enrichie en Ni, Cu et Zn tandis que la région d'Odessa s'est enrichie en Co et Cd. La pollution des sédiments du plateau allait de « non polluée » à « très fortement polluée ». Depuis la fin du 20e siècle, la teneur en métaux lourds des sédiments du plateau est restée stable dans la région d'Odessa mais a augmenté dans la région du delta du Danube.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished

Community models predict the effects of stress on community structure and processes. I tested the Menge and Sutherland 1987 model in an estuarine epifaunal community experiencing low oxygen stress, termed hypoxia. Epifauna, animals living on the surfaces of substrates, are ecologically important in many estuaries where hypoxia occurs, yet little is known about the effects of hypoxia on these communities. Epifauna formed dense communities in the York River, a tributary of the Chesapeake Bay, USA, despite frequent hypoxia. Abundance and species composition was similar in two areas of the river, even though the downstream study area often experienced lower oxygen concentrations during hypoxic episodes than the upstream study area. Many dominant species exposed to high and low oxygen in the laboratory had a median lethal time (LT50) in hypoxia greater than the duration of typical hypoxic episodes in the York River, suggesting that hypoxia may cause little mortality for many species in this system. Predation by a variety of taxa decreased during hypoxia in the laboratory, because many mobile predators had higher mortality than sessile prey, and because predation rates decreased. Peak recruitment of dominant taxa, and of total epifauna, in the York River occurred during neap tides in the downstream study area, coinciding with the lowest oxygen concentrations. In the laboratory, low oxygen decreased recruitment of dominant taxa, but some recruitment continued in hypoxia for most taxa, indicating that larvae of dominant epifauna are tolerant of hypoxia. Larval tolerance of hypoxia may allow communities to persist even though the recruitment season of many epifaunal species coincides with the peak season of hypoxia. These findings supported some predictions of the consumer stress model, but not all. as predicted, when stress increased, the importance of disturbance for determining community structure increased, while the importance of predation decreased. Unlike predictions, stress changed recruitment rates in the laboratory. There were few effects of stress on abundance and diversity, possibly because in this system hypoxia is mild, brief, and because the species in this community can tolerate stress, colonize disturbed areas quickly, and grow quickly enough to complete life-cycles between hypoxic episodes.

The development of hypoxia represents one of the most common and ecologically detrimental effects of anthropogenic nutrient enrichment in coastal marine ecosystems. Due to the physiological importance of oxygen as a key component of metabolic processes, the development and persistence of hypoxia can reduce the distribution of important species, modify food webs, decrease diversity and richness, and sub-lethally affect growth and reproductive rates. While many recent studies have focused on the global increase in hypoxia and highlighted the need for nutrient reduction strategies, some key processes associated with hypoxia remain understudied. of particular importance is the resolution of the major carbon sources fueling hypoxia in tributary estuaries, which receive inputs from the upland watershed, internal primary production, and advection from the main estuary, which may also be a source of hypoxic water. Development of well-constrained, intermediate complexity ecosystem models is also needed to provide realistic predictions of the response of hypoxia to nutrient reduction strategies, and to understand the interactive effects of these load reductions with ongoing climate change. The recent implementation of high spatial and temporal resolution water quality sampling instruments has confirmed the importance of the spring-neap tidal cycle and its effect on the formation and disruption of stratification and hypoxia in the York River estuary (YRE). However, these results have indicated that the advection of high-salinity hypoxic water into the lower YRE from the Chesapeake Bay (CB) may be as important as internal oxygen consumption. Additionally, previous studies have suggested that phytoplankton production in the YRE and similar tributary estuaries may be insufficient to explain the magnitude of hypoxia observed. This study synthesized in-situ measurements and high resolution water quality monitoring with an intermediate complexity model to examine the significance of these factors and how they interact to cause hypoxia within the YRE. Simulations were used to determine the magnitude of nutrient and/or organic matter (OM) reductions required to reduce the extent and severity of hypoxia in the presence of increasing temperatures resulting from climate change. A comparison of in-situ and computed oxygen concentrations for the YRE indicated that internal respiration was sufficient to drive hypoxia under stratified conditions, without the need for advection of hypoxic water from the CB. Phytoplankton production was the major source of organic carbon to the YRE and 1.5 times greater than advective inputs from CB, which were roughly balanced by exports. Watershed sources and microphytobenthos contributed comparatively little carbon to the whole system. Model simulations indicated that lower portions of the YRE tributaries are strongly influenced by watershed OM loading during summer, while the low mesohaline region is influenced by internal primary production and OM from the tributaries. The high meso- and polyhaline regions responded primarily to advected dissolved organic carbon from CB. Results indicate that different regions of the YRE require separate management strategies to control hypoxia, with the key issue in the lower estuary being the "far field" source of labile OM from outside the system. The model predicted increasing primary production under warmer conditions in winter and spring throughout most of the YRE, but decreasing production in summer and fall in the lower estuary. These changes together with increasing respiration resulted in increased autotrophy in the upper YRE, while NEM was predicted to decrease throughout the rest of the estuary. Warmer temperatures increased both the temporal and spatial extent of hypoxia in the model, suggesting the need for additional nutrient and OM load reductions in order to achieve the same level of improvement predicted without warming.

The ocean is the most extended biome present on our planet. Recent decades have seen a dramatic increase in the number and gravity of threats impacting the ocean, including discharge of pollutants, cultural eutrophication and spread of alien species. It is essential therefore to understand how different impacts may affect the marine realm, its life forms and biogeochemical cycles. The marine nitrogen cycle is of particular importance because nitrogen is the limiting factor in the ocean and a better understanding of its reaction mechanisms and regulation is indispensable. Furthermore, new nitrogen pathways have continuously been described. The scope of this project was to better constrain cause-effect mechanisms of microbially mediated nitrogen pathways, and how these can be affected by biotic and abiotic factors. This thesis demonstrates that meiofauna, the most abundant animal group inhabiting the world’s seafloors, considerably alters nitrogen cycling by enhancing nitrogen loss from the system. In contrast, larger fauna such as the polychaete Marenzelleria spp. enhance nitrogen retention, when they invade eutrophic Baltic Sea sediments. Sediment anoxia, caused by nutrient excess, has negative consequences for ecosystem processes such as nitrogen removal because it stops nitrification, which in turn limits both denitrification and anammox. This was the case of Himmerfjärden and Byfjord, two estuarine systems affected by anthropogenic activities, such as treated sewage discharges. When Byfjord was artificially oxygenated, nitrate reduction mechanisms started just one month after pumping. However, the balance between denitrification and nitrate ammonification did not favor either nitrogen removal or its retention. Anoxia is also present in aggregates of the filamentous cyanobacteria Nodularia spumigena. This thesis shows that even in fully oxic waters, millimetric aggregates can host anaerobic nitrogen processes, with clear implications for the pelagic compartment. While the thesis contributed to our knowledge on marine nitrogen cycling, more data need to be collected and experiments performed in order to understand key processes and regulation mechanisms of element cycles in the ocean. In this way, stakeholders may follow and take decisions in order to limit the continuous flow of human metabolites and impacts on the marine environment.

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.

Hypoxia and ocean acidification (OA) are amongst the major environmental threats to marine ecosystems worldwide. Biofilms, the signpost to guide larval settlement of many benthic invertebrates, are known to be responsive to environmental changes and thus can become the crucial factor for the response of benthic invertebrate communities. This study aimed at investigating the individual and interactive effects of hypoxia and OA on biofilms and the subsequent effects on larval settlement. Biofilms collected from two sites (clean, hypoxic) were treated with a factorial design of low dissolved oxygen and/or low pH conditions in microcosms and the bacterial cell density and viability (by LIVE/DEAD® cell viability assays) were analyzed. Larval settlement preference was tested with the marine invertebrate, Crepidula onyx. The total bacterial cell densities of biofilms of the hypoxia and hypoxia and OA combination treatment were lower than that of the control biofilms for both sites. There was generally no significant difference in cell viability among control and different treatments for both sites. While the larval settlement rate on hypoxia and hypoxia and OA combination treated biofilms was significantly lower. In conclusion, this study revealed that hypoxia and OA are likely to affect larval settlement by alteration of biofilms, and this may lead to alterations in future coastal communities.

The aim of this thesis was to quantify experimentally in the field the effects of different timing regimes of hypoxia on the structure of benthic communities in a transitional habitat. The experiment was performed from 8 July to 29 July 2019 in a shallow subtidal area in Pialassa Baiona (Italy), a lagoon characterized by mixing regimes dominated by the tide. The benthic community was isolated using cylinders 15,5Cm x 20Cm size. Hypoxic conditions were imposed by covering the treated cylinders with a black plastic bag while control cylinders were left uncovered. We created 4 different timing regimes of hypoxia by manipulating both the duration of hypoxia (4 or 8 days) as well as the ratio between the duration of subsequent periods of hypoxia and the duration of a normoxic period between subsequent hypoxic events (D4R3/2, D8R3/2). At the end of each experimental trial, the benthic communities within each pot were retrieved, sieved in the field and subsequent analyzed in the laboratory where organisms were identified and counted. Results showed that benthic organism were generally negatively affected by hypoxic stress events. As expected, longer hypoxic events caused a stronger decrease of benthic community abundance. When the hypoxic events were interrupted by the normoxic event there were two different results. If the hypoxic period was too long, the normoxic period didn’t cause a positive recovery effect, and further decline of the benthic community was observed. Conversely normoxia had positive effects if the period of hypoxia was short enough not to compromise the benthic community. This resulted in a statistically significant interaction between the tested factors Duration and Ratio. Amphipods were the most sensitive organisms to hypoxia. We conclude that the effects of hypoxia can be greatly relieved by short normoxic periods if they happen frequently enough.

Seasonal hypoxia in the northern Gulf of Mexico has been mapped extensively and is known to overlap the habitat of the brown shrimp Farfantepenaeus aztecus on the Louisiana continental shelf. Yet the impacts of Gulf hypoxia on the profitable brown shrimp fishery in Louisiana remain largely unknown. The problem is primarily attributable to nonpoint source pollution in the Mississippi River, but awareness of the problem has not resulted in an effective policy solution to stem this pollution to date. Using the combination of a quantitative data analysis to look for a correlation between Farfantepenaeus aztecus and hypoxic water, a survey mailed to shrimp fishers in Louisiana, and qualitative interviews with shrimp fishers and environmental activists and lawyers in Louisiana, I will examine the potential of a legal tool, the Public Trust doctrine, to slow nonpoint source pollution into the Mississippi River.

Face à la perte massive de la biodiversité, il devient urgent d'accroître nos connaissances sur les populations afin de pouvoir mettre en place rapidement des mesures de conservation efficaces. Les oiseaux marins figurent parmi les plus menacées et méconnues des espèces d'oiseaux. Nombre de leurs espèces nichent sur des territoires isolés, en terriers ou dans des zones inaccessibles pour l'homme. Les méthodes de prospection traditionnelles sont majoritairement utilisées pour le suivi des populations d'oiseaux marins, mais leur efficacité reste incertaine. Par ailleurs, les nouveaux outils d'acquisition de données et d'analyses proposent de nouvelles perspectives mais restent trop peu éprouvés.Cette thèse propose, dans le contexte de la Réserve naturelle nationale des Terres australes françaises, d'explorer l'efficacité des méthodes traditionnelles pour l'estimation des populations de deux espèces à nidification côtière, de tester la bioacoustique comme nouvelle méthode d'estimation des oiseaux marins à nidification hypogée, et d'utiliser des données de comptage et de suivi démographique d'une population en déclin pour tester l'efficacité de la mise en place de mesures de conservation.Les méthodes de prospection traditionnelles sont les plus robustes et peuvent être utilisées efficacement pour l'estimation et le suivi des populations d'oiseaux marins. Ces méthodes peuvent être complétées par l'utilisation des nouvelles technologies, notamment la bioacoustique pour les prospections des espèces à nidification hypogée. La combinaison des méthodes et la compréhension solide de la biologie et de l'écologie des espèces permettra de les conserver efficacement sur le long-terme
Facing the massive loss of biodiversity, it is crucial to increase our knowledge about populations in order to rapidly implement effective conservation measures. Seabirds are among the most threatened and little-known of all bird species. Many of their species nest in isolated territories, in burrows or in areas inaccessible to man. Traditional survey methods are mainly used to monitor seabird populations, but their effectiveness remains uncertain. In addition, new data acquisition and analysis tools offer new perspectives, but remain too untested.This thesis proposes, in the context of the Réserve naturelle nationale des Terres australes françaises, to explore the effectiveness of traditional methods for estimating the populations of two coastal-nesting species, to test bioacoustics as a new method for estimating burrowing seabirds populations, and to use count and demographic monitoring data from a declining population to test the effectiveness of implementing conservation measures.Traditional survey methods are the most robust and can be used effectively to estimate and monitor seabird populations. These methods can be supplemented by the use of new technologies, in particular bioacoustics for surveys of hypogean nesting species. The combination of these methods and a sound understanding of the biology and ecology of the species will enable them to be conserved effectively over the long term

Les contraintes physiologiques cérébrales rendent le cerveau humain vulnérable à l'hypoxie, qu’elle soit environnementale (haute altitude) ou en lien avec une pathologie hypoxémiante. Parmi ces pathologies, et en raison de sa forte prévalence dans la population générale, le syndrome d'apnées obstructives du sommeil (SAOS) est un modèle physiopathologique reconnu des effets délétères de l'hypoxie sur le cerveau. Les épisodes cycliques d'apnées et d'hypopnées survenant au cours du sommeil qui caractérisent le SAOS entraînent une hypoxie intermittente, une fragmentation du sommeil et des fluctuations de la pression intra-thoracique, tous trois facteurs déclenchant des mécanismes intermédiaires contribuant au développement de maladies cardio-métaboliques ainsi que des répercussions cérébrales (troubles cognitifs et accidents vasculaires cérébraux (AVC)). Ce travail de thèse explore la relation bidirectionnelle entre les syndromes d’apnées du sommeil (SAS) et le cerveau. Le premier axe se concentre sur les conséquences neurocognitives du SAOS au travers du contrôle de la marche. Les répercussions neurocognitives du SAOS sont à ce jour bien décrites et des troubles de la marche ont récemment été mis en évidence, avec une relation de type dose-réponse entre la gravité du SAOS et la sévérité des troubles de la marche. Il a ainsi été suggéré que la marche pouvait représenter un marqueur des répercussions cérébrales du SAOS. Les effets du traitement par pression positive continue (PPC) sur le contrôle de la marche ont été investigués au cours de ce travail de Thèse, avec des résultats contrastés. Dans une première étude prospective contrôlée, 8 semaines de traitement par PPC entraînaient une amélioration du contrôle de la marche chez des patients atteints de SAOS sévère (Baillieul et al., 2018, Plos One). Afin de valider ces résultats et d'étudier les corrélats neurophysiologiques du lien entre marche et SAOS, nous avons mené un essai contrôlé randomisé étudiant l'impact de 8 semaines de traitement par PPC comparativement à la Sham-PPC (Baillieul et al., 2020, Soumis). Contrairement à notre hypothèse, nous n'avons constaté aucune amélioration du contrôle de la marche dans le groupe PPC, résultat corroboré par l'absence d'impact de la PPC sur les déterminants du contrôle de la marche. Le deuxième axe est centré sur les répercussions cérébro-vasculaires des SAS. SAS et AVC sont deux pathologies graves et étroitement liées, le SAS pouvant être à la fois cause et conséquence potentielle des AVC. Le présent travail est axé sur l'identification des traits phénotypiques de SAS chez les patients post-AVC, afin d'en améliorer le diagnostic (Baillieul et al., en préparation). Le dépistage du SAS post-AVC est crucial en raison du risque élevé de morbi-mortalité et de conséquences fonctionnelles associées au SAS après AVC, mais il ne peut être effectif sans une identification plus précise des patients à risque de SAS. Le troisième axe a été conçu comme une perspective qui servira au développement du deuxième axe. Dans ce dernier axe, le potentiel de l'imagerie cérébrale et en particulier de l'imagerie par résonance magnétique pour développer des marqueurs de récupération et étudier les mécanismes physiopathologiques des déficiences liées aux AVC sont présentés, au travers de la marche et de son contrôle. Les corrélats neuronaux de la marche comme activité post-AVC sont mis en évidence, en utilisant une approche de type Voxel-based lesion-symptom mapping (Baillieul et al., 2019, Hum. Mov. Sci.). Les marqueurs d'imagerie basés sur l’utilisation du Diffusion tensor imaging pour prédire la récupération de la marche post-AVC sont également présentés (Soulard et al., 2019, Neurology). Ces travaux sur les marqueurs d'imagerie cérébrale de la récupération post-AVC serviront à développer des outils pour les recherches à venir sur les corrélats neuronaux des SAS post-AVC
The human brain is a perfect example of our dependence on oxygen. Brain physiological constraints render it vulnerable to hypoxia, such as encountered in environmental conditions (high altitude exposure) or pathological hypoxemic conditions. Among those pathological conditions, and due to its high prevalence in general population and the various levels of hypoxia resulting of the different degrees of severity of the pathology, obstructive sleep apnoea syndrome (OSAS) is a pathophysiological model of choice to investigate the detrimental effects of hypoxia on the brain. The cyclical, repeated episodes of apnoea and hypopnea during sleep that characterize OSAS result in intermittent hypoxia, sleep fragmentation and fluctuations in intrathoracic pressure, which are stressors that triggers mechanisms contributing to the initiation and progression of life-threatening cardiometabolic diseases, as well as several brain repercussions, such as cognitive impairment and stroke. This Thesis work explores the bidirectional relationship between sleep apnoea syndromes (SAS) and the brain. The first axis is focused on the neurocognitive consequences of OSAS through the lens of gait control. The neurocognitive signature of OSAS has been thoroughly investigated but recently, gait impairments have been highlighted in severe OSAS, with dose-response relationship between OSAS severity and the magnitude of gait impairments. As gait control relies at least partly on frontal lobe functions, it has been suggested that gait could represent a marker of OSAS brain repercussions. We investigated the effects of continuous positive airway pressure (CPAP) treatment on gait control, with contrasting results. In a first prospective controlled study, eight weeks of CPAP improved gait control in severe OSAS patients (Baillieul et al., 2018, Plos One). In order to validate those results and investigate the neurophysiological correlates of the link between gait control and OSAS, we conducted a randomized controlled trial which investigated the impact of an 8-week CPAP treatment compared to sham-CPAP on gait control in severe OSAS patients (Baillieul et al., 2020, Submitted). Contrary to our hypothesis, we found no improvement in gait control in the CPAP group and this result is substantiated by the absence of impact of CPAP on the determinants of gait control, further illustrating the complexity of the OSAS-neurocognitive relationship. The second axis is focused on the cerebrovascular repercussions of SAS. SAS and stroke are both severe intertwined conditions, SAS being both cause and potentially consequence of stroke. The present work is focused on the identification of phenotypic traits of SAS in post-stroke patients, to improve diagnosis of SAS following stroke (Baillieul et al., in preparation). Screening stroke patients for SAS is crucial due to the high risk of morbimortality and functional consequences associated to SAS following stroke but cannot be achieved without a more accurate identification of patients at risk to develop SAS following stroke. The third axis has been conceived as a perspective that will serve the development of the second axis. In this last axis, the potential of brain imagery and in particular magnetic resonance imagery to develop markers of stroke recovery as well as investigate the pathophysiological mechanisms underlying stroke-related deficiencies are presented, with a specific focus on gait and walking activity. The neural correlates of walking activity following stroke are highlighted, using a voxel-based lesion-symptom mapping approach (Baillieul et al., 2019, Hum. Mov. Sci.). Imagery markers of walking recovery following stroke using diffusion tensor imaging are also presented (Soulard et al. 2019, Neurology). This work on brain imagery markers of stroke recovery will further serve the development of investigations focused on the neural correlates of SAS following stroke

Crustacean muscle fibers are some of the largest cells in the animal kingdom, with fiber diameters in the giant acorn barnacle (Balanus nubilus) exceeding 3 mm. Sessile animals with extreme muscle sizes and that live in the hypoxia-inducing intertidal zone – like B. nubilus – represent ideal models for probing the effects of oxygen limitation on muscle cells. We investigated changes in metabolism and structure of B. nubilus muscle in response to: normoxic immersion, anoxic immersion, or air emersion, for acute (6h) or chronic (6h exposures twice daily for 2wks) time periods. Following exposure, we immediately measured hemolymph pO2, pCO2, pH, Na+, Cl-, K+, and Ca+ then excised tergal depressor (TD) and scutal adductor (SA) muscles to determine citrate synthase (CS) activity, lactate dehydrogenase (LDH) activity, and D-lactate levels. We also prepared a subset of SA and TD muscles from the chronic barnacles for histological analysis of fiber diameter (Feret’s), cross-sectional area (CSA), mitochondrial distribution and relative density, as well as nuclear distribution and myonuclear domain size. There was a significant decrease in hemolymph pO2 and pCO2 following acute and chronic anoxic immersion, whereas air emersion pO2 and pCO2 was comparable to normoxic levels. Fiber CSA and diameter did not change significantly in either tissue, while myonuclear domain size in SA muscle was significantly lower in the anoxic and emersion groups than the normoxic control. Neither CS, nor LDH activity, showed any significant treatment effect in either tissue, whereas both muscles had significantly higher D-lactate levels after air emersion following acute (though not chronic) exposure. Thus far, our findings indicate that B. nubilus experience a general reduction in aerobic metabolism under anoxia, emersion is only mildly oxygen-limiting, and that muscle plasticity is occurring during chronic emersion and anoxia.

The high productivity of Eastern Boundary Upwelling Ecosystems (EBUE), some of the most productive ecosystems in the globe, is attributed to the nutrient rich waters brought up through upwelling. Climate change scenarios for coastal upwelling systems, predict an intensification of coastal upwelling winds. Associated with intensification in upwelling are biogeochemical changes such as ocean hypoxia and ocean acidification. In recent years, the California Current System (CCS) has experienced the occurrence of nearshore hypoxia and the novel rise of anoxia. This has been attributed to changes in the intensity of upwelling wind stress. The effects of some of the more severe hypoxia and anoxia events in the CCS have been mass mortality of fish and benthic invertebrates. However, the impacts on zooplankton in this system are not known. Meroplankton, those organisms which have a planktonic stage for only part of their life cycle, are an important component of zooplankton communities. The larval stage of benthic invertebrates forms an important link between benthic adult communities and planktonic communities. Larvae serve to disperse individuals to new locations and to link populations. They are also food for fish and planktonic invertebrates. This important life stage can spend long periods in the plankton (from days to months) where environmental conditions can affect larval health, subsequent settlement and recruitment success, and juvenile health. This research assesses the role of hypoxia and larval survivorship, and the relationship between individual abundance and community structure of larvae to environmental factors in the field. In laboratory experiments (Chapter 2), a suite of 10 rocky intertidal invertebrate species from four phyla were exposed to low oxygen conditions representative of the nearshore environment of the Oregon coast. Results revealed a wide range in tolerances from species with little tolerance (e.g. the shore crab Hemigrapsus oregonensis) to species with high tolerance (e.g. the California mussel Mytilus californianus). The differential responses across larvae to chronic hypoxia and anoxia potentially could affect their recruitment success and consequently, the structure and species composition of intertidal communities. Field studies (Chapter 3 & 4) explore the relationship between environmental variables and larval abundance and community structure. Chapter 3 focuses on broad taxonomic groups, while Chapter 4 focuses on larval decapods in particular. Fine focus was devoted to decapod larvae, due to laboratory findings of heightened sensitivity to hypoxia of decapod crabs. A finding that is also supported in the literature. The goal of field studies was to identify the environmental parameters that structure meroplankton and larval decapod communities and identify which of these parameters play a significant role in influencing larval abundance. A number of environmental variables contributed to meroplankton assemblage structure and larval decapod assemblage structure. These included distance from shore, depth, date, upwelling intensity, dissolved oxygen, and cumulative wind stress. Some of these factors occurred frequently in larval abundance models. In Chapter 3, individual abundance across broad taxonomic groups was most commonly explained by upwelling intensity while in Chapter 4, individual abundance of different decapod species was explained by cumulative wind stress, which is a proxy for upwelling intensity. The prominent role of upwelling related factors in explaining individual abundance is important considering climate change projections of an increased intensification of upwelling winds in EBUE.
Graduation date: 2012
Access restricted to the OSU Community at author's request from Jan. 6, 2012 - Jan. 6, 2013

Natural cycles of environmental variability and long-term deoxygenation in the ocean impose oxygen deficiency (hypoxia) on marine communities. My research exploits a naturally occurring hypoxia cycle in Saanich Inlet, British Columbia, Canada where I combined spatial surveys with remotely operated vehicles, ecological time-series from the subsea cabled observatory VENUS, and lab-based respirometry experiments to examine the influence of seasonally variable oxygen conditions on epibenthic communities. In situ oxygen thresholds established for dozens of fish and invertebrate species in this system show they naturally occur in lower oxygen levels than what general lethal and sublethal thresholds would predict. Expansion of hypoxic waters induced a loss of community structure which was previously characterized by disjunct distributions among species. Communities in variable hypoxia also have scale-dependent structure across a range of time scales but are primarily synchronized to a seasonal oscillation between two phases. Time-series revealed timing of diurnal movement in the slender sole Lyopsetta exilis and reproductive behavior of squat lobster Munida quadrispina in the hypoxia cycle. Hypoxia-induced mortality of sessile species slowed the rate of community recovery after deoxygenation. The 10-year oxygen time-series from VENUS, revealed a significant increase in the annual low-oxygen period in Saanich Inlet and that deoxygenation has occurred in this system since 2006. Differences in the critical oxygen thresholds (O2crit) and standard metabolic rates of key species (spot prawn Pandalus platyceros, slender sole, and squat lobster) determined the lowest in situ oxygen at which populations occurred and explained disproportionate shifts in distributions and community respiration. Finally, a meta-analysis on global O2crit reported for crustaceans showed that hypoxia tolerance differs among major ocean basins. Long-term trends of deoxygenation suggest a future regime shift may occur when the duration at which a system remains below critical oxygen levels exceeds the time needed for communities to recover. Species-specific traits will determine the critical threshold and the nature of the community response in systems influenced by variable states of oxygen deficiency. However, oceanographic and evolutionary history provides context when determining the regional response of benthic communities influenced by rapidly changing environments.
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La séquence sédimentaire de la carotte COR0503-CL05-37PC, prélevée dans le chenal Laurentien au centre du Golfe du Saint-Laurent (48°20'N - 61°29'W, à 408 m de profondeur), a fait l'objet de plusieurs analyses géochimiques et micropaléontologiques dans le but de reconstruire les variations de la productivité pélagique et des conditions du milieu benthique dans le golfe du Saint-Laurent pendant les derniers millénaires. Les analyses géochimiques (Corg et Corg/N) et isotopiques(δ13Corg et δ15N) montrent des variations importantes de flux de matière organique, notamment une augmentation du taux de carbone organique, depuis environ 2300 ans cal. BP, que l'on estime être liée à l'augmentation de l'apport en matière organique d'origine marine. Les kystes des dinoflagellés permettent également de reconstruire une augmentation de la productivité biogénique dans les eaux de surface. Par ailleurs, plusieurs changements ont été enregistrés dans les assemblages de foraminifères benthiques calcaires. Ces changements se manifestent essentiellement par l'augmentation de l'abondance relative, vers 2000 ans cal. BP, de Brizalina subaenariensis, Globobulimina auriculata et Bulimina exilis, des espèces tolérant de faibles taux d'oxygène et des flux de carbone organique élevés. La variation de l'abondance de Nonionellina labradorica indique des changements dans l'apport des eaux du Courant du Labrador. En outre, les analyses isotopiques (δ18O) sur les tests de foraminifères benthiques Bulimina marginata, montrent des variations de la température et/ou de l'origine des eaux profondes qui sont indépendantes de la variation de la productivité pélagique et de l'augmentation de l'apport en matière organique. ______________________________________________________________________________ MOTS-CLÉS DE L’AUTEUR : Golfe du Saint-Laurent, hypoxie, dinokystes, productivité, foraminifères benthiques, géochimie, isotopes stables, Holocène.

Continental shelves located along eastern boundary currents occupy relatively small volumes of the world’s oceans, yet are responsible for a large proportion of global primary production. The Oregon coast is among these ecosystems. Recent analyses of dissolved oxygen at shallow depths in the water column has suggested increasing episodes of hypoxia and anoxia, events that are detrimental to larger macro-faunal species. Microbial communities, however, are metabolically diverse, capable of utilizing alternative electron donors and acceptors, and can withstand transient periods of low dissolved oxygen. Understanding the phylogenetic and metabolic diversity of microorganisms in these environments is important for assessing the impact hypoxic events have on local and global biogeochemistry. Several molecular ecology tools were used to answer questions about the distribution patterns and activities of microorganisms residing along the coast of Oregon in this dissertation. Ribosomal rRNA fingerprinting and sequence analyses of samples collected during 2007-2008 suggested that bacterial community structure was not substantially influenced by changes in dissolved oxygen. However, substantial depth dependent changes were observed, with samples collected in the bottom boundary layer (BBL) displaying significant differences from those collected in the surface layer. Phylogenetic analyses of bacterial rRNA genes revealed novel phylotypes associated with this area of the water column, including groups with close evolutionary relationships to putative or characterized sulfur oxidizing bacteria (SOB). Analysis of metagenomes and metatranscriptomes collected during 2009 suggested increasing abundances of chemolithoautrophic organisms and their activities in the BBL. Thaumarchaea displayed significant depth dependent increases during the summer, and were detected at maximal frequencies during periods of hypoxia, suggesting that nitrification maybe influenced by local changes in dissolved oxygen. Metagenomic analysis of samples collected from 2010 revealed substantial variability in the metabolic potential of the microbial communities from different water masses. Samples collected during the spring, prior to upwelling clustered independently of those collected during the summer, during a period of upwelling, and did not display any clear stratification. Samples collected during the summer did cluster based on depth, consistent with previous observations, and increases in the relative abundances of chemolithotrophic gene suites were observed in the BBL during stratified conditions, suggesting that the metabolic potential for these processes is a repeatable feature along the Oregon coast. Overall, these observations suggest that depth impacts microbial community diversity, metabolic potential, and transcriptional activity in shallow areas of the Northeast Pacific Ocean. The increase in lithotrophic genes and transcripts in the BBL suggests that this microbial community includes many organisms that are able to use inorganic electron donors for respiration. We speculate that the dissolved organic material in the BBL is semi-labile and not available for immediate oxidation, favoring the growth for microorganisms that are able to use alternative electron donors.
Graduation date: 2012