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To date, two main vent faunal assemblages have been described on active sulfide edifices along the northern Mid-Atlantic Ridge (nMAR): one dominated by bathymodiolin mussels in low temperature areas and the other dominated by alvinocaridid shrimp in warmer habitats. In this study, we describe the ecology of new types of assemblage, dominated by gastropods, that are recurrent in several nMAR vent fields, from ~830 m to 3500 m depth. We assessed and compared the composition, abundance, diversity and trophic niche of these assemblages from three vent fields (Menez Gwen, Lucky Strike and Snake Pit) and characterized their habitats in terms of key environmental conditions. These assemblages, first seen during the Momarsat cruise in 2012 at the Lucky Strike vent field, were investigated during several subsequent cruises. They appear to be widespread along the nMAR, forming two distinct assemblages, one dominated by Lepetodrilus atlanticusat the shallowest vent field Menez Gwen, and the other by Peltospira smaragdina at the other investigated fields. Our data seem to indicate that these gastropods dominate an intermediate habitat at MAR vents and may play an important ecological role in these communities.

So far, the natural recovery of vent communities at large scales has only been evaluated at fast spreading centres, by monitoring faunal recolonisation after volcanic eruptions. However, at slow spreading ridges, opportunities to observe natural disturbances are rare, the overall hydrothermal system being more stable. In this study, we implemented a novel experimental approach by inducing a small-scale disturbance to assess the recovery potential of vent communities along the slow-spreading northern Mid-Atlantic Ridge (nMAR). We followed the recovery patterns of thirteen Bathymodiolus azoricus mussel assemblages colonising an active vent edifice at the Lucky Strike vent field, in relation to environmental conditions and assessed the role of biotic interactions in recolonisation dynamics. Within 2 years after the disturbance, almost all taxonomic richness had recovered, with the exception of a few low occurrence species. However, we observed only a partial recovery of faunal densities and a major change in faunal composition characterised by an increase in abundance of gastropod species, which are hypothesised to be the pioneer colonists of these habitats. Although not significant, our results suggest a potential role of mobile predators in early-colonisation stages. A model of post-disturbance succession for nMAR vent communities from habitat opening to climax assemblages is proposed, also highlighting numerous knowledge gaps. This type of experimental approach, combined with dispersal and connectivity analyses, will contribute to fully assess the resilience of active vent communities after a major disturbance, especially along slow spreading centres targeted for seafloor massive sulphide extraction.

The recovery of populations and their ability to recolonise a disturbed habitat is mainly dependent on their reproductive biology (e.g., fecundity, frequency of reproduction, and time to maturity) and recruitment success. To assess recolonisation processes and connectivity of vent communities, and infer their resilience to natural and anthropogenic disturbances, we studied the life-history traits of two dominant species of vent gastropods from the northern Mid-Atlantic Ridge: Protolira valvatoides and Pseudorimula midatlantica. Gonad morphology, gametogenesis, and reproductive outputs related to shell length were described using histological analyses, and population structure was assessed from individuals’ size–frequency distributions. Samples were collected at different locations of the Montségur and Eiffel Tower edifices (Lucky Strike vent field) in April 2015, July 2017, and August 2018 to inform on spatial and temporal variations of their reproductive outputs and demography. All stages of oocyte development were found in the gonads of both species, suggesting a continuous gametogenesis and asynchronous reproduction. However, the two species showed contrasting reproductive strategies. Indeed, while P. midatlantica is gonochoric with a fecundity of up to 327 mature oocytes, P. valvatoides is hermaphrodite with an extremely low fecundity including a maximum of eight vitellogenic oocytes. Maximum oocyte size was 176 µm for P. midatlantica and 272 µm for P. valvatoides. We infer from previous knowledge and our results that both species exhibit a lecithotrophic development of larvae. There was no evidence of temporal variability of reproductive traits, but environmental conditions seem to affect gametogenetic maturity and oocyte size of P. midatlantica limpets. Variations in population structure at the edifice scale suggest habitat selection of individuals related to biotic and abiotic factors.

Hydrothermal vent sites along the Juan de Fuca Ridge in the north-east Pacific host dense populations of Ridgeia piscesae tubeworms that promote habitat heterogeneity and local diversity. A detailed description of the biodiversity and community structure is needed to help understand the ecological processes that underlie the distribution and dynamics of deep-sea vent communities. Here, we assessed the composition, abundance, diversity and trophic structure of six tubeworm samples, corresponding to different successional stages, collected on the Grotto hydrothermal edifice (Main Endeavour Field, Juan de Fuca Ridge) at 2196 m depth. Including R. piscesae, a total of 36 macrofaunal taxa were identified to the species level. Although polychaetes made up the most diverse taxon, faunal densities were dominated by gastropods. Most tubeworm aggregations were numerically dominated by the gastropods Lepetodrilus fucensis and Depressigyra globulus and polychaete Amphisamytha carldarei. The highest diversities were found in tubeworm aggregations characterised by the longest tubes (18.5 ± 3.3 cm). The high biomass of grazers and high resource partitioning at a small scale illustrates the importance of the diversity of free-living microbial communities in the maintenance of food webs. Although symbiont-bearing invertebrates R. piscesae represented a large part of the total biomass, the low number of specialised predators on this potential food source suggests that its primary role lies in community structuring. Vent food webs did not appear to be organised through predator–prey relationships. For example, although trophic structure complexity increased with ecological successional stages, showing a higher number of predators in the last stages, the food web structure itself did not change across assemblages. We suggest that environmental gradients provided by the biogenic structure of tubeworm bushes generate a multitude of ecological niches and contribute to the partitioning of

Biodiversity conservation and response of benthic communities to pressures, related to global change and human activities, are a research priority. They also constitute a useful indicator to define the environmental status of the Marine Strategy Framework Directive as part of the European Water Framework Directive (WFD). The biogeographic features of the Aquitanian coast (in terms of rocky substrate communities) induce the interest to have a monitoring and research site. Indeed, it includes the habitat characteristics as well as fauna and flora (the Habitats Directive). The aim of the BIGORNO project is to implement new tools to assess the rocky biocenosis in the southern marine subregion of the Bay of Biscay by integrating fauna communities assessment (flora communities already monitored in WFD). This provides further information about the intertidal biodiversity and the spatial distribution of benthic communities. A standardized procedure for collecting information on benthic communities provides relevant indicators of intertidal habitat. Global environmental change (including climate evolution) should be considered while entering these indicators in European framework in order to compare results over longer periods. Using a spatially stratified random sampling plan, the mobile fauna (true abundance), fixed fauna and macroalgae (percentage cover) were measured in quadrats of 0.1 m² spread both on upper and lower mediolittoral zones. The analysis highlight dependent faunistic and floristic structuration according to microhabitats which confirms the need of a such stratification. Species/taxa or species associations were also identified as indicators of microhabitats. These various analysis improve knowledge for monitoring the Basque intertidal rocky shore in a sustainable way. Finally, within the Marine Strategy Framework Directive, these results allow to take into account biogeographic specificities to harmonize protocol metrics at the scale of the sub-area Bay of

To determine good ecological status and conservation of the Sub-Marine area of the Bay of Biscay, the implementation of a new rocky intertidal habitats monitoring is needed. A protocol has been adapted from the Brittany protocol for the water body FRFC11 "Basque coast" for the two indicators "intertidal macroalgae" and "subtidal macroalgae" under the Water Framework Directive to qualify the ecological. However no protocol has been validated for fauna in front of meridional characters of the benthic communities. Investigations carried out on macroalgae communities on intertidal area in WFD framework, since 2008, constitute an important working basis. This is the aim of the Bigorno project (Intertidal Biodiversity of the south of the Bay of Biscay and Observation for New search and Monitoring for decision support), financed by the Agency of Marine Protected Areas and the Departmental Council. To implement knowledge, a sampling protocol has been used in 2015 on the boulder fields of Guéthary. This site is part of Natura 2000 area "rocky Basque coast and offshore extension "It constitutes also a Znieff site and restricted fishing area. The sampling strategy considers the heterogeneity of substrates and the presence of intertidal microhabitats. Two main habitats are present: "mediolittoral rock in exposed area habitat" and "boulder fields". Habitat "intertidal pools and permanent ponds" is also present but, it is not investigated. Sampling effort is of 353 quadrats of 0.1 m², drawn randomly according to a spatially stratified sampling plan, defined by habitat and algal belts. Taxa identification and enumeration are done on each quadrat. The objective of this work is to expose results from data collected during 2015 sampling program. The importance of characterizing benthic fauna communities spatial distribution belonging to the Basque coast according to algal belts defines during the WDF survey was highlighted. Concurrently, indicators of biodiversity were studied.

In a context of several directives for transitional and coastal waters masses (Water Framework Directive, Marine Strategy Framework Directive and Bathing Water Directive), the good ecological status of water but also conservation of marine ecosystem should be quickly achieved. The Basque coastline (SE of the Bay of Biscay) is subject to strong human pressures with a high density of population over the year, and especially during the summer with tourist activities. It generates nutrient discharge, water and sediment pollution and impacts the quality of the marine environment. Such pressure and the way to reduce its impacts are the main concerns of authorities Locally, in order to improve wastewater treatment process, an alternative technical solution is applied for one year to reduce bacteriological pollution fluxes. This treatment is based on the injection of a biocidal compound (performic acid) into the outlet pipe before the discharge into the sea. The impact of this new treatment on the environment was considered from chemical and biological aspects on a sedentary benthic population of Mytilus spp settled above the sewage outfall. Samples were collected at different periods of the year between February and August on this location and on an undisturbed reference site located on the artificial intertidal shore. Chemical aspects were apprehended by metal concentrations in biota. Biological aspects were based on physiological status and biometrics indicators. It enables us to look for the phenotypic variations of morphological patterns of the mussel shells in relation to environmental changes. It was completed by an ecological characterisation of the present benthic communities. The communication proposes a detailed description of the method and a presentation of first results which actually does not show any modification of the biological indicators in relation to the new process.