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International audience; The Thau lagoon is a Mediterranean coastal lagoon used for shellfish farming. It is periodically affected by anoxia events that trigger oyster mortality. To investigate the effects of an anoxia event focussed on nutrient dynamics and the responses of the microbial planktonic communities a 13-day in situ experiment was performed in September 2020. Transparent mesocosms (270 L) were placed at a depth of 4 m, inserted in the sediment, and kept closed throughout the experiment. The experiment comprised three treatments: i) Natural environment (N), i.e. in the natural water outside the mesocosms containing a rope of 30 oysters (Crassostrea gigas), ii) Control mesocosm (C) filled with natural water with no oysters, and iii) Oyster mesocosm (O) filled with natural water containing a rope of 30 oysters. Oyster respiration in the oyster mesocosm depleted oxygen after 54 h. All the oysters from O mesocosm were dead after nine days and decomposition of their flesh combined with releases from the water-sediment interface increased dissolved inorganic nitrogen (dominated by ammonium), phosphates, and ∑H2S up to 390, 17 and 295 μmol·L−1, respectively. Phytoplankton biomass consequently increased by 20 (11.8 μg chlal −1) and abundance by 4.5 (186 × 106 cells·L1) dominated largely by green algae \textless5 μm. During the oyster mortality period (day 6 to day 9) high abundances of heterotrophic flagellates and large ciliate specimens were observed. This shift in the community towards small phytoplankton favours the microbial loop and is detrimental to shellfish farming. In a context of global warming in which the risk of anoxia is higher, the results of the present investigation demonstrate that anoxia triggers shellfish mortality and that the change in the plankton community disrupts the normal functioning of the ecosystem, causing serious financial losses. In this context, it is crucial to predict possible hypoxia and anoxia events using high frequency measurements of dissolved oxygen, by avoiding using shallow zones for oyster production and by reducing shellfish stocks, or by mechanically lifting the oysters out of the water during the night to reduce oxygen respiration in the ecosystem.

Disease emergence is accelerating in response to human activity-induced global changes. Understanding the mechanisms by which host populations can rapidly adapt to this threat will be crucial for developing future management practices. Pacific Oyster Mortality Syndrome (POMS) imposes a substantial and recurrent selective pressure on oyster populations (Crassostrea gigas). Rapid adaptation to this disease may arise through both genetic and epigenetic mechanisms. In this study, we used a combination of whole exome capture of bisulfite-converted DNA, next-generation sequencing, and (epi)genome-wide association mapping, to show that natural oyster populations differentially exposed to POMS displayed signatures of selection both in their genome (single nucleotide polymorphisms) and epigenome (CG-context DNA methylation). Consistent with higher resistance to POMS, the genes targeted by genetic and epigenetic variations were mainly related to host immunity. By combining correlation analyses, DNA methylation quantitative trait loci, and variance partitioning, we revealed that a third of the observed phenotypic variation was explained by interactions between the genetic sequence and epigenetic information, ∼14% by the genetic sequence, and up to 25% by the epigenome alone. Thus, as well as genetic adaptation, epigenetic mechanisms governing immune responses contribute significantly to the rapid adaptation of hosts to emerging infectious diseases.

Ce qu'il faut retenir : Le climat change sur la lagune de Thau comme ailleurs, avec des conséquences sur le cycle de vie des coquillages, leur croissance, les mortalités. L’été est une période cruciale avec des températures élevées, des forts risques d’anoxie, aux conséquences désastreuses. Grâce au réseau national d’observations conchylicoles ECOSCOPA, des mesures de développement et de nombreux paramètres environnementaux sont suivis depuis 30 ans sur des huîtres standardisées, avec un site d’observation à Marseillan. Le SIPEN et le ROL permettent eux aussi de disposer de mesures précises sur la croissance des huîtres en plusieurs points de la lagune. Les résultats présentés par le Cepralmar montrent que l’été 2022 s’est plutôt bien passé. Enfin, côté solutions pratiques d’adaptation, le CRCM a présenté les deux systèmes d’oxygénation testés dans le cadre du projet « table du 21e siècle ».

Avis sur la stratégie de remembrement des filières conchylicoles sur les lotissements des Aresquiers de Sète-Marseillan

The use of detritivores under sea farms is a promising avenue to mitigate the benthic impacts of marine fish farms. Sea cucumbers are interesting candidates for integrated multi-trophic aquaculture (IMTA) due to their prevalence in the marine environment, their diversified diet and their economic value. Yet limited information is available regarding their capacities to be stocked and reared underneath aquaculture cages and the associated effects on their survival, growth rate and body composition. This study focused on Holothuria tubulosa, a Mediterranean sea cucumber species candidate for rearing in the vicinity to marine fish cages. We investigated its potential for co-culture on the seabed more or less influenced by marine fish cages. The farm's waste footprint was predicted using a dispersion model (NewDEPOMOD) to estimate the farm's influence along a transect where we also sampled sediment at four distances from the cages (0 m, 25 m, 100 m from the cages, plus a reference site at 150 m). Organic composition of the sediment was analysed (TOC, TON, TOP, OM, stable isotope signature) and linked to the results from the dispersion model. Based on the model simulation, the maximum flux of matter reached almost 17 kg solids.m−2.year−1 below the cages, and gradually decreased with distance from the cages. An isotopic gradient was also found in the sediments according to the distance from the farm, with an enrichment in δN15 and a depletion in δC13 with increasing proximity to the farm. In parallel we investigated the response of adult sea cucumbers placed at varying distances from the fish cages for a period of one month, measuring their proximate composition, isotopic concentration, and fatty acid and protein composition. We found that despite good survival, growth was null over the experiment. While the isotope signature of the sea cucumbers was significantly affected by distance from the cage, this did not follow the pattern found in sediment. There was a clear difference in fatty acid composition between sites, with sea cucumbers closer to the cages having lower levels of short-chain fatty acids. The protein content was also lower in sea cucumbers reared right below the cages. These results suggest that while adult H. tubulosa can survive the environmental conditions below marine aquaculture cages, they do not nutritionally benefit from fish waste over short periods in the stocking conditions we tested. Their use in IMTA requires further investigation to find optimal stocking conditions.

Background As a major threat to the oyster industry, Pacific Oyster Mortality Syndrome (POMS) is a polymicrobial disease affecting the main oyster species farmed across the world. POMS affects oyster juveniles and became panzootic this last decade, but POMS resistance in some oyster genotypes has emerged. While we know some genetic loci associated with resistance, the underlying mechanisms remained uncharacterized. So, we developed a comparative transcriptomic approach using basal gene expression profiles between different oyster biparental families with contrasted phenotypes when confronted to POMS (resistant or susceptible). Results We showed that POMS resistant oysters show differential expression of genes involved in stress responses, protein modifications, maintenance of DNA integrity and repair, and immune and antiviral pathways. We found similarities and clear differences among different molecular pathways in the different resistant families. These results suggest that the resistance process is polygenic and partially varies according to the oyster genotype. Conclusions We found differences in basal expression levels of genes related to TLR-NFκB, JAK-STAT and STING-RLR pathways. These differences could explain the best antiviral response, as well as the robustness of resistant oysters when confronted to POMS. As some of these genes represent valuable candidates for selective breeding, we propose future studies should further examine their function.

After an exceptional hydroclimatic year, a “massive green algae bloom”, dominated by the autotrophic picoeukaryote Picochlorum sp, was observed in 2018–2019 in the Thau lagoon. Oyster farmers informed decision-makers and scientists about an alarming halt to growth, loss of flesh weight and abnormal mortality in commercial size oysters, whereas mussel farmers observed no adverse effects. Two hypotheses were tested. First, Mediterranean mussels are capable of filtering Picochlorum spp. whereas Pacific and flat oysters are not. Second, picophytoplankton filtration rates vary with the size of the oyster, spats having greater ability than juveniles and commercial size oysters. A series of experiments was conducted in February 2019 using a series of beakers containing (i) three different types of solutions (1 L): water from the Thau lagoon containing Picochlorum at a rate of 45.2 million cells.L−1 (P) or diluted to 20.7 million cells. L−1 (D) and a solution of Isochrysis galbana containing 9.4 million cells. L−1; (ii) in the absence or presence of five types of bivalves: Pacific oysters of three different sizes (spat, juvenile and commercial size), flat oysters, and Mediterranean mussels of commercial size. Four separate water samples were taken during a 40-min incubation period to measure fluctuations in picophytoplankton and nanophytoplankton abundances using flow cytometry. Retention efficiency (%) was compared according to species and size. In contrast to Pacific and flat oysters, mussels with an average weight of 21.4 g, were able to drastically reduce the abundances of Picochlorum and depleted nearly one million Picochlorum cells per minute at 11.4 °C. These results suggest that the 1228 t of mussels that died in 2018 before the Picochlorum bloom could have helped limit the Picochlorum bloom if they had survived the heat and anoxia.

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 (

International audience; Significant mortality of Crassostrea gigas juveniles is observed systematically every year worldwide. Pacific Oyster Mortality Syndrome (POMS) is caused by Ostreid Herpesvirus 1 (OsHV-1) infection leading to immune suppression, followed by bacteraemia caused by a consortium of opportunistic bacteria. Using an in-situ approach and pelagic chambers, our aim in this study was to identify pathogen dynamics in oyster flesh and in the water column during the course of a mortality episode in the Mediterranean Thau lagoon (France). OsHV-1 concentrations in oyster flesh increased before the first clinical symptoms of the disease appeared, reached maximum concentrations during the moribund phase and the mortality peak. The structure of the bacterial community associated with oyster flesh changed in favour of bacterial genera previously associated with oyster mortality including Vibrio, Arcobacter, Psychrobium, and Psychrilyobacter. During the oyster mortality episode, releases of OsHV-1 and opportunistic bacteria were observed, in succession, in the water surrounding the oyster lanterns. These releases may favour the spread of disease within oyster farms and potentially impact other marine species, thereby reducing marine biodiversity in shellfish farming areas

International audience; The aim of the present work was to test the influence of juvenile oyster culture on benthic-pelagic coupling in the French Mediterranean Thau Lagoon. Although the period of juvenile growth induces the introduction of millions of juvenile oysters in the lagoon over a short duration, the effects on the water-column and on the benthic ecosystem are poorly known. Using a multiple ecological approach, water column parameters, sedimentation rates, sediment characteristics and benthic fluxes were monitored on three occasions during the oyster pre-growth period, from October to December, at two sites inside the farming area and two sites outside the farming area that served as reference. In the water column, results showed a significant lower ratio between total phytoplankton biomass (chlorophyll-a) and degraded pigments (pheopigment), a significant increase in the contribution of phytoplankton containing chlorophyll-b (ie chlorophytes) to total phytoplankton biomass and a significant decrease in the contribution of phytoplankton containing chlorophyll-c (mainly diatoms) to the total phytoplankton biomass at farm sites, attributed to the grazing pressure caused by juvenile oysters on large particles, as described for adult oysters. No significant effect was observed at the benthic interface during this period, due to the dispersion of the small quantities of biodeposit produced by juveniles in the low water temperature conditions. A multivariate approach supported the effect of juvenile oysters on the water column and revealed a trend in benthic conditions with farm sites being distinct from the reference ones. The long farming history of the lagoon could have shaped sediment properties such that farming areas maintain different benthic conditions.

WOS:000540002100014; International audience; Thau lagoon is a large Mediterranean coastal lagoons and it supports traditional shellfish farming activities. It has been subject to eutrophication leading to major anoxic events associated with massive mortalities of shellfish stocks. Since the 1970s, improvements have been made to wastewater treatment systems, which have gradually led to oligotrophication of the lagoon. The aim of our study was to determine how the decrease in nutrient inputs resulted in major ecological changes in Thau lagoon, by analysing five decades of time-series (1970–2018) of observations on pelagic and benthic autotrophic communities. We were able to identify two periods during the oligotrophication process. Period 1 (1970–1992) was considered a eutrophic period, characterised by the shift from seagrass dominance to dominance of red macroalgae. Period 2 (1993–2018), characterised by improved eutrophication status, was further divided into three: a transition phase (1993–2003) during which the water column continued to recover but the benthic community lagged behind in recovery and in partial resilience; a regime shift (2003–2006), after which the water column became oligotrophic and seagrass began to recover (2007–2018). Considering anoxia crises as indicators of ecosystem resilience and resistance, we used a generalised linear model to analyse meteorological and environmental data with the aim of identifying the triggers of summer anoxia over the study period. Among the meteorological variables studied, air temperature had the strongest positive effect, followed by the period and wind intensity (both negative effects) and by rainfall in July (positive effect). The risk of triggering anoxia was lower in period 2, evidence for the increasing resistance of the ecosystem to climatic stress throughout the oligotrophication process. At the ecosystem scale and in the long term perspective, the ecological gains related to oligotrophication are especially important in the context of climate change, with more frequent and severe heat waves predicted.

Global change causes fluctuations as rainfall deficits that in some cases amplifies the reduction in nutrient intakes required for water quality regulation. In this context, oligotrophication reduces the pelagic production of coastal ecosystems and promotes the return of benthic macrophytes such as Zostera meadows. It is now necessary to know and understand the potential benefits related to the return of seagrass beds associated with the environmental recovery of shellfish-exploited-ecosystems (SEE). The French–Japanese SAKURA project aimed to (1) clarify and compare relationships between dynamics of nutrient levels, phytoplankton, and oyster production in the Thau Lagoon (France) and Hiroshima and Aki bays, using historical data analysis and carrying capacity models, (2) highlight the Zostera spp. contribution to oyster life cycles studying the variability of larval recruitment, survival, growth of juveniles, and trophic regime of oysters in the presence or in the absence of Zostera spp. meadows, (3) describe and compare the dynamics of socio-ecosystems of SEE under oligotrophication. First results of the SAKURA project permitted to start to improve knowledge on the influence of oligotrophication processes on the ecological status of shellfish-exploited-ecosystems and on the oyster life cycle. Analysis is still ongoing. Now, the French and Japanese partners want to deepen the interdisciplinary approach and the knowledge of this major sea and coastal challenge by expanding their partnership in the international community to address more holistically the conservation of biodiversity and the sustainable use of resources in the changing coastal seas.

International audience; Harmonizing coastal fisheries with water-quality improvement has become an essential factor for the sustainable use of coastal ecosystem services. Here, we present the scope of our study based on an interdisciplinary approach including ecological actions, socio-economic actions and socio-psychological actions. We chose to focus on the interaction between oyster aquaculture and seagrass vegetation as a typical ecological action using the coastal ecosystem complex (CEC) concept. Coastal organisms have adapted their traits to the environment over a long period of time, so that restoration of the CEC represents reconstruction of the original process of coastal production. Subtidal seagrass vegetation with intertidal oyster reefs is the original CEC in Japan, which would be expected to enhance coastal production by improving the production efficiency without adding nutrients. A simple field experiment examining carbon and nitrogen contents and stable isotope ratios revealed that oyster spats cultivated on a tidal flat adjacent to seagrass beds had higher nitrogen contents and higher delta C-13 ratios than spats cultivated in an offshore area using only pelagic production. This result suggests that utilization of the CEC, which enables oysters to use both pelagic and benthic production, has potential to sustain a food provisioning service for humans, even in oligotrophic conditions.

International audience; In the context of increasing demand for environmental recovery, aquatic systems may face the challenge of evolving under oligotrophication. This is the case in Mediterranean lagoons, in particular the shellfish-farmed Thau lagoon in France, where we studied recruitment of the Pacific oyster Crassostrea gigas. Oyster spat and environmental parameters were monitored at several sampling sites for 3 yr (2012 to 2014) using an original method with a temporal overlap deployment of collectors to study pre- and post-settlement processes and to identify the best conditions for recruitment. Contrary to the ‘no Pacific oyster reproduction’ paradigm in Mediterranean lagoons, our study showed that recruitment of this introduced species is possible in the Thau lagoon at levels comparable to those in other traditional French breeding basins. We identified a favorable environmental window for recruitment characterized by high water temperature (>26.5°C) and high nanophytoplankton and Chaetoceros spp. abundances (>4.3 × 106 and 345 × 103 cells l-1, respectively). In these favorable conditions, we hypothesize that the ecosystem functions as an autotrophic system, in contrast to the heterotrophic system that characterizes unfavorable conditions. Under heterotrophic conditions, high abundances of mixotrophic and heterotrophic organisms (ciliates and dinoflagellates) limited the metamorphosis of C. gigas larvae, leading to poor recruitment. This study provides new knowledge on the reproduction of the Pacific oyster in a Mediterranean lagoon under warming and oligotrophication. The shellfish industry will profit from the discovery of spatfields to develop new nursery practices that are eco-friendly and limit risks of transfers with other spatfall areas.

International audience; The Thau lagoon, located at the South of France, provides 10% of the French pacific oyster Crassostrea gigas production. Despite this intensive shellfish production, the larval recruitment of this species within the Thau lagoon is still misunderstood and spat collection remains highly variable; as a consequence, shellfish farmers rely mostly on hatchery”s spat. The present study aimed to describe reproduction features of C. gigas within the Mediterranean, as well as to explore spawning triggers of this species. To achieve this goal, the reproduction cycle from gametogenesis to spawning was followed during two years in several locations within the Mediterranean Thau lagoon. The spawning behavior of C. gigas within the Thau lagoon appeared to be slightly different from the one of the Atlantic Coast, showing several spawning events from June to October. The minimal temperature for massive spawning was 23 °C, which is much higher than temperatures mentioned in the literature for this species. A strong relationship was found between phytoplankton communities and gametogenesis, the latter being improved by a higher diatom/dinoflagellate ratio. Temperature was insufficient alone to explain spawning, highlighting other potential triggers such as the moon or lightning strikes. We made the hypothesis of a hierarchy of events triggering spawning, encompassing locking, synchrony and intensifying triggers. Statement of relevance This paper is filling in a gap in the literature regarding to reproduction of C. gigas in the Thau lagoon, which is always associated to temperature. However, temperature may not be the only factor involved in oyster reproduction, that is what we tried to show in this paper.

International audience; The recent discovery of Pacific oyster Crassostrea gigas (also known as Magallana gigas) spatfields in a Mediterranean lagoon intensely exploited for shellfish farming (Thau lagoon) revealed significant contrasts in spatial patterns of recruitment. We evaluated the processes that drive spatial patterns in oyster recruitment by comparing observed recruitment, simulated hydrodynamic connectivity and ecological variables. We hypothesized that spatial variability of recruitment depends on (1) hydrodynamic connectivity and (2) the ecology of the larval supply, settlement, metamorphosis, survival and biotic environmental parameters. We assessed recruitment at 6-8 experimental sites by larval sampling and spat collection inside and outside oyster farming areas and on an east-west gradient, from 2012-2014. Hydrodynamic connectivity was simulated using a numerical 3D transport model assessed with a Eulerian indicator. The supply of large umbo larvae did not differ significantly inside and outside oyster farming areas, whereas the supply of pediveligers to sites outside shellfish farms was structured by hydrodynamic connectivity. Inside shellfish farming zones, unfavorable conditions due to trophic competition with filter-feeders jeopardized their settlement. In this case, our results suggest loss of settlement competence by oyster larvae. This confirms our hypothesis of top-down trophic control by the oysters inside farming zones of Thau lagoon in summer that fails to meet the ecological requirements of these areas as oyster nurseries. Knowledge of oyster dispersal, connectivity and recruitment in coastal lagoons will help local development of sustainable natural spat collection. On a global scale, our method could be transposed to other basins or used for other species such as mussels, clams or scallops, to better understand the spatial patterns of bivalve recruitment. Management of the oyster industry based on natural spat collection will help develop a sustainable activity, based on locally adapted oyster strains but also by reducing the risks of transferring pathogens between basins and the global carbon footprint of this industry.

Japanese oysters spatfields were recently discovered in the French Mediterranean Thau lagoon farmed for shellfish. This discovery led to interesting issues concerning research for a highly seashell exploited ecosystem. The analyses of various environmental parameters characterized favorable sites and periods for spat collection. Spat collection in Thau lagoon presented high temporal and spatial heterogeneities related to hydrodynamic, predation, competition and trophic resource variability. Here, the results highlight various explicative factors of the Japanese oysters recruitment within Thau lagoon and underline the high ecological variability in the studied system under oligotrophication.

Coastal environments of the world have been exposed to eutrophication for several decades. Recently the quality of coastal waters has been gradually and successfully improved, however the improvement has caused another issue in ecoastal ecosystem services, called oligotrophication. Local stakefolders have suggested that oligotrophication reduces pelagic productivity and moreover fishery production in coastal ecosystems, while oligotrophication with high transparency has recovered benthic macrophyte vegetation which have been depressed by phytoplankton derived from eutrophication. In particular, seagrass species is one of the most important coastal vegetation for climate change mitigation and adaptation, which has been welcomed by another stakefolders. Therefore, harmonizing coastal fishery with environmental conservation is now essential for the sustainable use of ecosystem services. Here, we just started some practice in field based on the interdisciplinary approach including ecological actions, socio-economical actions and moreover psychological actions to find the integrative coastal management maximizing well-beings of various stakefolders, which is essential to harmonize environmental conservation with sustainable fishery and aquaculture. Now we are focusing on the interaction between oyster aquaculture and seagrass vegetation as an ecological action.

WOS:000529795300004; The decrease of microbial and nutrient inputs from the watershed has long dominated lagoon ecosystem management objectives. Phytoplankton biomass and abundance have drastically decreased for more than a decade and Zostera meadow have gradually recovered, expressing lagoon ecosystem restoration such as Thau lagoon. Do the progressive achievement of the good ecological status of the Thau lagoon possibly threatens the shellfish industry in terms of production and oyster quality, by reducing the carrying capacity? To provide answers about the right balance to be achieved between conservation and exploitation, a new numerical tool was developed to help in decision-making. We hereby propose to incorporate a Dynamic Energy Budget type shellfish production model to an existing lagoon ecosystem box-model. The influence of different scenarios of nutrient inputs (related to projections of population growth or improvement of treatment plants) and shellfish stocks were tested on oyster performances (production, oyster condition index), carrying capacity of the lagoon and ecological status indices used within the EU Water Framework Directive. Model outputs demonstrated that shellfish production was mainly controlled by nutrient inputs, which depend on hydro-meteorological variability, and specifically by phosphorus and N:P ratios of nutrient inputs. Scenarios tested, however, demonstrated smaller differences of oyster production in comparison to inter-annual variability. The overall ecological status of the lagoon remained in a "good" status with acceptable lagoon-scale phytoplankton depletion, regardless of scenarios, setting the carrying capacity of this ecosystem to be sustainable.

International audience; Reproduction and recruitment of benthic invertebrates are influenced by the climate and by the ecological structure of marine ecosystems, along with local anthropogenic pressures such as eutrophication or oligotrophication. Using the Pacific oyster Crassostrea gigas as a biological model, we tested the hypothesis that the variability in prodissoconch II (PII) size (i.e. size at metamorphosis) depends on ecological functioning. Settlement and recruitment were assessed at 5 sampling sites on the French Mediterranean shellfish farmed Thau lagoon during the main summer recruitment events in 3 consecutive years (2012-2014). Hydrobiological and planktonic analyses were conducted at 3 sampling sites. Our results showed that recruitment was extremely heterogeneous, ranging from 0 to 260 +/- 27 SE ind. dm(-2) throughout the ecosystem and was linked with variability in PII size, which ranged from 180 to 296 mu m. The annual temporal pattern of PII sizes appeared to be controlled by temperature during the settlement period, whereas the spatial pattern depended on phytoplankton biomass and on the trophic functioning of the ecosystem. Smaller PII sizes were significantly correlated with the highest phytoplankton biomass, while larger PII sizes were positively correlated with mixotrophic cryptophyte abundance. We found an inverse relationship between PII size and survival after metamorphosis, showing that recruitment success was associated with smaller PII sizes. Regional climate conditions and local trophic functioning appear to be key factors in metamorphosis and consequently contribute to recruitment heterogeneity. Further studies should be performed in other ecosystems following an oligotrophication trajectory to generalize this result.

International audience; Marine intrusion was simulated in a choked and in a restricted coastal lagoon by using a 3D-hydrodynamic model. To study the spatiotemporal progression of seawater intrusion and its mixing efficiency with lagoon waters we define Marine Mixed Volume (VMM) as a new hydrodynamic indicator. Spatial patterns in both lagoons were described by studying the time series and maps of VMM taking into account the meteorological conditions encountered during a water year. The patterns comprised well-mixed zones (WMZ) and physical barrier zones (PBZ) that act as hydrodynamic boundaries. The choked Bages-Sigean lagoon comprises four sub-basins: a PBZ at the inlet, and two WMZ's separated by another PBZ corresponding to a constriction zone. The volumes of the PBZ were 2.1 and 5.4 millions m3 with characteristic mixing timescale of 68 and 84days, respectively. The WMZ were 12.3 and 43.3 millions m3 with characteristics mixing timescale of 70 and 39days, respectively.

An extra large aperture scintillometer (XLAS) was used over several months across the Thau Lagoon (South of France) to retrieve one-wavelength scintillation and, thence, sensible heat flux. We present the experiment with the XLAS, an eddy-covariance station and meteorological stations measuring on or near the Thau Lagoon. Changes implemented to adapt the scintillometry processing schemes to the above water conditions are presented together with a full error budget, including sensitivity tests to the relevant parameters of the scintillometer processing scheme. The XLAS error budget amounts to 16% (systematic part) +/- 50% (random part). Sensible heat fluxes obtained using the XLAS under unstable atmospheric conditions are then compared to eddy-covariance estimates used as a reference. The scintillometry technique proved to perform satisfactorily in such a watery environment. Some discrepancies observed between the XLAS and eddy-covariance measurements were investigated according to the lagoon fraction of the source area, to discriminate whether they were related to deviations from the Monin-Obukhov similarity theory or to different atmospheric conditions at the respective instrument locations. Local atmospheric conditions agreed well with the Monin-Obukhov similarity theory, especially measurements with source areas largely composed of the lagoon surface. Retaining only the measurements with almost only the lagoon surface in the source area improved the agreement between the XLAS and eddy-covariance measurements. The remaining discrepancies are interpreted as being due to significant location differences between the two instruments, resulting in different atmospheric conditions, and to size differences in the source areas.

We developed a growth model for the oyster Crassostrea gigas cultured in Thau Lagoon. The oyster standing stock in the lagoon ranged between 10,000 and 15,000 tons a year. Two culture methods are presently in use in Thau Lagoon which are used in about the same proportions. At seeding, initial size of oysters is different among methods. The model was calibrated on (1) growth data accounted for both culture methods and (2) hydrobiological data (temperature, salinity, suspended particulate matter and chlorophyll a ), both recorded in several sites in the lagoon between March 2000 and October 2001. The lagoon is slightly eutrophic: total chlorophyll a and total particulate matter averaged 1.2 μg l −1 and 2.2 mg l −1 , respectively. Organic content accounted for ca. 40–50% of particulate matter. There was no seasonal trend in seston, whereas temperature and salinity were minimal in winter. Oyster growth varied among sites in response to spatial variations in seston. Growth was maximal in summer and minimal in winter because of temperature seasonality. For each location, we modelled growth as a function of particulate organic matter and temperature. Chlorophyll a was left out of the model because of a weaker fit with growth. Growth was modelled as G = a POM b T c Y d , where G is the growth rate in shell length (mm day −1 ) or in mass (g day −1 ), POM is particulate organic matter (mg l −1 ), T is temperature (°C) and Y is either shell length (mm) or mass (total individual mass or dry flesh mass in g). Allometry ( Y d ) allowed us to use the same model for both culture methods. The model yielded a good fit with actual size, either as measured by shell length ( R 2 =0.96) or total individual mass ( R 2 =0.93).

The aquaculture of the oyster Crassostrea gigas is largely based on natural recruitment of the species on spat collectors along the french Atlantic coast, the remaining spat being provided by hatcheries and nurseries. Due to an increase in the climate/meteorological variability, during the last twenty years, the natural spat collection has become variable, from null to overabundant depending on the year. Since 2008, the oyster spat mortalities worsen the situation, reducing the available spat and increasing the cost of the latter. However, the temporal variability of the natural recruitment modalities in the Mediterranean do not allow the aquaculture industry to rely on native spats, making the understanding of natural recruitment in the Mediterranean a challenge for the future in order to fit the professional needs.

International audience; We present the first large-scale, high-resolution spatial and temporal pattern of disease mortality caused by Ostreid herpesvirus 1 (OsHV-1) that has affected Pacific oyster Crassostrea gigas in France every year since 2008, in relation to local farm use. Mortality was monitored in healthy oyster spat deployed at 106 locations in the Thau Mediterranean lagoon during the epizootic that occurred in spring 2011. We conducted a field survey of farming practices in the areas where the sentinel oysters were placed before the epizootic. We found that the risk of disease mortality in sentinel oysters was higher within the bivalve farming area than outside it, suggesting that the infection pressure is much higher in areas with intensive farming activity. The risk of mortality was higher in farms rearing spat, a developmental stage particularly susceptible to OsHV-1, than in farms rearing adult oysters, which are generally more resistant. Additionally, the mortality risk in farms rearing adult animals was similar to that in empty farms, which suggests that, during the study period, adult oysters did not vector the disease. Interestingly, the mortality risk for sentinel oysters deployed in mussel Mytilus galloprovincialis farms was lower than for those held in oyster farms or empty farms, suggesting that mussels reduced the infection pressure on susceptible oysters.

International audience; Although spatial studies of diseases on land have a long history, far fewer have been made on aquatic diseases. Here, we present the first large-scale, high-resolution spatial and temporal representation of a mass mortality phenomenon cause by the Ostreid herpesvirus (OsHV-1) that has affected oysters (Crassostrea gigas) every year since 2008, in relation to their energetic reserves and the quality of their food. Disease mortality was investigated in healthy oysters deployed at 106 locations in the Thau Mediterranean lagoon before the start of the epizootic in spring 2011. We found that disease mortality of oysters showed strong spatial dependence clearly reflecting the epizootic process of local transmission. Disease initiated inside oyster farms spread rapidly beyond these areas. Local differences in energetic condition of oysters, partly driven by variation in food quality, played a significant role in the spatial and temporal dynamics of disease mortality. In particular, the relative contribution of diatoms to the diet of oysters was positively correlated with their energetic reserves, which in turn decreased the risk of disease mortality.

The gametogenic cycle and the reproductive effort of the blacklip pearl oyster, Pinctada margaritifera, cultivated in Takapoto lagoon were studied for a 1-year period (March 1997-April 1998) by bimonthly observations of gonadal sections, dry tissue weights and gonadal index in a population of pearl oyster composed of three age-groups. pearl oysters attained sexual maturity in the end of their first year (height approximate to 40 mm), implying that P. margaritifera is a late-maturing species in comparison with other Pteriidae. This species was also confirmed to be a marked protandrous successive hermaphrodite in culture, with 100 % of males at first maturity and 75 % in older pearl oyster (height > 120 mm). The general pattern of gametogenic activity, fairly synchronous in both sexes, was comparable with that of other tropical bivalves: reproduction occurs continuously throughout the year with a maximal activity during the warm season (November-May). No resting period was observed. Quantitative growth data showed that P. margaritifera exhibits an annual synchronised polymodal spawning pattern, with two spawning peaks in age-group I (height approximate to 70 mm) and five in age-groups II (height approximate to 100 mm) and III (height approximate to 120 mm). Spawning was sometimes incomplete, nevertheless a clear relationship between gamete production (P-R, g) and size (height H, mm) was obtained: P-R = 5.26 x 10(-7) H-2.91 (R-2 = 0.99, p < 0.05). Estimation of P-R was used to calculate the annual reproductive effort in P. margaritifera. Reproductive effort (%) was similar to those calculated for temperate species and showed a progressive increase with the age of pearl oyster, from 7 % in age-group I to 38 % in age-group III. This study showed that, in a fairly stable tropical environment such as the Takapoto lagoon, P. margaritifera is a multiple spawner, which uses an opportunistic reproductive strategy, allowing investment all year around, of any surplus energy into gamete production. Surplus energy is ensured by the high pumping rates developed by this non-symbiotic bivalve to succeed in low seston conditions. (C) 2000 Ifremer/Cnrs/Inra/Ird/Cemagref/Editions scientifiques et medicales Elsevier SAS., Le cycle et l'effort de reproduction de l'huître perlière à lèvres noires, Pinctada margaritifera, cultivée dans le lagon de Takapoto, ont été étudiés sur une période annuelle (de mars 1997 à avril 1998), par un suivi bimensuel des stades de maturation et des indices gonadiques, sur une population en élevage composée de trois groupes d'âge (1, 2 et 3 ans). Cette étude a confirmé que P. margaritifera est une espèce qui se reproduit tardivement par rapport aux autres Ptériidés (elle atteint sa maturité sexuelle à la fin de sa première année, pour une hauteur approximative de 40 mm). Par ailleurs, elle présente, en élevage, une protandrie marquée (100 % de mâles à première maturité, 75 % pour des huîtres âgées de trois ans). Le schéma général de sa gamétogenèse, relativement synchrone dans les deux sexes, est comparable à celui des autres bivalves tropicaux : la reproduction est quasiment continue (aucune période de repos sexuel) avec une activité accrue en saison chaude (de novembre à mai). L'analyse des variations du poids de la gonade suggère l'existence de deux pics d'émission de gamètes chez les huîtres de un an et cinq pics chez les huîtres de 2 et 3 ans (cycle de reproduction de type polymodal). Bien que ces émissions de gamètes soient parfois incomplètes, une relation entre la production de gamètes (PR, en g) et la taille de l'huître perlière (hauteur, H, en mm) a été établie : PR = 5,26 x 10-7 H2,91 (R2 = 0,99, p < 0,05). Ces résultats ont permis de calculer l'effort de reproduction de P. margaritifera à Takapoto et sur un bilan annuel : il augmente avec l'âge de l'huître perlière de 7 % pour le groupe d'âge I à 38% pour le groupe d'âge III. Ces valeurs annuelles sont similaires à celles calculées chez les bivalves de milieux tempérés. Cette étude a donc montré que, dans un environnement tropical assez stable tel que celui du lagon de Takapoto, P. margaritifera présente une reproduction continue, avec plusieurs pics d'émission de gamètes par an. Cette stratégie « opportuniste » permet l'investissement, de tout surplus d'énergie en production de gamètes, tout au long de l'année. Malgré la faible concentration du milieu en nourriture, ces surplus d'énergie sont assurés par les fortes capacités de filtration développées par cette espèce tropicale non-symbiotique.

La conchyliculture, et principalement l’élevage de l’huître creuse, Crassostrea gigas, constitue la principale activité aquacole française. Cette activité repose, en grande partie, sur le recrutement naturel de l’espèce qui assure 70% des besoins en jeunes huîtres (naissain) : cette activité de collecte s’appelle le captage. Les deux principaux centres de captage en France sont les bassins d’Arcachon et de Marennes-Oléron. Or, depuis une dizaine d'années, sur le Bassin d'Arcachon, le captage devient très variable: à des années de captage nul (par exemple les années 2002, 2005, 2007) ou faible (2009, 2010, 2011) succèdent des années excellentes voire pléthoriques (les années 2003, 2006, 2008, 2012). A Marennes-Oléron, cette variabilité existe, mais s’avère beaucoup moins marquée. En outre, à la faveur du lent réchauffement des eaux, le captage peut désormais se pratiquer de plus en plus vers le nord. Ainsi, la Baie de Bourgneuf, mais aussi la Rade de Brest sont devenues, depuis quelques années, des secteurs où un nombre croissant d’ostréiculteurs pratiquent avec succès le captage, mais avec, là aussi, des irrégularités dans le recrutement qu’il convient de comprendre. Enfin, depuis la crise des mortalités de 2008, il se développe aussi sur l’Etang de Thau une volonté de pratiquer le captage. Afin de mieux comprendre les facteurs de variations du captage, l’Ifremer a mis en place, à la demande du Comité National de la Conchyliculture, un réseau national de suivi de la reproduction : le Réseau Velyger. Créé en 2008 sur fonds européens et financé désormais par la Direction des Pêches Maritimes et de l’Aquaculture, ce réseau apporte, chaque année, sur les écosystèmes cités précédemment, une série d’indicateurs biologiques (maturation, fécondité, date de ponte, abondance et survie larvaire, intensité du recrutement, survie du naissain) dont l’analyse croisée avec des indicateurs hydrologiques et climatiques permet progressivement de mieux appréhender les causes de variabilité du recrutement de l’huître creuse en France, modèle biologique et espèce clé de la conchyliculture française. Ce rapport présente donc les résultats 2012 de ce réseau d’observation et fait appel, pour la partie hydro-climatique, à des observations acquises par d’autres réseaux. Il détaille toutes les caractéristiques par secteur du cycle de reproduction de l’huitre creuse : maturation des adultes, période de ponte, abondance et survie des larves, intensité du captage et mortalités précoces. Il fournit ensuite une interprétation et une synthèse des résultats 2012 par secteur et à la lueur des résultats des années antérieures. Ainsi, pour l’année 2012, on retiendra les faits majeurs suivants : • Après une année 2011 particulièrement sèche, l’année 2012 s’avère plutôt humide à partir du printemps avec un gradient Nord-Sud marqué. Ce gradient se retrouve dans les concentrations phytoplanctoniques : elles sont élevées en Rade de Brest et Baie de Bourgneuf, normales à faibles du Bassin de Marennes Oléron à l’Etang de Thau. En outre, jusqu’au mois de juillet, les températures de l’air et de l’eau sont tout justes dans les normes voire déficitaires pour les sites les plus au nord. • Ces conditions hydro-climatiques ont imprimé un gradient dans la maturation et la fécondité des huitres adultes avec un indice de condition très élevé en Rade de Brest, élevé en Baie de Bourgneuf, normal à faible du Bassin de Marennes Oléron à l’Etang de Thau. En outre, le manque de chaleur jusqu’en juillet s’est traduit par une ponte plutôt tardive sur l’ensemble des écosystèmes de la côte atlantique, à l’exception de la ponte en Baie de Bourgneuf qui a été très précoce compte tenu des conditions. A contrario, le Bassin d’Arcachon enregistre la ponte la plus tardive jamais observée sur ce secteur (au-delà du 15 août). A ce titre, le mécanisme du déclenchement de la ponte mériterait une attention particulière dans les années à venir. • Cela dit, grâce à un mois d’août favorable sur le plan thermique, les cohortes larvaires présentes à cette période ont eu une survie normale à bonne (e.g. 0.3 à 0.8 %), ce qui s’est traduit par un captage modéré à bon sur l’ensemble des secteurs de la façade atlantique. Il convient cependant de noter que la ponte trop précoce en Baie de Bourgneuf, qui s’est déroulée dans des eaux fraîches, n’a pas permis une bonne évolution de la cohorte larvaire (survie de 0.03 %) et le captage reste, sur ce secteur, faible à modéré en 2012. • En conséquence, l’année 2012, très différente de l’année 2011, se caractérise par un captage qualifié de « bon » dans le Bassin d’Arcachon (~230 naissains/coupelle), modéré dans le Bassin de Marennes-Oléron (~ 180 naissains/coupelle) et en Rade de Brest (~ 100 naissains/coupelle), modéré à faible en Baie de Bourgneuf (15 naissains/coupelle) et faible à nul dans l’étang de Thau (~ 5 naissains/coupelle). Ce rapport montre aussi que l’étang de Thau joue un rôle d’exception. Malgré une ponte d’intensité normale, des abondances de larves élevées et une bonne survie des cohortes, le captage 2012 y est très faible ce qui positionne l’étang de Thau en dehors du modèle de fonctionnement admis pour le cycle de reproduction de l’huître creuse sur la façade atlantique. Il semble que, sur ce site, la métamorphose constitue un verrou biologique qu’il convient d’étudier.

The THAUMEX measurement campaign, carried out during the summer of 2011 in Thau, a coastal lagoon in southern France, focused on episodes of marine breezes. During the campaign, three intensive observation periods (IOPs) were conducted and a large amount of data were collected. Subsequently, standalone modelling using the FLake lake model was used, first to assess the surface temperature and the surface energy balance, and second to determine the energy budget of the water column at the measurement site. Surface fluxes were validated against in situ measurements, and it was determined that heat exchanges are dominated by evaporation. We also demonstrated that the model was sensitive to the light extinction coefficient at Thau, due to its shallowness and clarity nature. A heat balance was calculated, and the inclusion of a radiative temperature has improved it, especially by reducing the nocturnal evaporation. The FLake lake model was then evaluated in three-dimensional numerical simulations performed with the Meso-NH mesoscale model, in order to assess the changing structure of the boundary layer above the lagoon during the IOPs more accurately. We highlighted the first time ever when Meso-NH and FLake were coupled and proved the ability of the coupled system to forecast a complex phenomenon but also the importance of the use of the FLake model was pointed out. We demonstrated the impact of the lagoon and more precisely the Lido, a sandy strip of land between the lagoon and the Mediterranean Sea, on the vertical distribution of turbulent kinetic energy, evidence of the turbulence induced by the breeze. This study showed the complementarities between standalone and coupled simulations. Keywords: lake modelling, surface energy budget, coastal breeze, water heat storage, surface temperature (Published: 31 October 2013) Citation: Tellus A 2013, 65 , 20951, http://dx.doi.org/10.3402/tellusa.v65i0.20951 This publication is part of a Thematic Cluster entitled "Parameterization of lakes in numerical weather prediction and climate models". Read the other papers from this thematic cluster here

Experiments were conducted to investi- gate the effects of farming sites, seeding date, age of oysters, ploidy level and rearing structures on mor - tality, pathogens and energetic reserves of oysters Crassostrea gigas. Mortality of oysters coincided with infections involving, primarily, the ostreid herpesvirus OsHV-1 and, secondarily, bacteria of the group Vibrio splendidus, when seawater temperature was between 17 and 24°C. Mortality started in early May and lasted until late September, with an interruption in July and August when seawater temperatures rose above 24°C. Mortality varied with farming site: oysters maintained in the open Mediterranean Sea showed no mortality and OsHV-1 was occasionally detected at a low level only, while oysters maintained in the farming area of the Thau lagoon exhibited mass mortality events concomitantly with outbreaks of OsHV-1. Mortality of oysters maintained in the Thau lagoon but outside the oyster farming area was sporadic and co incided with currents coming from the farming area where mortality was occurring. Mortality of oysters also varied with farming practices. Naive oysters remained susceptible to pathogen-related mortality pressure during their first 2 yr, whereas oysters that had survived a mass mortality event in the past appeared resistant. Mor - talities of diploid (2n) and triploid (3n) oysters were similar during spring but mortalities of 2n oysters were double those of 3n oysters during summer and autumn. Finally, oysters grown in Australian baskets suffered 80% mortality compared with only 30% among oysters cemented onto ropes. The effects of farming practices on mortality are discussed in relation to pathogens and energetic reserves of oysters. OPENEN ACCESSCESS

Les travaux présentés dans le présent rapport finalisent l’étude engagée dans le cadre de l’Axe 1 : « Amélioration des connaissances et compréhension des processus concernant le fonctionnement et le devenir du littoral méditerranéen » de la Convention Cadre entre l’Ifremer et l’Agence de l’Eau Rhône Méditerranée et Corse portant sur le « Développement de modèle hydrodynamiques et hydrosédimentaires sur les masses d’eau de transition ». Au terme de cette étude, nous disposons d’une méthode de jaugeage des débits en canal, opérationnelle dans la majorité des configurations de profils verticaux de courant rencontrées. Le chenal portuaire de Port-La-Nouvelle, unique voie de communication entre la lagune de Bages-Sigean et la Méditerranée, a été retenu comme site pilote pour la mise au point de la méthode. La définition d’un protocole de déploiement des appareils de mesures, la mise à jour des besoins en terme de stratégie d’échantillonnage et la chaine de traitement automatisée des données acquises sont aujourd’hui disponibles et font que cette méthode est applicable sur d’autres sites lagunaires. Cette étude à également mis à jour les ordres de grandeur des volumes journaliers entrants (23 m3s-1 en moyenne) et sortants (29 m3s-1 en moyenne) de la lagune de Bages-Sigean sous différents régimes hydro-climatiques. En particulier, une première estimation des temps de renouvellement à montrer que l’événement pluvieux exceptionnel du début d’année 2006 serait susceptible de renouveler la totalité des eaux de la lagune (73 Mm3 exportés en quinze jours pour un volume lagunaire de 60 Mm3). L’acquisition de connaissances et de données sur le fonctionnement des échanges hydriques entre une lagune et la mer, ainsi que le développement de méthodes robustes et applicables en routine d’acquisition de données de débits dans les graus des lagunes présentent, dans les contextes européen (Directive Cadre sur l’Eau) et régional (Réseau de Suivi Lagunaire), un intérêt indéniable. Ainsi, l’Agence de l’Eau, la Région Languedoc-Roussillon et l’Ifremer ont lancé une nouvelle étude visant à construire puis évaluer, sur les lagunes méditerranéennes identifiées par la DCE comme masses d’eau de transition, des indicateurs hydro-morphologiques ou plus précisément selon la terminologie DCE, « des facteurs physiques ayant un impact potentiel sur la structure et la composition des peuplements biologiques ». Les connaissances et données maintenant disponibles sur la lagune de Bages-Sigean viendront largement alimenter cette étude.

Lagarde Franck. Sports de nature et protection de l'environnement. D'une logique de conflit à une logique de conciliation. In: Les Cahiers de l'INSEP, n°37, 2006. Sport, environnement et développement durable. pp. 165-166.

Le bassin de Marennes-Oléron est le premier site ostréicole européen. Depuis 1988 des mortalités chroniques apparaissent sur les élevages conchylicoles. En 1996, un programme de suivi de mortalité et des paramètres environnementaux a été développé. Les différentes conclusions retenues au cours des années antérieures font état de brusques variations de température suspectées dans l'apparition de ces mortalités. Une approche écophysiologique des huîtres par l'intermédiaire de la mesure de respiration a permis de déterminer l'effet significatif d'un stress thermique (∆T=28°C). La maturité sexuelle apparaît également comme un facteur prépondérant dans la demande métabolique. A l'issue de cette étude, il apparaît que les huîtres matures et de surcroît stressées par le facteur température peuvent refléter un certain niveau de "détresse physiologique".

La conchyliculture est une activité importante dans le bassin de Marennes-Oléron. L'aquaculture extensive de l'huitre japonaise (Crassostrea gigas ) représente, avec 130 000 tonnes par an, la première ressource marine nationale (Héral, 1986) dont 40 % se fait dans ce bassin. Il est donc important de connaître et d'étudier l'ensemble des phénomènes du bassin. Depuis 1970, on observe un ralentissement de la croissance des huîtres sur le bassin et une baisse de productivité sur certains bancs ostréicoles exploités tels que Ronce les Bains (25 % de mortalité en Juillet 1996 ) et le banc de Perquis (25 % de mortalité en Juillet 1996) (REMORA 1995). Depuis 1988, les élevages ostréicoles de Charente-Maritime subissent en périodes printanières et estivales, des mortalités significatives et anormales. Un suivi de ces cheptels a donc été mis en place par IFREMER afin de rechercher les relations éventuelles entre la productivité, la mortalité et les paramètres environnementaux et biomètriques. Voilà maintenant 4 années que des doutes sont émis par les exploitants professionnels du bassin sur la qualité des eaux de rejet de la station d'épuration de la Tremblade. La mise en route de l'installation permettant le traitement physicochimique (traitement estivale) a été mise en cause dans le problème de mortalité qui se produit aux débuts du mois de Juin. Etant donné l'inquiètude des ostréiculteurs face à ce problème, l'IFREMER a donc jugé nécessaire de mettre en place un test de toxicité des effluents de la station d'épuration de la Tremblade sur les huîtres (Crassostrea gigas ) ainsi que sur leurs larves Cette expérience a été effectuée à la station d'IFREMER de La Tremblade.

Ces tables rondes « Recherche & Conchyliculture », coanimées par le CRCM et l’Ifremer sont nées d’une double volonté : casser le plafond de verre entre le monde de la R & D et les professionnels de la conchyliculture ; renforcer les échanges et la communication entre les conchyliculteurs, les porteurs de recherche, d’expérimentations techniques, d’innovation… Elles découlent d’une attente exprimée par les professionnels dans le cadre de la construction du Contrat de filière conchylicole Occitanie, et ont pour but de transmettre et valoriser la connaissance existante qui touche de près ou de loin la conchyliculture méditerranéenne et ses écosystèmes de production. En effet, les écosystèmes exploités en Occitanie, sentinelles du changement climatique, sont riches mais ils sont également sous pression et en évolution. La compréhension et l’anticipation de ces évolutions sont importantes pour le développement des filières. L’observation, la modélisation, la restauration, l’adaptation, la protection de ces écosystèmes structurent les projets que l’Ifremer mène avec les parties prenantes du contrat de filière. Ces tables rondes sont donc une opportunité de renforcer les échanges entre tous les acteurs du contrat de filière. Elles permettent de valoriser les projets de recherche, d’expertise et d’innovation, d’anticiper les enjeux à venir pour renforcer la résilience des écosystèmes et la durabilité de la filière conchylicole. Maladie de Charcot, « eaux vertes » et écrins éco-conçus : cette table-ronde a abordé des sujets très différents qui éclairent plusieurs aspects de la profession conchylicole, de son extrême dépendance au milieu jusqu’à sa capacité d’innovation. Dans un premier temps, le professeur Camu du CHU de Montpellier a fait le point sur la Sclérose Latérale Amyotrophique (SLA) ou maladie de Charcot, liée à une neurotoxine présente dans les plans d’eau. Puis Franck Lagarde de l’Ifremer a détaillé l’intense travail de recherche partenariale qui a été mené depuis 2018 pour comprendre le phénomène des eaux vertes. Enfin, Philippe et Christophe Cambon, conchyliculteurs à Loupian, ont présenté un projet d’innovation qui modernise l’ensemble de la pratique de culture.

The Mediterranean lagoons host many anthropogenic uses including shellfish aquaculture since the beginning of the 20th century. Imported in 1970, the cupped oyster, Crassostrea gigas, has adapted perfectly and has allowed an important development of oyster’s industry until 2008, the year of occurrence of spat outbreak related to the emergence of an Ostreid Herpes-virus µvar. The scarcity of spat and the speculation on this product changed regionally cultural practices and the supply of juveniles, with in particular the wish of developing a local activity of spat collection. However the cycle of reproduction of the species and the recruitment of juveniles were poorly known in lagoon environments. In this context, the objective of this thesis is to characterize the variability of the reproduction and the recruitment of the cupped oyster in Mediterranean lagoon. The lagoon configuration offers the advantages of productivity for the broodstock and hydrodynamic connectivity for the larvae. However, it appears that the ecological functioning of the system at the base of the primary production plays an essential role on the trophic resources for the success of oyster reproduction. This work takes place in the context of the ecological restoration under oligotrophication, which first effects we describe on the oyster's reproductive cycle.; Les lagunes méditerranéennes hébergent de nombreux usages anthropiques dont la conchyliculture depuis le début du 20ème siècle. Importée en 1970, l’huître creuse, Crassostrea gigas, s’y est adaptée parfaitement et a permis un développement ostréicole important jusqu’en 2008, année d’apparition des surmortalités de naissain liées à l’émergence d’un variant du virus OsHv1. La raréfaction du naissain et la spéculation sur ce produit ont modifié régionalement les pratiques culturales et les modes d’approvisionnement en juvéniles, avec notamment la volonté de développer une activité locale de captage. Mais le cycle de reproduction de l’espèce et le recrutement de juvéniles étaient mal connus en milieu lagunaire. Dans ce contexte, l’objectif de cette thèse est de caractériser la variabilité de la reproduction et du recrutement de l’huître creuse en lagune méditerranéenne. La configuration lagunaire offre des atouts de productivité pour le cheptel de géniteurs et de connectivité hydrodynamique pour les larves. Cependant, il apparaît que le fonctionnement écologique du système à la base de la production primaire joue un rôle essentiel sur les apports trophiques pour le succès de la reproduction des huîtres. Ce travail s’inscrit, en outre, dans le contexte d’un écosystème en voie de restauration écologique entamant un processus d’oligotrophisation dont nous décrivons les premiers effets sur le cycle de reproduction de l’huître.

L’Ifremer coordonne, sur l’ensemble du littoral métropolitain, la mise en œuvre des réseaux d’observation et de surveillance de la mer côtière. Ce bulletin contient une synthèse et une analyse des données collectées par l’ensemble des réseaux : de contrôle microbiologique (REMI), d’observation et de surveillance du phytoplancton et de l’hydrologie dans les eaux littorales (REPHY), de surveillance des phycotoxines dans les organismes marins (REPHYTOX), d’observation de la contamination chimique (ROCCH), d’observations conchylicoles (ECOSCOPA) des départements des Pyrénées Orientales, de l’Aude, de l’Hérault, du Gard du littoral de la région Occitanie en 2021. Des représentations graphiques homogènes, assorties de commentaires, donnent des indications sur les niveaux et les tendances des paramètres mesurés.

Malaïgue sur la lagune de Thau, expertise de la température de l'eau de la lagune de Thau.

LES CHANGEMENTS CLIMATIQUES ET LA BIODIVERSITÉ DU LITTORAL Impact du changement climatique sur le phytoplancton des lagunes côtières méditerranéennes

The Thau lagoon, located at the South of France, provides 10% of the French pacific oyster Crassostrea gigas production (Robert et al., 2013). Despite this intensive shellfish production, the larval recruitment of this species within the Thau lagoon is still misunderstood and spat collection remains highly variable; as a consequence, shellfish farmers rely mostly on hatchery's spat. In 2012, the PRONAMED II project was launched, aiming to improve our understanding of the C. gigas reproductive cycle within Mediterranean lagoons and to explore the feasibility for oyster-spat collecting. In addition to an extensive sampling work encompassing both spatial and temporal features of larval recruitment, we tried to understand the contribution of larval dispersal on recruitment patterns. For this purpose, we developed new approaches using the MARS 3D hydrodynamical model. While most models keep looking at the location of larvae after a simulation duration specified by the user, we propose here a new approach considering the pelagic larval duration and focusing on the duration of the larval competency for settlement. A new index was set up to take into account the binary possibility for the larvae to settle or not during its final pelagic stage, the settlement probability increasing with the amount of larvae passing through a model mesh.