The aim of this study was to summarise the literature reporting the risk factors for mortality in the mussel speciesMytilus edulisandMytilus galloprovincialisin order to identify potential science-based solutions to prevent or mitigate mussel mortality outbreaks. We followed the PRISMA methodology: Preferred Reporting Items for Systematic Reviews and Meta-Analyses. The studied corpus of 91 publications (114 studies) was highly heterogeneous with respect to the methodological approaches used to define or estimate mussel mortality and the related putative risk factors. Results showed that the mortality risk of both mussel speciesM. edulisandM. galloprovincialisvaried across the seasons, increased with an elevated seawater temperature above a thermal threshold of 20 and 24 degrees C, respectively, decreased by protecting mussels from predation, and was associated with the presence of pathogens inM. edulis. ForM. galloprovincialis, using mussel spat from the same area where the farming is carried out and farming them together with another mussel species appears to reduce the mortality risk. However, forM. edulis, this could be achieved by using pure crosses and in particular mussel spat having a selected genotype. For wild bed conservation, sand accumulation and anthropogenic sedimentation should be minimised. Our analysis showed that current approaches to this research topic are limited and are unlikely to yield actionable evidence to identify mussel mortality prevention or mitigation strategies. Therefore, recommendations are offered to increase the ability of future eco-epidemiological research to identify multiple exposures associated with mussel mortality, underpinned by standardised efforts and cooperative initiatives.

In the context of global change, increasing mariculture production has raised particular concerns regarding its environmental impact and sustainability. Molluscs and particularly blue mussel account for a significant part of this total production. Although blue mussels are considered to be pretty resilient to environmental disturbances, we report in this study an unprecedented mussel mortality event that occurred during the winter of 2014 in the Breton Sound. 9000metrictonnes of mussels were lost and mortality rates up to 100% were recorded at some farming areas. Through a coupling approach, the present work aims to better understand the potential environmental drivers associated with those mortalities. Firstly, we analysed long-term in situ and satellite data from environmental monitoring networks (available since 1998) to characterize the variability of seawater masses of the sound during the winter of 2014. Secondly, we used modelling simulations to study the possible relationship between seawater hydrodynamics and observed spatio-temporal patterns of mussel mortalities. From January to April 2014 at the long-line culture site where mortalities started, seawater temperatures ranged from 8.3 to 13.3°C (10.2±0.8°C). Salinity and turbidity values showed successive and short drops (below 16; 29.3±2.3) and numerous peaks (above 70 NTU; 17.4±13.4 NTU) respectively. Winter conditions of 2014 were encountered along the entire French Atlantic coastline and linked to the sixth highest positive North Atlantic Oscillation (NAO+) index recorded since 1865. These particular environmental variations characterized the winter of 2014 but also others whereas no comparable mussel mortality rates were reported. Exact causes of the 2014 mortality event are still unknown but we showed these environmental variations could not alone be responsible. These have likely affected the sensitivity of the blue mussel populations that were already weakened by early spawning. Meanwhile, these may have facilitated the apparition of a pathogenic strain of Vibrio splendidus isolated on moribund mussels at that time. Our modelling simulations suggested that this pathogenic strain could spread through hydrodynamic patterns and drive the observed mussel mortalities. If this pathogenic strain recurs in future years, particularly with the added stress associated with climate change, mussel mass mortality events may exceed the resilience of this species.

Depuis 1992, la surveillance de la sante des mollusques marins du littoral francais est assuree par le reseau de Pathologie des Mollusques (Repamo). Ses activites s’inscrivent dans le cadre de la Directive Europeenne 2006/88/CE. Depuis son evaluation par la plateforme nationale d’epidemiosurveillance en sante animale en 2012, l’objectif de surveillance est la detection precoce des infections dues a des organismes pathogenes exotiques et emergents affectant les mollusques marins sauvages et d’elevage. L’annee 2015 est la premiere annee de transition pour laquelle un debut d’evolution des modalites de surveillance de la sante des mollusques marins animees par l’Ifremer a ete amorce. Un dispositif hybride de surveillance a ete mis en place, s’appuyant sur l’existant et integrant des debuts d’evolution. La surveillance evenementielle a constitue l’activite principale du dispositif en 2015 et s’est appuyee sur des reseaux existants : (1) la surveillance des mortalites observees sur des animaux sentinelles deployes sur les sites ateliers des reseaux Ifremer RESCO 2 (12 sites) pour l’huitre creuse Crassostrea gigas et MYTILOBS 2 (8 sites) pour la moule bleue Mytilus edulis. Pour l’huitre creuse Crassostrea gigas, la mortalite cumulee moyenne etait de 50,3% (ecart-type 10,9%) pour le naissain standardise Ifremer (NSI), de 11,0% (ecart-type 9,1%) pour les huitres de 18 mois et de 7,3% (ecart-type 5,6%) pour les huitres de 30 mois. Les mortalites ont ete observees principalement entre le debut du mois de mai et la mi-juillet. Lors de ces episodes de mortalite, des prelevements d’animaux ont ete realises en vue d’analyses diagnostiques : 7 prelevements pour le NSI, 2 pour les huitres de 18 mois et 1 pour les huitres de 30 mois. Aucun agent reglemente n’a ete detecte dans les echantillons d’huitres creuses preleves et analyses...

This study aimed to describe seasonal variations of phytoplankton abundances in relation to the physical and chemical (nutrients and metals) environment under the influence of freshwater input in the Charente river estuary (Marennes-Oléron bay, France) over three years, from 2011 to 2014. Phytoplankton abundances were determined using microscopy and flow cytometry. Considering high frequency temperature and salinity data, breakpoints in each series led to the identification of two local hydroclimatic periods: the first (2011 and early 2012) being warmer and higher in salinity than the second (from spring 2012 to the beginning of 2014). A multiblock PLS analysis highlighted the significant contribution of the physical environment (temperature, salinity and Photosynthetically Active Radiation (PAR)) on phytoplankton abundances. Two partial triadic analyses (PTA) were run in order to visualize seasonal variations of i) phytoplankton groups and ii) nutrients and trace elements, irrespective of spatial gradient: picoeukaryote occurrence showed a difference between year 2011 and the years 2012 and 2013 (as did cadmium, nickel and silica levels). However, both PTA revealed greater differences between year 2013 and the years 2011 and 2012, as shown by occurrences of cryptophytes, dinoflagellates and nanoeukaryotes, as well as copper and phosphate levels. These results showed a shift between the hydroclimate breakpoint and some phytoplankton responses, suggesting that their development and succession might depend on conditions early in the year. Finally, a STATICO analysis was performed on the paired PTA in order to examine the relations of phytoplankton with nutrients and metals more closely. Most phytoplankton groups were represented on the first axis, together with cadmium on the one hand, and nitrates, silica and nickel on the other. This analysis revealed the separation of phytoplankton groups on the second axis that represented phosphates and copper. Hydroclimatic conditions and the nature of freshwater inputs, especially phosphates and copper content, might be key factors driving phytoplankton structure in the Charente estuary.

Suite aux surmortalites observees au sein des elevages de moules touchant a la fois les animaux adultes et les juveniles au mois de mars, dans les Pertuis Charentais, la Direction Generale de l’Alimentation (DGAl) a adresse une saisine a Ifremer le 04 avril 2014. Celle-ci fait etat d’une demande d’appui scientifique et technique d’Ifremer pour tenter d’apporter des elements de reponse par rapport a cette mortalite subite et importante. L’Ifremer a propose un projet d’action permettant l’expertise du phenomene de surmortalite de la moule bleue, selon 3 axes : l’impact environnemental, la pathologie et les caracteristiques physiologiques des moules. Il s’agit de mettre en evidence la presence ou non d’agents infectieux connus ou emergents, d’apporter des elements de comprehension au regard des fluctuations environnementales, de la reponse physiologique et biochimique des moules dans la mer des Pertuis Charentais et des interactions entre ces trois volets.

Ce rapport a comme objectif principal de preciser les connaissances sur les relations existantes entre les apports terrigenes quantitatifs et qualitatifs des fleuves (Lay, Sevre niortaise, Charente, Seudre, Loire, Gironde) et la conchyliculture (ostreiculture et mytiliculture) dans les Pertuis Charentais. Si la quantite d’eau douce, representee par la variable salinite, semble avoir un impact faible sur la conchyliculture, sa qualite, determinee par les substances particulaires et dissoutes associees, peut au contraire jouer un role majeur dans les equilibres biogeochimiques et microbiens. En automne-hiver, les grandes quantites d’eau douce transportees ont un effet « globalement positif » en amendant l’ecosysteme conchylicole. Les elements dissous mineraux ou organiques associes determinent alors en grande partie la qualite de la croissance printaniere a venir, elle-meme determinante pour les rendements annuels des elevages conchylicoles. Toutefois en hiver, leurs effets sont differes car les charges minerales importantes et les faibles luminosites et temperatures limitent la production primaire. Les apports printaniers, plus directement lies aux usages territoriaux des bassins versants (i.e. agriculture, tourisme) sont quant a eux variables en quantite mais aussi en qualite d’une annee a l’autre. De nombreuses substances peuvent alors se retrouver diluees dans les exutoires naturels comme les Pertuis Charentais. En partie benefiques, elles peuvent neanmoins, selon leur nature et leur proportion, representer un risque encore mal estime. De plus, leur dangerosite sur le monde animal et vegetal etant principalement evaluee en laboratoire, leurs impacts sur l’ecosysteme estuarien, en milieu naturel, peuvent etre differents et restent de ce fait meconnus. Plusieurs etudes ont pu mettre en evidence l’effet direct et indirect significatif de contaminations anthropiques (i.e. cadmium, pesticides) sur la conchyliculture...

Plankton and dissolved organic carbon (DOC) dynamics in fractionated estuarine microbial communities (<150 mu m, <60 mu m and <20 mu m), incubated with allelopathic (lytic) or non allelopathic (non-lytic) Alexandrium tamarense filtrates were investigated over a period of 48 h. Additionally, the amount of dissolved organic matter (DOM) available for bacterial growth in the treatments was measured via bacterial seawater culture experiments immediately and 6 h after addition of A. tamarense filtrates. The lack of effect on DOC concentrations and plankton community composition in lytic treatments indicated that allelochemicals did not inhibit the growth of the microbial community. Nevertheless, bacterial seawater culture experiments provided evidence that lytic filtrate addition provoked the release of bioavailable DOM from the microbial community. Since DOM was only released from the largest seawater fraction, microorganisms >60 mu m were probably most sensitive towards allelochemicals. (C) 2011 Elsevier B.V. All rights reserved.

Alexandrium tamarense (M. Lebour) Balech strains isolated in spring 2007 from a single bloom in Thau lagoon have been grown in nonaxenic artificial media. For three strains showing large oscillations in biomass (crashes followed by recoveries) on a scale of several days, a significant relationship was observed between changes in cell densities (as in vivo fluorescence) and changes in nitrate concentrations. Increases in cell densities were accompanied by decreases in nitrate, while decreases in cell densities corresponded to increases in nitrate, presumably due to nitrification. Net increases in nitrate could reach up to 15 μmol N · L −1 · d −1 indicating a very active nitrifying archaeal/bacterial population. However, following population crashes, algal cells can recover and attain biomass levels similar to those reached during the first growth phase. This finding indicates that those archaea/bacteria do not compete for nutrients or do not hamper algal growth under those conditions. In contrast to diatoms, dinoflagellates such as A. tamarense do not excrete/exude dissolved organic matter, thus preventing excessive bacterial growth. This mechanism could help explain the recovery of this species in the presence of bacteria.

This study investigates the effect of filtrates from an allelopathic dinoflagellate, Alexandrium tamarense, on four microbial food webs that have been manipulated experimentally from natural seawater by modifying the availability of resources in the form of dissolved organic carbon with additions of peptone, and by altering the grazing pressure with size fractionation. Bacterial production was generally not affected by allelochemicals, but bacteria showed higher net growth in all food webs when allelochemicals were added, whereas heterotrophic nanoflagellates > 7 μm and ciliates were constrained in all food webs. Allelochemicals had the largest negative effects on microbial communities with low grazing pressure. In food webs with high grazing pressure and additional resources, phytoplankton and small nanoflagellates were positively affected by the addition of allelochemicals, suggesting that those were interfering with trophic interactions in the microbial communities. By the lysis of organisms sensitive towards allelochemicals, resources are made available and grazing pressure on certain microorganisms is reduced. However, the intensity of these interactions is modulated by both the availability of resources and the biomass of grazers in the initial food web.