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Bivalves are regulators of coastal lagoons and provide a wide range of ecosystem services. However, coastal lagoons are sensitive to climate change. Our objective was to describe the drivers of the cascade of ecological events that occurred during a summer heatwave and which resulted in recruitment failure of the Pacific oyster Crassostrea gigas. Results show that elevated temperatures and salinity caused a shift in planktonic food availability toward smaller taxa. These trophic changes did not affect food accumulation by oyster larvae or their fatty acid composition but did affect post-metamorphosis success, with up to 24% fewer young metamorphosed postlarvae at some sites and no development of juveniles at all sites. This resulted in the failure of oyster recruitment and in the development of tubeworms, a trophic and spatial competitor that can better ingest small particles. This knowledge suggests that, in the context of marine heatwaves, the ecological limits of oyster larvae are narrower than their physiological limits.

In the framework of the Sustainable Development Goals (SDGs) led by the United Nations, coastal management methods are required to achieve both sustainable food production and environmental conservation as a climate change countermeasure. Oyster farming is an important food production method now being developed in coastal areas around the world. Recently, climate change has caused several negative effects on oyster aquaculture such as poor spat collection due to oligotrophication, ocean acidification, and poor spat growth and survival due to frequent anoxic events derived from high seawater temperature. The oysters cultivated in many regions of the world are intertidal species inhabiting intertidal zones such as sandy/muddy tidal flats and estuaries, where seagrass beds are often distributed in adjacent lower intertidal and subtidal areas. Seagrass vegetation is one of the most important ecosystems functioning as a countermeasure for global climate change. Not only does it mitigate greenhouse gas emissions by sequestration and storage of blue carbon derived from atmospheric CO2, but it also functions as an adaptation measure providing a buffering function against ocean acidification and water quality mprovement. Based on the concept of aquaculture supported by natural ecosystem interactions between oysters and seagrass beds, our project examined whether aquaculture techniques that take into account both mitigation and adaptation to climate change are effective for both sustainable use of coastal areas and environmental conservation. We conducted field experiments in both the French Mediterranean Sea and the Seto Inland Sea of Japan to clarify the effect of eelgrass beds on (1) natural oyster spat collection and (2) growth and survival of oyster spat. The results of our experiments revealed that spat recruitment was significantly higher in areas without eelgrass distribution, while spat growth and survival rate after the settlement were significantly higher in eelgrass beds even when anoxic events occurred in the study areas. Therefore, our results indicate a possibility that seagrass vegetation contributes to sustainability of oyster aquaculture by mitigating environmental degradation during cultivation

In the framework of the Sustainable Development Goals (SDGs) led by the United Nations, coastal management methods are required to achieve both sustainable food production and environmental conservation as a climate change countermeasure. Oyster farming is an important food production method now being developed in coastal areas around the world. Recently, climate change has caused several negative effects on oyster aquaculture such as poor spat collection due to oligotrophication, ocean acidification, and poor spat growth and survival due to frequent anoxic events derived from high seawater temperature. The oysters cultivated in many regions of the world are intertidal species inhabiting intertidal zones such as sandy/muddy tidal flats and estuaries, where seagrass beds are often distributed in adjacent lower intertidal and subtidal areas. Seagrass vegetation is one of the most important ecosystems functioning as a countermeasure for global climate change. Not only does it mitigate greenhouse gas emissions by sequestration and storage of blue carbon derived from atmospheric CO2, but it also functions as an adaptation measure providing a buffering function against ocean acidification and water quality improvement.Based on the concept of aquaculture supported by natural ecosystem interactions between oysters and seagrass beds, our project examined whether aquaculture techniques that take into account both mitigation and adaptation to climate change are effective for both sustainable use of coastal areas and environmental conservation. We conducted field experiments in both the French Mediterranean Sea and the Seto Inland Sea of Japan to clarify the effect of eelgrass beds on (1) natural oyster spat collection and (2) growth and survival of oyster spat. The results of our experiments revealed that spat recruitment was significantly higher in areas without eelgrass distribution, while spat growth and survival rate after the settlement were significantly higher in eelgrass beds even when anoxic events occurred in the study areas. Therefore, our results indicate a possibility that seagrass vegetation contributes to sustainability of oyster aquaculture by mitigating environmental degradation during cultivation.

After having been eutrophied until the 2000s, the Thau lagoon has been now on an ecological restoration trajectory according to the oligotrophisation process. However, 2018 appeared to be a turning point in its ecological functioning due to its disturbed hydroclimatic context with the appearance of an unprecedented phenomenon of green water significantly impacting the oyster farming sector in its productivity and socio-economy. Already described in the 1980s in the Leucate lagoon, this phenomenon had never been observed in Thau and the need to observe in order to understand led to the carrying out of environmental monitoring from January 2019 to January 2020 on 3 stations. The description of the spatio-temporal dynamics of phytoplankton and of the species causing green water was associated with the biogeochemistry of lagoon waters (nitrogen, phosphorus and silicate nutrients) and the dynamics of autotrophic and heterotrophic eukaryotic communities. This monitoring was completed by a diagnosis of macrophytes (including macroalgae and seagrass beds) carried out in 2019 in order to measure the impact of disturbances on this compartment. The hypothesis of a link between the remarkable hydroclimatic context of 2018 and this green water phenomenon was put forward. Our results showed that the hydroclimatic context of the Thau Basin has evolved with a trend towards an increase in annual average water temperatures (+1.6°C over 20 years between 2000 and 2019), with an increase in annual average salinities (+2 PSU between 2000 and 2019) and more intense periods of oversalinity (up to 110 days in 2012 and maximums of 42.9 in 2016). Winter storms, spring thunderstorms, summer heatwave, anoxy and autumn Mediterranean episodes of intense precipitation make 2018 an atypical year. This succession of extreme events resulted in the phenomenon of green water. This project helped to find the origin: a bloom of Picochlorum, a picophytoplankton known to be thermotolerant and halotolerant. Picochlorum dominated the phytoplankton from December 2018 to April 2019 followed by minor developments until August 2019. The return to more typical phytoplankton communities occurred in September 2019 until the end of monitoring in February 2020. This bloom could be linked to massive inputs of nitrogen and phosphate nutrients linked successively to heavy rainfall in spring, to the summer anoxy and to the autumn rainfall of the Mediterranean episodes. The population dynamics of Picochlorum was controlled by nutrients but probably also by a parasitoid predator of the Aphelidium type that was identified in a preliminary way. The precise identification of this parasitoid remains a prospect. The impacts of the malaigue and green water phenomena on the macrophytes have led to a decrease in the total macrophyte cover. The relative cover of reference species, such as eelgrass, has also decreased, with a greater impact on the western part of the basin. The extreme hydroclimatic events of 2018 induced a cascade of ecological effects with changes in phytoplankton communities. It took one year after the 2018 anoxy to return to the previous state, as in the case of the green waters of the Leucate lagoon in the 1980s. Mediterranean lagoon systems represent high-stakes areas with ecological, environmental, societal and economic issues. The ecological restoration of Mediterranean lagoons and the associated goods and services requires an understanding of the dynamics of the various physical, chemical, biological and societal compartments, in order to support the marine culture sectors and anticipate their future trajectories, particularly in a context of anthropisation and global change., Après avoir été eutrophisée jusque dans les années 2000, la lagune de Thau est depuis sur une trajectoire de restauration écologique selon le processus d’oligotrophisation. Cependant, l’année 2018 est apparue comme une année de bascule dans son fonctionnement écologique par son contexte hydroclimatique perturbé avec l’apparition d’un phénomène inédit d’eaux vertes impactant significativement la filière ostréicole dans sa productivité et sa socio-économie. Déjà décrit dans les années 1980 dans la lagune de Leucate, ce phénomène n’avait jamais été observé dans Thau et le besoin d’observer pour comprendre a abouti à la réalisation d’un suivi environnemental de janvier 2019 à janvier 2020 sur 3 stations. La description de la dynamique spatio-temporelle du phytoplancton et de l’espèce à l’origine d’eaux vertes a été associée à la biogéochimie des eaux lagunaires (nutriments azotés, phosphorés et silicatés) et à la dynamique des communautés eucaryotes autotrophes et hétérotrophes. Ce suivi a été complété par un diagnostic des macrophytes (incluant les macroalgues et les herbiers) réalisé en 2019 afin de mesurer l’impact des perturbations sur ce compartiment. L’hypothèse du lien entre le contexte hydroclimatique remarquable de l’année 2018 et ce phénomène d’eaux vertes a été posée. Nos résultats ont montré que le contexte hydroclimatique du bassin de Thau a évolué avec une tendance à l’augmentation des moyennes annuelles de températures de l’eau (+1.6°C sur 20 ans entre 2000 et 2019), avec l’augmentation des moyennes annuelles de salinités (+2 PSU entre 2000 et 2019) et des périodes de sursalinités plus intenses (jusqu’à 110 jours en 2012 et des maximums à 42,9 en 2016). Les tempêtes hivernales, les orages printaniers, la canicule estivale, la malaïgue et les épisodes méditerranéens automnaux de précipitations intenses font de l’année 2018, une année atypique. Cette succession d’évènements extrêmes a eu pour conséquence le phénomène d’eaux vertes. Ce projet a permis d’en trouver l’origine : une efflorescence de Picochlorum, picophytoplancton connu pour être thermotolérant et halotolérant. Picochlorum a dominé le phytoplancton de décembre 2018 à avril 2019 suivi de développements mineurs jusqu’en aout 2019. Le retour à des communautés phytoplanctoniques plus typiques s’est fait en septembre 2019 jusqu’à la fin du suivi en février 2020. Cette efflorescence a pu être reliée à des apports massifs en nutriments azotés et phosphatés liés successivement à des précipitations fortes au printemps, à la malaïgue estivale et aux précipitations automnales des épisodes méditerranéens. La dynamique de population du Picochlorum a été contrôlée par les nutriments mais probablement aussi par un prédateur parasitoïde de type Aphelidium identifié de façon préliminaire. L’identification précise de ce parasitoïde reste une perspective. Les impacts des phénomènes de malaïgue et d’eaux vertes sur les macrophytes ont induit une diminution du recouvrement total en macrophytes. Le recouvrement relatif en espèce de référence, comme les zostères, a également diminué avec un impact plus important sur la partie ouest du bassin. Les évènements extrêmes hydroclimatiques de 2018 ont induit une cascade d’effets écologiques avec changement de communautés phytoplanctoniques. Il aura fallu une année après la malaïgue de 2018 pour voir le retour à l’état antérieur, comme dans le cas des eaux vertes de la lagune de Leucate dans les années 1980. Les systèmes lagunaires méditerranéens représentent des zones à forts enjeux avec des problématiques écologiques, environnementales, sociétales et économiques. La restauration écologique des lagunes méditerranéennes et des biens et services associés nécessite de comprendre la dynamique des différents compartiments physiques, chimiques, biologiques et sociétaux, pour accompagner les filières des cultures marines et d’anticiper leurs trajectoires futures, en particulier dans un contexte d’anthropisation et de changement global.

In the framework of the Sustainable Development Goals (SDGs) led by the United Nations, coastal management methods are required to achieve both sustainable food production and environmental conservation as a climate change countermeasure. Oyster farming is an important food production method now being developed in coastal areas around the world. Recently, climate change has caused several negative effects on oyster aquaculture such as poor spat collection due to oligotrophication, ocean acidification, and poor spat growth and survival due to frequent anoxic events derived from high seawater temperature. The oysters cultivated in many regions of the world are intertidal species inhabiting intertidal zones such as sandy/muddy tidal flats and estuaries, where seagrass beds are often distributed in adjacent lower intertidal and subtidal areas. Seagrass vegetation is one of the most important ecosystems functioning as a countermeasure for global climate change. Not only does it mitigate greenhouse gas emissions by sequestration and storage of blue carbon derived from atmospheric CO2, but it also functions as an adaptation measure providing a buffering function against ocean acidification and water quality improvement. Based on the concept of aquaculture supported by natural ecosystem interactions between oysters and seagrass beds, our project examined whether aquaculture techniques that take into account both mitigation and adaptation to climate change are effective for both sustainable use of coastal areas and environmental conservation. We conducted field experiments in both the French Mediterranean Sea and the Seto Inland Sea of Japan to clarify the effect of eelgrass beds on (1) natural oyster spat collection and (2) growth and survival of oyster spat. The results of our experiments revealed that spat recruitment was significantly higher in areas without eelgrass distribution, while spat growth and survival rate after the settlement were significantly higher in eelgrass beds even when anoxic events occurred in the study areas. Therefore, our results indicate a possibility that seagrass vegetation contributes to sustainability of oyster aquaculture by mitigating environmental degradation during cultivation.

In the framework of the Sustainable Development Goals (SDGs) led by the United Nations, coastal management methods are required to achieve both sustainable food production and environmental conservation as a climate change countermeasure. Oyster farming is an important food production method now being developed in coastal areas around the world. Recently, climate change has caused several negative effects on oyster aquaculture such as poor spat collection due to oligotrophication, ocean acidification, and poor spat growth and survival due to frequent anoxic events derived from high seawater temperature. The oysters cultivated in many regions of the world are intertidal species inhabiting intertidal zones such as sandy/muddy tidal flats and estuaries, where seagrass beds are often distributed in adjacent lower intertidal and subtidal areas. Seagrass vegetation is one of the most important ecosystems functioning as a countermeasure for global climate change. Not only does it mitigate greenhouse gas emissions by sequestration and storage of blue carbon derived from atmospheric CO2, but it also functions as an adaptation measure providing a buffering function against ocean acidification and water quality improvement. Based on the concept of aquaculture supported by natural ecosystem interactions between oysters and seagrass beds, our project examined whether aquaculture techniques that take into account both mitigation and adaptation to climate change are effective for both sustainable use of coastal areas and environmental conservation. We conducted field experiments in both the French Mediterranean Sea and the Seto Inland Sea of Japan to clarify the effect of eelgrass beds on (1) natural oyster spat collection and (2) growth and survival of oyster spat...

En 2018, dans un contexte climatique exceptionnel, la lagune de Thau a été impactée par une « malaïgue » causant des mortalités estivales massives puis par un phénomène d’eaux vertes, dominé par le picophytoplancton du genre Picochlorum durant l’automne 2018, entrainant un arrêt de croissance chez les huîtres. Inquiétés par le phénomène et les impacts engendrés sur la production, les acteurs locaux ont co-construit le projet de recherches « Overte » pour (i) mettre en place un suivi environnemental pour décrire ce phénomène et mieux le comprendre et (ii) mettre en évidence les interactions entre Picochlorum et les coquillages en élevage. Ce rapport aborde le second objectif. Ainsi une série d’expérimentations a été réalisée en février 2019 pour tester les capacités de différentes espèces et tailles de bivalves en élevage, à diminuer les abondances de Picochlorum par leur activité de filtration. Ces expériences ont été réalisées en laboratoire à l’aide d’une série de béchers contenant (i) 3 types de solutions (1L) : l’eau de la lagune de Thau contenant Picochlorum à 45,2 millions de cellules.L-1 (P) ou à 20,7 millions de cellules.L-1 (D) et une solution d’Isochrysis galbana ; (ii) en absence ou en présence de 5 types de bivalves : des huîtres creuses (Crassostrea gigas) de taille T6, T20 ou C3, des huîtres plates, Ostrea edulis, ou des moules méditerranéennes (Mytilus galloprovincialis). Une série de prélèvements d’eau a été réalisée au cours des 40 minutes d’incubation pour déterminer la variabilité temporelle des concentrations de picophytoplancton et de nanophytoplancton par cytométrie en flux. Cette étude a permis de démontrer qu’au contraire des huîtres creuses et des huîtres plates, les moules d’un poids moyen de 21,4 g sont capables de diminuer drastiquement les abondances de Picochlorum et ainsi de retenir près de 1 millions de cellules de Picochlorum par minute à 11,4°C. Ces résultats suggèrent que les 1228 tonnes de moules, mortes en 2018 avant l’efflorescence de Picochlorum, auraient pu participer au contrôle de l’efflorescence de Picochlorum si elles avaient survécu aux chaleurs et à l’anoxie. Au vu de ces résultats, il serait sans doute important d’assurer une gestion de stock des moules dans la lagune de Thau. Néanmoins il serait très important de récolter les moules en élevage avant les chaleurs estivales, pour éviter des mortalités massives telles que celles observées ces dernières années en lien avec le réchauffement climatique. Les perspectives de ce travail sont de tester les capacités de filtration des moules selon un gradient de température et de concentrations cellulaires pour déterminer le seuil de concentration à partir duquel Picochlorum ne serait plus ingéré et assimilé mais rejeté dans la colonne d’eau sous forme de pseudofécès.