Evaluation de la qualité des zones de production conchylicole. Département d'Ille-et-Vilaine. Période 2021-2023

Après un rappel des objectifs, du fonctionnement et de la méthode d’interprétation des résultats du réseau de surveillance microbiologique des zones de production conchylicoles (REMI) et du réseau d’observation de la contamination chimique (ROCCH), ce rapport inclut un bilan national et décrit le programme annuel du département de l’Ille-et-Vilaine (35). Il présente l’ensemble des résultats obtenus, en particulier l’estimation de la qualité microbiologique et chimique des zones de production de coquillages classées. L’année 2023 présente des résultats très similaires à l’année dernière. Ils font suite à plusieurs années de stabilisation des bruits de fond des contaminations. Aujourd’hui, l’essentiel des points ne montre plus de tendance significative à l’augmentation ou à la diminution des niveaux de contamination (95% des points de suivi – annexe 4). Le bilan de la qualité sanitaire des zones conchylicoles dans le département d’Ille-et-Vilaine est donc assez stable Les estimations de la qualité sanitaire 2023 montrent une amélioration (B vers A) pour la zone 35.04 « Sud Cézembre » (groupe 2) et une dégradation (A vers B) pour la zone 35.08 « Stockage Cancale » (groupe 3), mais cette dernière est due à un pic de contamination relevé en 2021. En baie du Mont-Saint-Michel, l’ensemble des points continue de présenter un faible bruit de fond des contaminations, malgré sept résultats qui restent déclassants. En baie de Saint-Malo et en Rance, l’état sanitaire s’était considérablement dégradé au début des années 2010. Mais depuis 2015, l’essentiel des zones suivies connaît un regain de qualité et permet un classement généralisé en qualité moyenne. Cette qualité sanitaire se maintient en 2023 avec quelques éléments : positifs (passage en bonne qualité de « Sud Cézembre », zone « Pointe de Saint-Suliac » toujours très proche du A) et négatifs (retour des alertes en Rance, toujours des alertes sanitaires parfois importantes au niveau de Dinard en lien avec son assainissement). La stratégie du suivi REMI a évolué en 2023 avec le sous-découpage de zones au centre de la baie du Mont-Saint-Michel (zones 35.06.02, 35.06.03, 35.16 et 35.17) afin de circonscrire les secteurs les plus touchés. Ces résultats sont issus d’une étude sanitaire toujours en cours sur le groupe 3 pour la partie plus au large (zones 35.11 et 35.13). De nouveaux contaminants chimiques ont été pris en compte dans l’estimation de la qualité sanitaire des zones. Il s’agit des PFAS (composés perfluoroalkylés), sans qu’aucun résultat ROCCH n’ait présenté de dépassement de seuil sur le département

Environmental drivers modelling the mangrove Kinorhyncha community along an urban-to-natural gradient in French Guiana (western Atlantic Ocean)

Meiofaunal communities are dominated by free-living nematodes and harpacticoid copepods. Most of the studies defining the environmental drivers that influence meiofauna focus on these profuse groups. The ‘rare’ meiofaunal organisms (densities <5%) are frequently overlooked, making it difficult to know which abiotic variables influence their populations, or their ecological role in the benthic ecosystems. In this study, we explore the ecological factors that characterize a low human-polluted mangrove in French Guiana (western Atlantic Ocean) along an urban-to-natural gradient, focusing on the ‘rare’ phylum Kinorhyncha. This taxon seems to be mainly influenced by sediment size, being more abundant in silty sediments. The presence of microorganisms also enhances the community, likely due to a higher food availability. Oppositely, some trace metals of both lithogenic and anthropogenic origin, as well as predominance of coarser sediment particles, seem to negatively affect their populations. The importance of including the lesser abundant groups of meiofauna in ecological studies is evident, since it allows to have a better idea of the ecological state of marine benthic environments

Evidence to inform spatial management of a western Pacific Ocean tuna purse seine fishery

Fisheries can have profound impacts on co-occurring species exposed to incidental capture, particularly those with life history traits that make them vulnerable to elevated mortality levels. Fisheries spatial management holds substantial potential to balance socioeconomic benefits and costs to threatened bycatch species. This study analyzed observer program data for a western Pacific Ocean tuna purse seine fishery to estimate the effect of the spatial and temporal distribution of fishing on catch rates of target and at-risk species by fitting spatially-explicit generalised additive multilevel regression models within a Bayesian inference framework. Mean field prediction surfaces defined catch rate hotspots for principal market tunas, silky sharks, rays and whale sharks, informing the development of candidate area-based management strategies. Due to sample size limitations, odontocete and marine turtle catch geospatial patterns were summarized using 2D hexagonal binning of mean catch rates. Effort could be focused in two areas within core fishing grounds in the Solomon and Bismark Seas to reduce overlap with hotspots for silky sharks, rays and whale sharks without affecting target catch. Effort could also be shifted outside of core fishing grounds to zones with higher target tuna catch rates that would also reduce overlap with hotspots for at-risk species. However, two tuna warmspots overlapped silky and whale shark warmspots. Sparse and small marine turtle and whale shark hotspots occurred across the fishing grounds. Research on the economic and operational viability of alternative spatial management strategies is a priority. A small subset of sets had disproportionately large odontocete captures. Real time fleet communication and move-on rules and avoiding sets on dolphin schools might reduce odontocete catch rates. Management of informative operational predictors such as set association type and mesh size present additional opportunities to balance catch rates of at-risk and target species. A transition to employing output controls that effectively constrain the fishery would alter the spatial management strategy to focus on zones with the lowest ratio of at-risk bycatch to target tuna catch. Findings inform the design of alternative spatial management strategies to avoid catch rate hotspots of at-risk species without compromising the catch of principal market species

Deep seafloor hydrothermal vent communities buried by volcanic ash from the 2022 Hunga eruption

Mass mortality of marine animals due to volcanic ash deposition is present in the fossil record but has rarely been documented in real time. Here, using remotely-operated vehicle video footage and analysis of ash collected at the seafloor, we describe the devastating effect of the record-breaking 2022 Hunga submarine volcanic eruption on endangered and vulnerable snail and mussel species that previously thrived at nearby deep-sea hydrothermal vents. In contrast to grazing, scavenging, filter-feeding, and predatory vent taxa, we observed mass mortality, likely due to smothering during burial by thick ash deposits, of the foundation species, which rely on symbiotic chemosynthetic bacteria for the bulk of their nutrition. This is important for our broad understanding of the natural disturbance of marine ecosystems by volcanic eruptions and for predicting the effects of anthropogenic disturbance, like deep-sea mining, on these unique seafloor habitats

Dynamics of molybdenum and barium in the Bay of Brest (France) explained by phytoplankton community structure and aggregation events

Primary producers are essential organisms for marine ecosystems because they form the basis of food webs, produce half of atmospheric oxygen and are involved in various biogeochemical cycles. At the end of a bloom event, phytoplankton cells are known to produce organic compounds that act as a ‘cement’, allowing the cells to stick together and form large sinking structures called aggregates. These aggregates are microenvironments with chemical properties that are very different from the surrounding water. The main objective of this study was to determine how the temporal variations in cell assemblages over time and the formation of aggregates following a bloom affect the concentrations of molybdenum (Mo) and barium (Ba) in the water column, which are elements typically measured within accretionary hard tissues (e.g., mollusc shells) to track phytoplankton dynamics in the environment. To do so, we performed an environmental monitoring from March to October 2021 at Lanvéoc in the Bay of Brest (France) during which several biological (e.g., variations in phytoplankton assemblages) and chemical (e.g., chemical properties of the water column) parameters were measured once to twice per week. Our results show that spring and summer blooms of Gymnodinium, known to be enriched in Mo, could be one of the reasons explaining the particulate Mo enrichments in the water column. In addition, large phytoplankton aggregates transported a significant amount of Mo to the seafloor and associated suspension feeders. In contrast, the temporal variations in dissolved and particulate Ba concentration were strongly influenced by the formation of diatom blooms. Interestingly, there was a significant shift in Ba from the dissolved to the particulate fraction during the largest diatom bloom in late spring, associated with a significant Ba transport to the seafloor, which may be explained by the adsorption of this element onto diatom frustules. This study therefore highlights the impacts of phytoplankton on the dynamics of these elements in coastal ecosystems. Previous article in issu

DELMOGES. Description des activités de pêches, cartographie et typologie des stratégies opérant dans le golfe de Gascogne

Depuis les années 1990, la France connaît régulièrement des épisodes de mortalités importantes de dauphins, qui entraînent des pics d’échouages sur le littoral Atlantique en hiver. Depuis 2016, les échouages de petits cétacés dans le golfe de Gascogne présentant des traces de capture, atteignent des niveaux inédits. Si les données scientifiques actuelles permettent d’évaluer globalement le risque induit par ces captures accidentelles pour la conservation de la population de dauphins communs, elles sont toutefois trop lacunaires pour comprendre les déterminants écosystémiques et halieutiques à l’origine de ces captures. En concertation avec l’Office français de la biodiversité, les professionnels de la pêche et l’Etat, La Rochelle Université-CNRS et l’Institut français de recherche pour l’exploitation de la mer (Ifremer) ont construit le projet Delmoges (Delphinus Mouvements Gestion). Il vise, dans un premier temps, à combler ces lacunes en allant chercher des nouvelles données sur les habitats des dauphins, sur leurs interactions trophiques dans l’écosystème et leurs interactions techniques avec les engins de pêche. Ensuite, le projet propose d’intégrer les connaissances sur l’ensemble du socioécosystème pour envisager une diversité de scénarios de diminution des captures accidentelles incluant des solutions technologiques et, enfin, d’en évaluer les conséquences biologiques et socioéconomiques. Le livrable L311 est produit dans le contexte du « WP3 – Interactions spatio-temporelles et techniques dans les engins de pêche ». Il vise à proposer une description des activités de pêche et une typologie des flottilles opérant dans le golfe de Gascogne. La première partie présente un ensemble de chiffres et indicateurs clés de l’activité de pêche professionnelle observée dans le golfe de Gascogne en 2022 ainsi que les tendances observées sur la période 2012-2022. Ces éléments contextualisent l’analyse typologique présentée par la suite qui caractérise les stratégies (à l ‘échelle annuelle) et les tactiques (à l’échelle des marées) de pêches utilisés dans le Golfe de Gascogne entre 2000 et 2022. Ces ensembles correspondent à des groupements de navires/marées présentant des comportements de pêche (activités spatiales, temporelles, composition des prises, caractéristiques des engins et navires de pêches, volumes d’activités) homogènes. Ces stratégies/tactiques sont ensuite mises en relations avec les captures accidentelles, afin d’identifier les groupements les plus à risques au vu des données actuelles

Direct observation of North Atlantic nutrient transport and biological pump variability linked to the Meridional Overturning Circulation

The ocean biological carbon pump (BCP) plays a pivotal role in the global carbon cycle. The BCP magnitude is determined by the fraction of nutrients utilised in biological production and remineralised at depth, with the remainder being subducted into the interior unused as ‘preformed’ nutrients. This fraction is currently around 50% and subject to the interaction of biological processes and global scale circulation. Consequently, changes in circulation can potentially impact biological carbon storage. Here we provide observational evidence that the reduction in the Atlantic Meridional Overturning Circulation (AMOC) that occurred over the 2004-2018 period has been accompanied by substantial changes in nutrient transports and associated carbon storage. Persistent southward net nutrient transport across 26.5°N exceeded nutrient sources, except by the end of the period when the system approached balance. This transient net loss of nutrients from the North Atlantic was accompanied by increases in the ratio of remineralized to preformed nutrients, indicating an increasing BCP efficiency (and carbon storage). Our results thus demonstrate observable transient changes in large scale nutrient transports linked to AMOC changes over interannual - decadal timescales, with implications for future ocean carbon storage

Patchiness of plankton communities at fronts explained by Lagrangian history of upwelled water parcels

The transport of plankton by highly dynamic (sub)mesoscale currents–-often associated with fronts and eddies–-shapes the structure of plankton communities on the same time scales as biotic processes, such as growth and predation. The resulting bio-physical couplings generate heterogeneities in their finescale distributions (1-10 km), or "patchiness." Here, we test the hypothesis that cross-frontal plankton patchiness at a front found 200-250 km offshore in the California Current System was influenced by wind-driven upwelling conditions upstream of the front. We show that in situ Eulerian measurements (cross-frontal transects) can be interpreted in a Lagrangian framework by using satellite-derived current velocities to trace water parcels backward in time to their coastal origins. We find that the majority of the water parcels sampled at this front originated along the central California coast during different episodic wind-driven upwelling pulses and followed various trajectories before converging temporarily at the front. In response to nutrient injections at the coast, plankton communities transformed during their journeys from the coast to the sampling zone, with a succession of phytoplankton and zooplankton blooms. The cross-frontal sampling captured the convergence of these distinct water parcels at different points in their biological histories, which resulted in the observed spatial patchiness. Our results suggest that identifying the processes controlling frontal plankton communities requires understanding them in the context of their spatial and temporal histories, rather than as two-dimensional responses to local frontal processes. In particular, Lagrangian approaches should be more widely applied to understand critical ecological patterns in highly dynamic systems