Impacts of oyster culture on microphytobenthos and its photosynthetic performances, benthic macrofauna and the role of estuarine erosion in oyster spat Crassostrea gigas mortality related to OsHV-1 μVar

Depuis 2008, la mortalité des naissains d'huîtres Pacifique Crassostrea gigas est principalement liée au virus de l’ostreid herpesvirus 1 µvariant (OsHV-1 µVar). Les paramètres environnementaux favorisant la persistance et la diffusion du virus pourraient jouer sur sa dynamique de propagation. Les objectifs de cette thèse étaient d’étudier in situ les interactions entre les naissains d’huîtres et leur environnement benthique lors d’un épisode de surmortalité. La photobiologie du microphytobenthos sous les tables semblait en meilleur état comparé à celle observée dans les allées pouvant favoriser la résilience virale. Les tables à huîtres atténuaient la lumière, la température et la dessication évitant ainsi une thermo et photoinhibition du microphytobenthos. Les indices de la qualité environnementale de l’habitat benthique basés sur la communauté macrozoobenthique ont traduit une dégradation du milieu suite aux apports de matière organique induits par les mortalités de naissains d’huîtres puis des échouages des macroalgues. Une quantité importante d’OsHV-1 était retrouvée à la surface du biofilm avant les mortalités. Sa remise en suspension était préférentiellement associée aux particules microphytobenthiques avec de faibles courants.Since 2008, the mortality of Pacific oyster spat Crassostrea gigas has mainly been linked to the Ostreid herpesvirus 1 μvariant (OsHV-1 μVar). Environmental parameters favoring the persistence and spread of the virus could affect its propagation dynamics. The objectives of this thesis were to study in situ the interactions between oyster spat and their benthic environment during an episode of mortality. The photobiology of the microphytobenthos beneath the tables seemed to be in a better state compared to that observed in aisles that could promote viral resilience. The oyster tables attenuated light, temperature and desiccation, thus avoiding thermo and photoinhibition of the microphytobenthos. The benthic habitat environmental quality indices based on the macrozoobenthic community showed a degradation of the environment following the influx of organic matter induced by oyster spat mortalities and strandings of the macroalgae. A significant amount of OsHV-1 was found on the surface of the biofilm before the mortalities. Resuspension was preferentially associated with microphytobenthic particles with small currents

Changes in benthic macrofauna in oyster parks during an OsHV-1 μVar oyster spat mortality outbreak

International audienceIn intertidal areas, oyster farming creates a crosshatching pattern between oyster tables and aisles. Tables provide a refuge from the current and solar irradiance and the oysters facilitate the accumulation of OM, thereby structuring the spatial organization of the associated macrozoobenthic community at mesoscale. The aim of this study was to describe the quality of the oyster table environment at small scale and the response of the macrozoobenthic community to OsHV-1 µvar oyster mortality. The species assemblage was dominated by Golfingia vulgaris, Tubificoides benedii, Capitella capitata and Scoloplos armiger. The table habitat appeared to be in a bad ecological state throughout the 2-month survey (May and June 2017), whereas in the aisle, eutrophication occurred lately and was clearly related to be due to the massive stranding of dead seaweed at the end of the survey (in early July). So, this disturbance of the species assemblage seemed to occur in two phases: 1) after oyster spat mortality and 2) after seaweed stranding resulted in a bad ecological status, as revealed by macrofaunal indicators. Large quantities of OsHV-1 DNA were also found in some species, including small crabs and amphipods, one week after the mortality crisis, but there is no apparent virus reservoir found in the benthic species

Non-Technical Survey Tool - Performance Evaluation Report: TIRAMISU deliverable D.220.4

During the Non-Technical Survey, information on a Suspected Hazardous Area (SHA) is collected and analysed for assessment and reduction/inclusion purposes. This phase focuses on a scale that is more local than the Advanced General Survey (WP210). Unlike the Technical Survey, the Non-Technical Survey does not involve entering the SHA physically. This deliverable, entitled Non-Technical Survey Tool Performance evaluation report, is the fourth deliverable devoted to tools for Non-Technical Survey (NTS). It includes a description of the tools developed, the rationale and guidelines for using them and an evaluation of their performance. It is meant to be a reference document on WP220 tools and services after the project lifetime. Therefore, it is as much as possible self-contained and includes more than the description of the work achieved during the last period.TIRAMISU D220.4info:eu-repo/semantics/nonPublishe

Changes in planktonic microbial components in interaction with juvenile oysters during a mortality episode in the Thau lagoon (France)

Oysters modify the planktonic microbial community structure by their filtration and NH4 excretion activities. While many studies have been conducted on this subject with adult oysters, none had been carried out in situ with juveniles. Pacific oyster juveniles (Magallana gigas, previously Crassostrea gigas) died massively all over the world since 2008 in relation with OsHV-1 infection. During mortality episodes, sick and dead oysters are not separated from healthy live ones, and left to decay in the surrounding environment, with unknown consequences for the nutrient cycle and planktonic microbial components (PMC). The present study aimed to elucidate for the first time the interactions between oyster juveniles and PMC during a mortality episode. Innovative 425-L pelagic chambers were deployed weekly in situ around oyster lanterns along a stocking-density gradient in the Thau Mediterranean lagoon (France) before, during and after an oyster mortality episode, from April to May 2015. This study reveals (i) significant changes of planktonic microbial community structure during mortality episodes, with a proliferation of picoplankton (<3 μm) and ciliates (Balanion sp., Uronema sp.) within 2 weeks when mortality rates and numbers of moribund juvenile oysters were highest. These changes were probably induced by oyster tissue leaching, decomposition and mineralization, which probably began during the moribund period, as suggested by an increase of PO4 concentration and N:P ratio decrease, (ii) oyster juveniles mainly retained 3–20 μm plankton. In contrast to adults, picophytoplankton and small heterotrophic flagellates (<3 μm) were significantly depleted in the presence of oyster juveniles. Depletion of picoplankton occurred only at the starting of the mortality episode and during the moribund phase. (iii) Oyster juvenile filtration and mortality shifted the planktonic microbial structure toward a heterotrophic microbial system, where ciliates and heterotrophic flagellates acted as a trophic link between picoplankton and oysters. The next stage of our investigation is to examine the effect of a mortality episode on pathogen fluxes in the water column, exploring their relationships with planktonic components and dead oyster flesh