Hétérogénéité sédimentaire et micro-habitats benthiques : approches in situ et expérimentale

The eastern Bay of Seine is characterized by complex sediment structures with high spatial and vertical heterogeneities. Its variability involves complexes animal-sediment relationships still unclear. A new approach based on the individual scale combines in situ sampling with experiments. The macrofauna vertical distribution and quantification of biogenics structures have shown the main concentration of the fauna in the first 10 cm depth, such as 98 % of the total abundance. Two abundance pics have been observed, the first close to the surface between 0 and 2 cm depth and the other one between 5 to 7 cm depth, mainly due to two species, Kurtiella bidentata and Chaetozone spp., which represent almost 80 % of the abundance from 5 to 7cm. Their presence within this depth is still unknown. Experiments have shown the great sediment reworking skill of the worm Lagis koreni and its ability to destroy the sediment structure, which can be linked to the in situ samples. This preliminary work has shown the interest of the techniques used and makes us going further.La sédimentologie de l’estuaire de la baie de Seine orientale se caractérise par une complexité de structures sédimentaires favorisant les hétérogénéités spatiales et verticales. Ce système fortement variable implique des relations macrofaune benthique - sédiments complexes qui n’ont cependant pas été précisément définies. Une nouvelle approche pour l’étude de ces relations a combiné prélèvements in situ et expérimentations en mésocosmes à l’échelle réduite de l’individu. La dimension tridimensionnelle du sédiment a été prise en compte afin de caractériser la distribution verticale de la macrofaune en présence de sédiments hétérométriques. Couplées à des analyses de volumes de vie par CAT-Scan, 98 % de la macrofaune étudiée se situe dans les dix premiers centimètres de sédiment avec la présence de deux pics d’abondance, en surface entre 0 et 2 cm de profondeur et entre 5 et 7 cm de profondeur. Cette forte abondance entre 5 et 7 cm concerne principalement deux espèces, le bivalve Kurtiella bidentata et les polychètes Chaetozone spp. représentant 80 % de l’abondance à cette profondeur. Cette forte abondance entre 5 et 7 cm n’a toutefois pas pu être expliquée. L’approche expérimentale a mis en évidence le potentiel de remaniement sédimentaire du polychète Lagis koreni et sa grande capacité à déstructurer une colonne sédimentaire, pouvant être reliée aux observations des prélèvements in situ. Ce travail préliminaire a montré le potentiel des méthodologies employées et les résultats suggèrent l’approfondissement de la méthodologie

Impact à court terme de la pêche à la fourche à cailloux sur la communauté benthique à Venus verrucosa

International audienceRecreational clam digging is a traditional activity on the large intertidal zone of the western coasts of the Cotentin (western English Channel). A variety of fishing gears are used to harvest the target species the warty venus Venus verrucosa (Linnaeus, 1758). In this note, the immediate effect (i.e., four days) of fork harvesting was studied during the March 2012 spring tide, following a control-impact design with a control station and three impacted stations using pebble forks. An immediate significant decreases of coarse sand and gravel benthic macrofauna is observed in fishing area. In the future, it is recommended that pebble fork fishing should be prohibited to harvest this target species.La pêche à pied est une activité récréative traditionnelle sur les larges estrans découverts lors des grandes marées de la côte ouest du Cotentin (bassin occidentale de la Manche). Une très grande diversité d'engins de pêches est utilisée pour pêcher la praire Venus verrucosa (Linnaeus, 1758). Dans cette note, les effets à court-terme de la pêche à la fourche à cailloux ont été étudiés lors de la marée d'équinoxe du mois de mars 2012 selon un protocole de comparaison entre stations impactées et une station de contrôle non impactée par la pêche à la fourche. Une décroissance significative de la macrofaune benthique de ces fonds sablo-graveleux est enregistrée en quatre jours dans les zones pêchées à la fourche. Il est proposé que cette technique de pêche destructive soit supprimée au profit d'outils de pêche moins pénalisant pour la faune benthique

Does the hydrodynamic, morphometric and sedimentary environment explain the structure of soft-bottom benthic assemblages in the Eastern Bay of Seine (English Channel)?

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Coupling high frequency monitoring and bioassay experiments to investigate a harmful algal bloom in the Bay of Seine (French-English Channel)

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Marine artificial reefs, a meta-analysis of their design, objectives and effectiveness

International audienceArtificial Reefs (AR) show a wide diversity and vary in their construction materials, shape and purpose, as illustrated by the present analysis of 127 scientific papers. AR have been deployed for different purposes, including fisheries improvement, ecological restoration of marine habitats, coastal protection or purely scientific research. Statistical analyses using 67 variables allow us to characterize the design, objectives and monitoring strategies used for AR. An effectiveness indicator comprised of three categories (low, moderate and high) was adapted from previous studies and applied to the present dataset in terms of the objectives defined in each scientific paper. The effectiveness of various monitoring approaches was investigated and recommendations were formulated regarding environmental parameters and the assessment of ecological processes as a function of AR type. These analyses showed that inert materials like concrete associated with biomimetic designs increase the benefits of reefs to the local environment. This study also compared effectiveness between the different economic, ecological or scientific objectives of AR projects and reveals that fisheries projects showed the highest efficiencies but points out the weakness of environmental assessments for this type of project. In conclusion, the analyses presented here highlight the need to use a panel of complementary monitoring techniques, independently of the initial purpose of the artificial structures, to properly assess the impact of such structures on the local environment. It is recommended to adopt approaches that associate structural and functional ecology. An improved characterization of the role of AR should be integrated into future assessments, taking into account the complex framework of ecosystem structure and trophic relationships

Influence of nutrient enrichment on colonisation and photosynthetic parameters of hard substrate marine microphytobenthos

International audienceThis study aimed to assess the influence of nutrient enrichment on the development of microalgal biofilm on concrete and PVC cubes. Three mesocosms were utilized to create a nutrient gradient over a period of 28 days. Various parameters including biomass, photosynthetic activity, microtopography, and extracellular polymeric substances (EPS) were measured. Imaging PAM techniques were employed to obtain surface-wide data. Results revealed that nutrient availability had no significant impact on Chl a biomass and the maximum quantum efficiency of PSII (F_v/F_m). The photosynthetic capacity and efficiency were minimally affected by nutrient availability. Interestingly, the relationship between microphytobenthic (MPB) biomass and photosynthesis and surface rugosity exhibited distinct patterns. Negative reliefs showed a strong correlation with F_v/F_m, while no clear pattern emerged for biomass on rough concrete structures. Overall, our findings demonstrate that under conditions of heightened eutrophication, biofilm photosynthesis thrives in the fissures and crevasses of colonized structures regardless of nutrient levels. This investigation provides valuable insights into the interplay between nutrient availability and surface rugosity

Colonisation of artificial structures by primary producers: competition and photosynthetic behaviour

International audienceColonisation of artificial structures by primary producers is an important determinant for eco-engineering projects. In this context, interactions between the colonisation by microphytobenthic biofilm and macroalgae were explored on 48 samples of marine infrastructures (MI) immersed for one year in the English Channel. Marine infrastructures samples with smooth and rough surface were compared to evaluate the influence of surface micro-scale rugosity. Microphytobenthos biomass (MPB), macroalgal diversity and photosynthetic parameters of both were assessed during colonisation. No significant differences were found as a function of the surface rugosity of MI samples, which was unexpected, but can be explained by biogenic rugosity provided by barnacles. Marine infrastructures were largely colonised by a red encrusting alga, Phymatolithon purpureum, which showed poor photosynthetic capacity compared to the microphytobenthos present next to it. Colonisation by monospecific encrusting algae tended to reduce the primary productivity of hard substrate

Mapping benthic communities: An indispensable tool for the preservation and management of the eco-socio-system in the Bay of Seine

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