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International audience; The ecological niche sensu Hutchinson is defined as the set of environmental conditions allowing a species to grow, maintain, and reproduce. This conception of the niche, which is assimilated to a p-dimensional hypervolume, with p representing all environmental variables, has been widely applied in ecology. However, displaying the niche hypervolume has proved challenging when more than three environmental dimensions are considered simultaneously. We propose a simple method (implemented

Supplementary text describing the datasets we used and providing key references. Supplementary figures displaying the multidimensional ecological niches of the 117 plankton species we used in our study.

The apparently unimodal latitudinal diversity gradient of Paleozoic phytoplankton is shown to be a truncated bimodal gradient., The latitudinal diversity gradient (LDG)—the decline in species richness from the equator to the poles—is classically considered as the most pervasive macroecological pattern on Earth, but the timing of its establishment, its ubiquity in the geological past, and explanatory mechanisms remain uncertain. By combining empirical and modeling approaches, we show that the first representatives of marine phytoplankton exhibited an LDG from the beginning of the Cambrian, when most major phyla appeared. However, this LDG showed a single peak of diversity centered on the Southern Hemisphere, in contrast to the equatorial peak classically observed for most modern taxa. We find that this LDG most likely corresponds to a truncated bimodal gradient, which probably results from an uneven sediment preservation, smaller sampling effort, and/or lower initial diversity in the Northern Hemisphere. Variation of the documented LDG through time resulted primarily from fluctuations in annual sea-surface temperature and long-term climate changes.

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Climatic changes are disrupting otherwise tight trophic interactions between predator and prey. Most of the earlier studies have primarily focused on the temporal dimension of the relationship in the framework of the match-mismatch hypothesis. Using a novel timeseries model explicitly quantifying both timing and the abundance component for predator-prey relationship, we show that timing and abundance of food affect the northsea cod (Gadus morhua) recruitment. The system shows a strong effect of matchmismatch; however, it does so in association with food abundance. The food abundance was shown to have a threshold effect on recruitment. This non-linear effect may explain the difficulties to reveal the mechanisms by which environment variability affects marine ecosystems. As such the quantification of the combined effect of abundance and timing of prey on predator dynamics will improve our ability to detect the effect of environmental changes on trophic interactions.

Oceanic inflow is estimated to contribute more than 90% of the nutrient input into the North Sea (NSTF, 1993). Variability in the volume, chemical properties, biological content and source of the inflowing water is thus likely to have a considerable effect on North Sea ecosystems. Changes seen in the plankton over the last 40 years, measured by the Continuous Plankton Recorder (CPR) survey, allow clear periods to be identified that appear to be associated with variability in inflow. Monthly estimates of inflow, outflow and netflow across a section between Orkney and Utsira in Norway, have been derived from runs of the NORWECOM model for two depth intervals: surface to 150m and >150m. A comparison is made between these modelled physical measurements and plankton results for the period 1958 to 1999. Distinguished plankton periods that appear to reflect changing inflow events are discussed in relation to hydrometeorological and fisheries variability and earlier plankton studies over approximately the last 100 years.

Despite evidence from a number of Earth systems that abrupt temporal changes known as regime shifts are important, their nature, scale and mechanisms remain poorly documented and understood. Applying principal component analysis, change‐point analysis and a sequential t‐test analysis of regime shifts to 72 time series, we confirm that the 1980s regime shift represented a major change in the Earth's biophysical systems from the upper atmosphere to the depths of the ocean and from the Arctic to the Antarctic, and occurred at slightly different times around the world. Using historical climate model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and statistical modelling of historical temperatures, we then demonstrate that this event was triggered by rapid global warming from anthropogenic plus natural forcing, the latter associated with the recovery from the El Chichón volcanic eruption. The shift in temperature that occurred at this time is hypothesized as the main forcing for a cascade of abrupt environmental changes. Within the context of the last century or more, the 1980s event was unique in terms of its global scope and scale; our observed consequences imply that if unavoidable natural events such as major volcanic eruptions interact with anthropogenic warming unforeseen multiplier effects may occur.

Estuaries are among the most productive ecosystems on Earth. They constitute important areas for materials exchanges and play therefore a crucial part in biogeochemical cycles. Because of their location at the interface between marine and continental realms, they are also a migratory path for different fish species and in some cases, a nursery or a refuge area. These systems are submitted to many pressures, some of them being related to anthropogenic activities (e.g fisheries, maritime traffic, dredging, rising pollutions, etc.). These changes are superimposed to the intrinsic variability of these naturally complex ecosystems (characterized by a spatio-temporal variability of different environmental factors; i.e., temperature, salinity, turbidity, etc.). However, since recent years, an intensification of these alterations, being partly linked to the climatic component of Global Change, is observed. These changes, already documented, and their incidence on biological communities, constitute a challenge for the future management of estuarine ecosystems. However, due to a lack of long-term continuous time series, for now few studies on the long-term evolutions of estuarine systems have been reported. In this work, I used a unique set of data (35 years) provided by an ecological monitoring of the Gironde estuary (the model of estuary used in the study). The main objective of this thesis is to characterize the evolution of the whole system (physico-chemistry and biology) over the last three decades and to identify the forcings involved. The originality of this work comes from a global analysis of the main estuarine descriptors and from the evaluation of the relative contribution of global forcings. A study based on multivariate analyses, through an end-to-end approach, reveals two abrupt shifts in the state of the ecosystem of the Gironde estuary. These two changes appear to be linked to hydroclimatic fluctuations at a regional scale (North Atlantic basin) and also at a local scale. Some hypotheses on the possible mechanisms involved, linking the large-scale climatic alterations to the changes observed on the biological estuarine communities are discussed. A second study that uses a modeling approach based on the ecological niche concept of Hutchinson, identifies a change in the spatial (longitudinal) distribution of estuarine copepod populations in response to warming and marinisation processes. Finally, a third work focuses on an invasive copepod species: Acartia tonsa. A double modeling approach estimates both the fundamental and realized niches of the species. The results highlight the relative importance of climate changes that facilitated the colonization of the estuary by A. tonsa., Les estuaires font partie des écosystèmes les plus productifs de la planète. Ils constituent une zone d'importants échanges de matières et jouent à ce titre un rôle capital pour les cycles biogéochimiques. Du fait de leur position à l'interface entre domaines continental et marin, ils sont également un lieu de passage obligatoire pour de nombreuses espèces de poissons migrateurs ou encore de nourricerie et de refuge. Ces systèmes sont contraints par de nombreuses pressions de natures diverses dont certaines en lien avec les activités humaines locales (p. ex. pêcheries, transport maritime, dragages, pollutions accrues, etc.). Ces altérations viennent s'ajouter à la variabilité intrinsèque de ces systèmes naturellement complexes (variabilité spatio-temporelle de différents facteurs environnementaux ; c.-à-d. température, salinité, turbidité, etc.). Toutefois, une intensification de ces d'altérations, dont une partie est associée à la composante climatique des changements globaux, est observée depuis ces dernières décennies. Ces modifications avérées et leurs incidences sur les communautés biologiques constituent un enjeu en matière de gestion des écosystèmes estuariens. Cependant, par manque de chroniques de données continues suffisamment longues, peu d'études sur l’évolution à long terme de systèmes estuariens ont été réalisées à ce jour. Pour mes travaux, j'ai eu à ma disposition un ensemble de chroniques uniques - acquises depuis 35 ans - associées à plusieurs suivis écologiques de l'estuaire de la Gironde (notre site atelier). L'objectif général de cette thèse est de caractériser l’évolution globale du système dans son ensemble (physico-chimie et biologie) et d'identifier les forçages impliqués. L’originalité de ces travaux, par rapport aux études spécifiques déjà menées, consiste à analyser l’évolution de l’ensemble des descripteurs caractéristiques du fonctionnement d'un système modèle et de qualifier la part relative aux forçages globaux. L'étude des séries chronologiques, via une approche end-to-end basée sur des analyses multivariées, permet d'identifier deux changements abrupts d'état de l'écosystème de l'estuaire de la Gironde, associés à des fluctuations hydroclimatiques à large échelle (bassin Atlantique nord) mais aussi à une échelle plus locale (estuaire). Des hypothèses relatives aux possibles mécanismes impliqués, reliant les altérations climatiques à large échelle aux altérations observées à l'échelle des communautés biologiques estuariennes, sont discutées. Un deuxième volet de la thèse, au travers d’une approche modélisatrice basée sur le concept de niche d'Hutchinson, permet d'identifier un changement de répartition spatiale, longitudinale, des populations de copépodes estuariennes en réponse au réchauffement des masses d'eau et à la marinisation de l'estuaire. Enfin un troisième volet de mes travaux s'est consacré à l'étude spécifique d'une espèce de copépode invasive : Acartia tonsa. Suivant une double approche modélisatrice permettant d'estimer à la fois les niches fondamentale et réalisée de l'espèce, ces travaux illustrent l'importance relative des changements climatiques qui ont facilité la colonisation et l'essor de cette espèce dans l'estuaire.