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1. Marine Heat Waves (MHWs) are episodes of anomalous warming in the ocean that can last from a few days to months. MHWs have different characteristics in terms of intensity, duration, and frequency and generate thermal stress on marine ecosystems. In reef ecosystems, they are one of the main causes of decreased presence and abundance of corals, invertebrates, and fish. The deleterious capacity of thermal stress often depends upon biotic factors such as resource availability (bottom-up control on predators) and predation (top-down control on prey). Despite the evidence of thermal stress and biotic factors affecting individual species, the combined effects of both stressors on the entire reef ecosystems are far less studied. 2. Here, using a food-web modeling approach, we estimated the rate of change in species’ biomass due to different MHW scenarios based on their physical characteristics. Specifically, we modeled the mechanistic link between species’ consumption rate and seawater temperature (thermal stressor), simulating species’ biomass dynamics for different MHW scenarios under different trophic control assumptions (biotic factor). 3. We find that total reef ecosystem biomass declined by 10% ± 5% under MHWs with severe intensity and top-down control assumption. The bottom-up control assumption moderates the total ecosystem biomass reduction by 5% ± 5%. Irrespective of the MHW scenario and the trophic control assumption, the most substantial biomass changes occur among top, meso-predators, and corals (5% to 20% ± 10%).4. Since habitat degradation may lead to reef ecosystems governed by top-down control on prey, our findings point to the critical importance of protecting reef ecosystems as a pivotal strategy to alleviate the impacts of thermal stress induced by MHWs. Overall, our results provide a unified understanding of the interplay between abiotic stressors and biotic factors in reef ecosystems under extreme thermal events, offering insights into present baselines and future ecological states for reef ecosystems.

AbstractOcean warming is altering the biogeographical distribution of marine organisms. In the tropics, rising sea surface temperatures are restructuring coral reef communities with sensitive species being lost. At the biogeographical divide between temperate and tropical communities, warming is causing macroalgal forest loss and the spread of tropical corals, fishes and other species, termed “tropicalization”. A lack of field research into the combined effects of warming and ocean acidification means there is a gap in our ability to understand and plan for changes in coastal ecosystems. Here, we focus on the tropicalization trajectory of temperate marine ecosystems becoming coral‐dominated systems. We conducted field surveys and in situ transplants at natural analogues for present and future conditions under (i) ocean warming and (ii) both ocean warming and acidification at a transition zone between kelp and coral‐dominated ecosystems. We show that increased herbivory by warm‐water fishes exacerbates kelp forest loss and that ocean acidification negates any benefits of warming for range extending tropical corals growth and physiology at temperate latitudes. Our data show that, as the combined effects of ocean acidification and warming ratchet up, marine coastal ecosystems lose kelp forests but do not gain scleractinian corals. Ocean acidification plus warming leads to overall habitat loss and a shift to simple turf‐dominated ecosystems, rather than the complex coral‐dominated tropicalized systems often seen with warming alone. Simplification of marine habitats by increased CO2 levels cascades through the ecosystem and could have severe consequences for the provision of goods and services.

Abstract Tidal circulation and tidal stream resource in Alderney Race (Raz Blanchard) were assessed by using a towed acoustic Doppler current profiler (ADCP) system and tidal modeling. Optimal Interpolation (OI) was applied to process the underway velocity measurements recorded at neap tide flood and ebb flow. The interpolation technique allows reconstructing space-time evolution of the velocity field within the domain during surveying periods. The method employs velocity covariances derived from numerical simulations by a 2D hydrodynamic model MARS. Model covariances are utilized by the OI algorithm to obtain the most likely evolution of the velocity field under the constraints provided by the ADCP observations and their error statistics. The resulting velocity fields were used for assessing the tidal stream resource at site. The largest overall difference between the kinetic power density derived from simulated and interpolated velocity fields was found for ebb tide. Model simulations constrained by velocity measurements demonstrated a significant (up to 30%) decrease of power available in the flow. A significant change in spatial pattern of power density distribution was also identified. It is demonstrated that by merging high resolution velocity measurements at tidal energy site with modeling the tidal stream potential estimation becomes more accurate.

Integrated Multi-Trophic Aquaculture takes advantage of the mutualism between some detritivorous fish and phytoplankton. The fish recycle nutrients by consuming live (and dead) algae and provide the inorganic carbon to fuel the growth of live algae. In the meanwhile, algae purify the water and generate the oxygen required by fishes. Such mechanism stabilizes the functioning of an artificially recycling ecosystem, as exemplified by combining the euryhaline tilapia Sarotherodon melanotheron heudelotii and the unicellular alga Chlorella sp. Feed addition in this ecosystem results in faster fish growth but also in an increase in phytoplankton biomass, which must be limited. In the prototype described here, the algal population control is exerted by herbivorous zooplankton growing in a separate pond connected in parallel to the fish-algae ecosystem. The zooplankton production is then consumed by tilapia, particularly by the fry and juveniles, when water is returned to the main circuit. Chlorella sp. and Brachionus plicatilis are two planktonic species that have spontaneously colonized the brackish water of the prototype, which was set-up in Senegal along the Atlantic Ocean shoreline. In our system, water was entirely recycled and only evaporation was compensated (1.5% volume/day). Sediment, which accumulated in the zooplankton pond, was the only trophic cul-de-sac. The system was temporarily destabilized following an accidental rotifer invasion in the main circuit. This caused Chlorella disappearance and replacement by opportunist algae, not consumed by Brachionus. Following the entire consumption of the Brachionus population by tilapias, Chlorella predominated again. Our artificial ecosystem combining S. m. heudelotii, Chlorella and B. plicatilis thus appeared to be resilient. This farming system was operated over one year with a fish productivity of 1.85 kg/m2 per year during the cold season (January to April).

Le document présente une méthode originale d'étude de la répartition de la crépidule dans la baie de Saint-Brieuc (Manche occidentale), au moyen du sonar latéral et de l'imagerie sous-marine. Une évaluation quantitative du stock complète cette distribution et permet d'estimer la biomasse de crépidules dans la baie à environ 250 000 tonnes (poids frais). The aim of this report is to provide an original method of mapping slipper limpet beds in the bay of Saint-Brieuc (Western Channel) by means of side scan sonar and submarine video. An evaluation of the biomass completes the distribution of limpets. The total fresh weight is about 250 000 tons

In this study, we examined the effect of near future ocean acidification (OA) on the transcriptome of a sensory organ in contact with surrounding water, the tongue in adult European sea bass (Dicentrarchus labrax) by mean of RNAseq experiment. We acquired a total of 14.1 Mb quality-trimmed reads covering 18,703 expressed genes from the tongue of fish reared from two generations at actual (pH 8.0 condition) and predicted near-future seawater pH (pH 7.6 condition). Gene ontologies analyses of expressed genes support the evidence that the tongue exhibits biological processes related to the sensory system, tooth mineralization and immune defences among others. Our data revealed only 295 OA-induced regulated genes with 114 up- and 181 down-regulated by OA. Functions over-represented encompass processes involved in organic substance metabolic process, RNA metabolism and especially RNA methylation which, combined with the regulation of some hsp genes expression, suggest a molecular response to stress which might contribute to lingual cell homeostasis under OA. The immune system process is also found enriched within OA-induced regulated genes. With the exception of one fatty acid receptor, known taste perception effectors were not impacted by OA in the tongue. However, a complementary droplet digital PCR approach dedicated to genes involved in gustatory signal transduction revealed the down regulation by OA of pyrimidinergic receptor (p2ry4) transcript expression in the gills of the fish. Combined with scanning electron microscopy analysis, our RNAseq data revealed that OA has no impact on processes related to teeth development and mineralization. Altogether, our data reveal that multigenerational exposure to OA has not a substantially effect on the tongue transcriptome but emphasis should be placed on investigating the potential physiological consequences related to the regulation of genes related to cell stress, immune system and fatty acid sensitivity to conclude on species resilience in face of OA.

Comprendre la variabilité du recrutement est une problématique majeure en halieutique. Dans ce travail, nous explorons une nouvelle approche pour étudier les facteurs qui déterminent le recrutement, dans le cadre de la modélisation biophysique. Le schéma de ponte des adultes peut influencer la survie des larves car il détermine les conditions environnementales qu'elles rencontrent pendant cette période critique. Notre cas d'étude est l'anchois du golfe de Gascogne Engraulis encrasicolus, qui est une espèce à pontes multiples. L'objectif de la thèse est de comprendre l'effet de l'environnement vécu par un individu i) sur l'énergie disponible pour la reproduction et ii) sur l'étalement des pontes et ses conséquences sur la croissance, le développement et la survie des larves. Pour appréhender les processus métaboliques en jeu, la théorie Dynamic Energy Budget est un outil particulièrement adapté. Cette théorie permet d'identifier les processus communs et les spécificités de chaque stade. Nous apportons tout d'abord une révision de la courbe de croissance de l'anchois du golfe de Gascogne. Nous reproduisons la croissance des juvéniles en tenant compte du fait qu'ils expérimentent en moyenne une température plus élevée durant cette phase que celle vécue ensuite par les adultes. La croissance larvaire diffère de la croissance des juvéniles et des adultes. Nous proposons de considérer la relation entre prise de nourriture et longueur de l'individu pour expliquer cette croissance. Ce travail nous permet ensuite de mieux comprendre et de quantifier l'effet des conditions environnementales vécues par un individu sur la durée de sa saison de reproduction. Ces conditions déterminent d'une part la taille de l'individu donc son potentiel reproducteur et d'autre part la quantité d'énergie qu'il peut effectivement mettre en réserve pour la reproduction. En conditions limitantes de nourriture, cette énergie peut en effet être mobilisée pour sa survie. Ainsi la structure en taille de la population et les conditions limitantes rencontrées par les individus sont des facteurs déterminants des fenêtres de ponte. La thèse permet enfin d'identifier les conditions de nourriture nécessaires à la survie jusqu'au stade juvénile, pour des larves issues de fenêtres de pontes différentes. Nous obtenons ce résultat à partir de la sélection des scénarios environnementaux qui reproduisent l'âge et la taille de l'otolithe à la métamorphose en fonction de la date d'ouverture de la bouche. Le lien entre métabolisme du poisson et formation de l'otolithe (une pièce calcifiée de l'oreille interne) est explicitement modélisé. Nous démontrons le potentiel du modèle pour la reconstruction de la quantité d'énergie assimilée par un individu au cours de sa vie à partir des variations observées de l'opacité dans l'otolithe. L'approche développée dans ce travail est une approche déterministe du lien environnement – individu, au travers des processus bioénergétiques. Cette approche nous permet de proposer des mécanismes originaux sous-jacents à certaines observations classiques en halieutique telles que le découplage entre la croissance de l'otolithe et la croissance en longueur du poisson et la phase exponentielle de la croissance pendant le stade larvaire. Une meilleure compréhension des cycles de vie requiert également la prise en compte du comportement et des stratégies individuelles. Ce travail peut constituer la base sur laquelle de telles études pourront à l'avenir s'appuyer. Understanding the recruitment variability of fish populations is a major challenge in fishery sciences. In the present work, we explore a new approach to study the potential factors that determine this recruitment in the context of biophysical modelling. The adult spawning pattern might influence the survival of the larvae as it determines the environmental conditions they experience during this critical period. We apply our study to the Bay of Biscay anchovy Engraulis encrasicolus, which is a multiple-batch spawner. The objective of the study is to understand the effect of the environmental conditions experienced by an individual i) on the energy available for reproduction and ii) on the temporal distribution of the spawning events and its consequences on larval growth, development and survival. To study these processes, the Dynamic Energy Budget (DEB) theory is particularly suitable. This theory allows us to identify the common processes and the specificities of each life stage. First, we actualise the growth curve of Bay of Biscay anchovy. Juvenile growth is reproduced by taking into account they experience in average a higher temperature during this stage than the adults thereafter. Larval growth in fish typically deviates from later juvenile and adult growth. We suggest to consider how food intake depends on body length to explain the observed growth patterns. Second, the present work allows us to better understand and quantify the effect of environmental conditions experienced by an individual on the length of its spawning season. These conditions determine on one hand the length of the individual and thus its reproduction potential, and on the other hand the amount of energy that it can actually store for reproduction. In limiting conditions, this energy can be mobilised for survival. Hence, the length structure of the population and the limiting conditions encountered by the individuals are determinant factors of the spawning windows. Third, we are able to identify the food conditions that allow survival until the juvenile stage for larvae issued from different spawning windows. We obtain this result from the selection of environmental scenarios that reproduce the observed age and otolith radius at metamorphosis according to first feeding date. The link between fish metabolism and otolith formation (a complex crystal in the inner ear of the fish) is explicitly modelled. We show the potential of the model to reconstruct individual life history from the observed variations of opacity in the otolith. The approach we used is a deterministic approach of the link between the environment and the individual, through bioenergetic processes. It allows us to formulate original mechanisms underlying classical observations in fishery sciences. As a better understanding of fish life cycles requires the study of individual behavior and strategies in response to environmental variations, we suggest the present work can be used as a basis for such studies.