Lucky Strike Vent Field Research Articles

Diffuse Venting and Near Seafloor Hydrothermal Circulation at the Lucky Strike Vent Field, Mid‐Atlantic Ridge

AbstractWe report on a 3 years monitoring experiment of low to medium temperature diffuse venting at two vent sites (Tour Eiffel and White Castle) of the Lucky Strike, black smoker‐type hydrothermal field, Mid‐Atlantic Ridge. Diffuse vents account for a large part of the energy flux of mid‐ocean ridges hydrothermal fields and provide key habitats for the hydrothermal fauna. We document the time and space variability of diffuse venting temperature and chemistry, describe the effect of tidal loading and currents and discuss the extent of mixing, cooling of black smoker fluids, heating of entrained seawater and anhydrite precipitation/dissolution in the substratum. We emphasize the role of a thin (<2 m) volcaniclastic formation capping the brecciated basalt substratum. This formation is porous, but becomes impermeable when indurated by hydrothermal precipitates. It forms an intermediate layer between the vents at the seabed and the fluids as they discharge out of the brecciated basalts. Diffuse fluids inferred to discharge out of meter‐spaced cracks in the brecciated basalts beneath this volcaniclastic layer are hot (>80°C) and contain >10% of the hot endmember fluid component, over distances of up to 25 m from the black smokers. These results provide a geologically integrated framework in which to study site‐scale, near seafloor hydrothermal circulation and associated vent habitats at Lucky Strike and other black smoker‐type hydrothermal fields. They suggest diffuse heat fluxes in the upper range of previously published estimates at the two studied Lucky Strike hydrothermal vent sites.

Convolutional neural networks for hydrothermal vents substratum classification: An introspective study

The increasing availability of seabed images has created new opportunities and challenges for monitoring and better understanding the spatial distribution of fauna and substrata. To date, however, deep-sea substratum classification relies mostly on visual interpretation, which is costly, time-consuming, and prone to human bias or error. Motivated by the success of convolutional neural networks in learning semantically rich representations directly from images, this work investigates the application of state-of-the-art network architectures, originally employed in the classification of non-seabed images, for the task of hydrothermal vent substrata image classification. In assessing their potential, we conduct a study on the generalization, complementarity and human interpretability aspects of those architectures. Specifically, we independently trained deep learning models with the selected architectures using images obtained from three distinct sites within the Lucky-Strike vent field and assessed the models' performances on-site as well as off-site. To investigate complementarity, we evaluated a classification decision committee (CDC) built as an ensemble of networks in which individual predictions were fused through a majority voting scheme. The experimental results demonstrated the suitability of the deep learning models for deep-sea substratum classification, attaining accuracies reaching up to 80% in terms of F1-score. Finally, by further investigating the classification uncertainty computed from the set of individual predictions of the CDC, we describe a semiautomatic framework for human annotation, which prescribes visual inspection of only the images with high uncertainty. Overall, the results demonstrated that high accuracy values of over 90% F1-score can be obtained with the framework, with a small amount of human intervention.

Fluid chemistry alters faunal trophodynamics but not composition on the deep-sea Capelinhos hydrothermal edifice (Lucky Strike vent field, Mid-Atlantic Ridge)

The recently discovered deep-sea Capelinhos hydrothermal edifice, ~ 1.5 km of the main Lucky Strike (LS) vent field (northern Mid-Atlantic Ridge), contrasts with the other LS edifices in having poorly-altered end-member hydrothermal fluids with low pH and chlorine, and high metal concentrations. Capelinhos unique chemistry and location offer the opportunity to test the effects of local abiotic filters on faunal community structure while avoiding the often-correlated influence of dispersal limitation and depth. In this paper, we characterize for the first time the distribution patterns of the Capelinhos faunal communities, and analyze the benthic invertebrates (> 250 µm) inhabiting diffusive-flow areas and their trophic structures (δ13C, δ15N and δ34S). We hypothesized that faunal communities would differ from those of the nearest LS vent edifices, showing an impoverished species subset due to the potential toxicity of the chemical environment. Conversely, our results show that: (1) community distribution resembles that of other LS edifices, with assemblages visually dominated by shrimps (close to high-temperature focused-fluid areas) and mussels (at low-temperature diffuse flow areas); (2) most species from diffuse flow areas are well-known LS inhabitants, including the bed-forming and chemosymbiotic mussel Bathymodiolus azoricus and (3) communities are as diverse as those of the most diverse LS edifices. On the contrary, stable isotopes suggest different trophodynamics at Capelinhos. The high δ15N and, especially, δ13C and δ34S values suggest an important role of methane oxidation (i.e., methanotrophy), rather than the sulfide oxidation (i.e., thiotrophy) that predominates at most LS edifices. Our results indicate that Capelinhos shows unique environmental conditions, trophic structure and trophodynamics, yet similar fauna, compared to other LS edifices, which suggest a great environmental and trophic plasticity of the vent faunal communities at the LS.

Monitoring ecological dynamics on complex hydrothermal structures: A novel photogrammetry approach reveals fine‐scale variability of vent assemblages

AbstractWe set out to characterize the fine‐scale processes acting on interannual dynamics of deep‐sea vent fauna by using a novel approach involving a 5‐yr time series of 3D photogrammetry models acquired at the Eiffel Tower sulfide edifice (Lucky Strike vent field, Mid‐Atlantic Ridge). Consistently, with the overall stability of the vent edifice, total mussel cover did not undergo drastic changes, suggesting that they have been at a climax stage for at least 25 yr based on previous data. Successional patterns showed consistency over time, illustrating the dynamic equilibrium of the ecological system. In contrast, microbial mats significantly declined, possibly due to magmatic events. The remaining environmental variability consisted of decimeter‐scale displacement of vent outflows, resulting from their opening or closure or from the progressive accretion of sulfide material. As a result, vent mussels showed submeter variability in the immediate vicinity of vent exits, possibly by repositioning in response to that fine‐scale regime of change. As former studies were not able to quantify processes at submeter scales in complex settings, this pioneering work demonstrates the potential of 3D photogrammetry models for conducting long‐term monitoring in the deep sea. We observed that the ability of mussels to displace may enable them to cope with changing local conditions in a stable system. However, the long‐term stability of mussel assemblages questions their capacity to withstand large‐scale disturbances and may imply a low resilience of these “climax” communities. This suggests that they may be particularly vulnerable to the negative effects of mining activities in hydrothermal ecosystems.

The soundscape of two deep-sea hydrothermal vent sites

Hydrothermal vents are sea floor structures where geothermally heated seawater is discharged. The high-temperature, chemically rich environment is host to uniquely adapted marine life. Vent soundscapes may contain important bioacoustic cues as well as signals enabling passive acoustic monitoring of hydrothermal vent dynamics. Proposals for deep-sea mining of seafloor massive sulfides near hydrothermal vents have elicited concern over potential environmental impacts due to disturbance from industrial activity, including changes to the soundscape. This study assesses the baseline soundscape at two sites, the Main Endeavour Field on the Juan de Fuca Ridge and the Lucky Strike vent field on the Mid-Atlantic Ridge over 12 months and 3 months respectively. To facilitate comparison with future studies at other sites, the most recently proposed standard soundscape analysis methodologies are employed, including terminology in alignment with ISO 18405:2017. In accordance with the latest soundscape standard literature, metrics quantifying the amplitude, impulsiveness, periodicity, and uniformity are reported. Spectral probability densities, percentiles, and long-term spectrograms are computed in hybrid millidecade frequency bands. Finally, a qualitative analysis is included to describe the source types contributing to the hydrothermal vent sound field.

Integrated Study of New Faunal Assemblages Dominated by Gastropods at Three Vent Fields Along the Mid-Atlantic Ridge: Diversity, Structure, Composition and Trophic Interactions

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.

Age and Rate of Accumulation of Metal‐Rich Hydrothermal Deposits on the Seafloor: The Lucky Strike Vent Field, Mid‐Atlantic Ridge

AbstractHydrothermal venting at the Lucky Strike hydrothermal field, located on the Mid‐Atlantic Ridge, is associated with faulting linked to the tectonic dismemberment of a central axial volcano. Radium‐226/Ba dating of hydrothermal barite indicates that hydrothermal venting is at least 6,600 years old, and that Lucky Strike is one of the youngest known vent fields on the Mid‐Atlantic Ridge, which typically have ages exceeding 20 ka. Deposit volume calculations indicate that the total accumulated mass of the hydrothermal deposits on the seafloor at Lucky Strike is ∼1.3 ± 0.2 Mt, and that this mass accumulated at a maximum average rate of 194 ± 28 t/yr. This accumulation rate is comparable to other well characterized mid‐ocean ridge hydrothermal sites, such as TAG and Endeavor, but at Lucky Strike is concentrated within a relatively small area of <2.5 km2.

Effects of Substrate Composition and Subsurface Fluid Pathways on the Geochemistry of Seafloor Hydrothermal Deposits at the Lucky Strike Vent Field, Mid‐Atlantic Ridge

AbstractThe Lucky Strike vent field, located on the Mid‐Atlantic Ridge (MAR), is hosted on enriched mid‐ocean ridge basalt associated with the nearby Azores hotspot. In this study, we present bulk rock geochemistry coupled with in situ sulfur isotope analysis of hydrothermal samples from Lucky Strike. We assess the geological controls on the differences in the major and trace element content and sulfur isotopic composition of the hydrothermal deposits within the vent field. The hydrothermal deposits contain elevated concentrations of elements typically enriched in mid‐ocean basalt (E‐MORB), such as Mo, Ba, and Sr, compared to typical values for other hydrothermal deposits hosted on the MAR. The range in sulfur isotope compositions of hydrothermal marcasite and chalcopyrite (−2.5 to 8.7‰) is similar to the range recorded at other sediment‐free basalt‐hosted seafloor hydrothermal sites. However, at Lucky Strike, the Capelinhos vent, situated 1.4 km east of the main field, is enriched in 34S (by ∼3.5‰ for both marcasite and chalcopyrite), relative to the main field. This difference reflects contrasting subseafloor fluid/rock interactions at these two sites, including subseafloor sulfide precipitation at the main field that results in <20% of reduced sulfur within the upwelling hydrothermal fluid reaching the seafloor. We also compare the geochemistry of the hydrothermal deposits at Lucky Strike to other hydrothermal sites along the MAR and show that the average hydrothermal deposit Ba/Co is useful to discriminate between E‐MORB and other mafic/ultramafic hosted deposits.

Long-term monitoring reveals unprecedented stability of a vent mussel assemblage on the Mid-Atlantic Ridge

Understanding scales and drivers of ecological variability is essential to a full understanding of ecosystem functioning. At remote deep-sea hydrothermal vents, infra-annual dynamics remain poorly described. This study aims to characterise the factors that drive the dynamics of a vent faunal assemblage dominated by Bathymodiolus azoricus mussels from infra-daily to monthly time steps. We analysed a 7-year time series of images and environmental data collected at a depth of 1695 m at the base of the active Eiffel Tower edifice in the Lucky Strike vent field (Mid-Atlantic Ridge). Using images acquired by the TEMPO ecological module connected to the EMSO-Azores observatory, we assessed the dynamics of key species inhabiting the faunal assemblage in relation to changes in environmental conditions monitored daily.Our results show that habitat conditions were generally stable over the 7-year period, with small-scale variability related to tidal periodicity and local temperature anomalies. Likewise, the mussel and zoanthid assemblages exhibited remarkable stability. Changes in fluid exposure and substratum instability induced decimetre-scale movements of the mussel assemblage. Microbial mats displayed infra-annual changes characterised by aperiodic growth and decline. Their development patterns could not be entirely attributed to environmental conditions, because other factors, including biotic interactions, appeared to be involved. The crab population preferentially occupied the mussel habitat, but no predation was observed. Scales of variation and driving factors were compared with those governing intertidal zones. The outcomes question the assumption that vent fauna experience extreme and highly variable conditions. On the MAR, mussel assemblages appear to experience relatively stable and mild environmental conditions compared with their coastal counterparts.

Recovery of hydrothermal vent communities in response to an induced disturbance at the Lucky Strike vent field (Mid-Atlantic Ridge)

So far, the natural recovery of vent communities at large scales has only been evaluated at fast spreading centers, 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 centers targeted for seafloor massive sulphide extraction.

High environmental stress and productivity increase functional diversity along a deep-sea hydrothermal vent gradient.

Productivity and environmental stress are major drivers of multiple biodiversity facets and faunal community structure. Little is known on their interacting effects on early community assembly processes in the deep sea (>200m), the largest environment on Earth. However, at hydrothermal vents productivity correlates, at least partially, with environmental stress. Here, we studied the colonization of rock substrata deployed along a deep-sea hydrothermal vent gradient at four sites with and without direct influence of vent fluids at 1,700-m depth in the Lucky Strike vent field (Mid-Atlantic Ridge [MAR]). We examined in detail the composition of faunal communities (>20μm) established after 2 yr and evaluated species and functional patterns. We expected the stressful hydrothermal activity to (1) limit functional diversity and (2) filter for traits clustering functionally similar species. However, our observations did not support our hypotheses. On the contrary, our results show that hydrothermal activity enhanced functional diversity. Moreover, despite high species diversity, environmental conditions at surrounding sites appear to filter for specific traits, thereby reducing functional richness. In fact, diversity in ecological functions may relax the effect of competition, allowing several species to coexist in high densities in the reduced space of the highly productive vent habitats under direct fluid emissions. We suggest that the high productivity at fluid-influenced sites supports higher functional diversity and traits that are more energetically expensive. The presence of exclusive species and functional entities led to a high turnover between surrounding sites. As a result, some of these sites contributed more than expected to the total species and functional β diversities. The observed faunal overlap and energy links (exported productivity) suggest that rather than operating as separate entities, habitats with and without influence of hydrothermal fluids may be considered as interconnected entities. Low functional richness and environmental filtering suggest that surrounding areas, with their very heterogeneous species and functional assemblages, may be especially vulnerable to environmental changes related to natural and anthropogenic impacts, including deep-sea mining.

Contrasting reproductive biology of two hydrothermal gastropods from the Mid-Atlantic Ridge: implications for resilience of vent communities

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.

Exploring Environmental DNA (eDNA) to Assess Biodiversity of Hard Substratum Faunal Communities on the Lucky Strike Vent Field (Mid-Atlantic Ridge) and Investigate Recolonization Dynamics After an Induced Disturbance

Deep hydrothermal vent ecosystems face physical disturbances from naturally occurring volcanic and tectonic activities and are at increasing risk of mineral resource exploitation, raising concerns about the resilience of biological communities. Following destructive events, efficient and rapidly applicable surveys of organisms are required to monitor the state, evolution and recovery of these ecosystems to their baseline status. In this study, we explored the eDNA approach as a tool to assess biodiversity of communities associated with deep-sea hard substrata and tracked the recolonization dynamics of benthic communities living on the Montsegur edifice within the Lucky Strike vent field (Mid-Atlantic Ridge), after an induced disturbance. Hard substratum samples were collected prior to and one year after the disturbance, for environmental DNA (eDNA) metabarcoding using one marker of the mitochondrial Cytochrome Oxidase I (COI) gene and three markers of the nuclear 18S ribosomal RNA (18S) gene. We also generated a DNA barcoding inventory that consisted of taxa physically collected and morphologically identified. This inventory contained amplified barcodes from COI, 18S and the nuclear ribosomal RNA 28S gene. The resulting sequence information from the COI and 18S were used for eDNA taxonomic assignment. The eDNA datasets uncovered a high diversity of metazoan OTUs. Baseline data collected at the start of the experiment identified higher OTU richness at sites peripheral to the active edifice, as well as inactive sites. One year following the initial disturbance, analysis of recolonization data found no statistical difference in presence/absence from baseline communities. The eDNA protocols provide a reproducible strategy to quickly assess biodiversity associated with deep sea hard substratum, enabling comparisons across various habitats. To follow recolonization dynamics at small spatial scales however, we recommend an approach that uses both molecular and morphological-based traditional methods. Finally, we present original data on the “unseen” diversity of the fauna inhabiting the poorly studied inactive sites, locations that are targeted by commercial mining. Continued monitoring of these sites is currently ongoing and will bring new insight on recovery potential over time, with the ultimate goal of informing conservation and management decisions in relation to the protection of hydrothermal vent ecosystems.

Ecology and trophic role of Oncholaimus dyvae sp. nov. (Nematoda: Oncholaimidae) from the lucky strike hydrothermal vent field (Mid-Atlantic Ridge)

BackgroundNematodes are an important component of deep-sea hydrothermal vent communities, but only few nematode species are able to cope to the harsh conditions of the most active vent sites. The genus Oncholaimus is known to tolerate extreme geothermal conditions and high sulphide concentrations in shallow water hydrothermal vents, but it was only occasionally reported in deep-sea vents. In this study, we performed morphological, genetic and ecological investigations (including feeding strategies) on an abundant species of Oncholaimus recently discovered at Lucky strike vent field on the Mid-Atlantic Ridge at 1700 m water depth.ResultsWe described this species as Oncholaimus dyvae sp. nov.. This new species differs from all other members of the genus by the combination of the following characters: body length (up to 9 mm), the presence of a long spicule (79 μm) with a distally pointed end, a complex pericloacal setal ornamentation with one precloacal papilla surrounded by short spines, and a body cuticule with very fine striation shortly posterior to the amphid opening. Overall, O. dyvae sp. nov. abundance increased with increasing temperature and vent emissions. Carbon isotopic ratios suggest that this species could consume both thiotroph and methanotrophic producers. Furthermore sulfur-oxidizing bacteria related to Epsilonproteobacteria and Gammaproteobacteria were detected in the cuticle, in the digestive cavity and in the intestine of O. dyvae sp. nov. suggesting a potential symbiotic association.ConclusionsThis study improves our understanding of vent biology and ecology, revealing a new nematode species able to adapt and be very abundant in active vent areas due to their association with chemosynthetic micro-organisms. Faced by the rapid increase of anthropogenic pressure to access mineral resources in the deep sea, hydrothermal vents are particularly susceptible to be impacted by exploitation of seafloor massive sulfide deposits. It is necessary to document and understand vent species able to flourish in these peculiar ecosystems.

Prokaryote Communities at Active Chimney andIn SituColonization Devices After a Magmatic Degassing Event (37°N MAR, EMSO‐Azores Deep‐Sea Observatory)

AbstractThe mixing zone between high‐temperature hydrothermal fluids and seawater produces redox gradients, promoting the development of unique ecosystems based on chemotrophy. The structure of microbial communities depends on their environment, which can vary according to space and time. Hydrothermal circulation within the oceanic crust determines the chemical composition and flow of fluids, depending on underground events (earthquakes, volcanic episodes, etc.) and impacts the development of microbial communities. This link between hydrothermal vent communities and deep geological events is the focus of the present study, the first of its kind for slow‐spreading ridge. In this study, we present a unique set of multidisciplinary data collected from 2008 to 2011 on the Eiffel Tower hydrothermal site (Lucky Strike vent field, Mid‐Atlantic Ridge, MAR). We benefit from continuous geophysical monitoring (temperature and seismicity) of the site, annual sampling of hydrothermal fluids (hot and diffuse) for geochemistry analyses, sampling of hydrothermal chimneys, and an in situ microbial colonization experiment over a year for microbial study. The high CO2content and concentrations of major elements (Cl, Ca, and Si) and SO4in the end‐member fluids collected in 2010 indicate that a magmatic degassing occurred between 2009 and 2010 under the Lucky Strike hydrothermal field. This is supported by the large temperature variations observed in March–April 2010. These magmatic CO2inputs seem to have affected microbial communities colonizing the high‐temperature chimney, as well as the basalts in the more diffuse and mixed zone, promoting the development of thermophilic/anaerobicArchaeaandBacteria(Archaeoglobales, Nautiliales, and Nitratiruptoraceae).

Modelling the interactions of the hydrothermal mussel Bathymodiolus azoricus with vent fluid

In the 40 years since the discovery of the rich faunal community around hydrothermal vents, many studies have clearly shown that environmental conditions have a strong influence on species distribution in these habitats. Nevertheless, the mechanisms that determine the spatial and temporal dynamics of species’ responses to vent conditions remain elusive. Metabolic studies to assess faunal interactions with vent fluid are particularly difficult to perform in the deep sea and are generally executed in isolation ex situ. Available data mainly concern foundation species, which visually dominate these ecosystems. This work uses a modelling approach to integrate biotic and abiotic data that have been acquired through the years on Eiffel Tower, a large sulphide edifice located on the Lucky Strike vent field on the Mid-Atlantic Ridge, and particularly on its dominant species, Bathymodiolus azoricus. A carbon-flux model was built using seven state variables: the biomass of mussels and their associated thiotrophic (SOX) and methanotrophic (MOX) symbionts and the ambient concentrations of oxygen, dihydrogen sulphide, methane and (particulate and dissolved) organic carbon. Temperature of the surrounding water and mussel density were the forcing variables in the system. Results showed no statistically significant differences between predicted and observed mussel biomass and estimates of energy partitioning within the mussel were in the range of available data.Metabolic rates were generally rather low and greatly reduced by a temperature effect in the coldest samples. These low metabolic rates imply a long lifespan for B. azoricus. Simulations suggest that they would strongly hinder re-establishment and resilience of mussel biomass. However, because symbionts respond quickly to changes in vent fluid, mussels would be able to buffer strong variations in the hydrothermal fluid supply. The model showed that if mussels fed indifferently on both types of symbionts, coexistence of MOX and SOX cannot be reached, thereby likely favouring hypotheses of competition for space inside the mussel gills and/or a differential use of the production of each symbiont. Model predictions are highly dependent on current knowledge, and the results presented here highlight the need for more quantitative data on the biology of B. azoricus across different size classes, on its interactions with symbionts, and in varying environmental concentrations in its substrates.

Colonization of synthetic sponges at the deep-sea Lucky Strike hydrothermal vent field (Mid-Atlantic Ridge): a first insight

The main objective of the present study was to investigate invertebrate colonization processes at deep-sea hydrothermal vents in response to environmental factors and to the presence of complex artificial substrata (i.e., synthetic sponges). We set out a pilot experiment at 1700 m depth on the Lucky Strike vent field (Eiffel Tower, Mid-Atlantic Ridge). Synthetic sponges were deployed in 2011 at five sites along a gradient of hydrothermal activity and were recovered in 2013, and the composition of macro- and meiofauna was assessed on four of them. The influence of temperature and fluid inputs on colonizer faunal abundance and diversity was analyzed. Faunal abundance and diversity decreased with increasing distance from vent emission. The colonizers were represented by a subset of species characterizing the natural populations at the Eiffel Tower edifice. Some taxa (e.g., pycnogonids, ophiuroids, cnidarians, foraminiferans) represented new records not yet found on deployed substrata on the Eiffel Tower. Synthetic sponges harbored a high percentage (from 17.5% to 55%) of juveniles and larval stages of polychaetes, molluscs, and copepods. A mature nematode community (mainly Cephalochaetosoma and Halomonhystera) in a reproductive stage was found. Variability in faunal composition was significantly correlated with distance from fluid emission. We hypothesize that the complex structure of inorganic sponge substrata may have favored settlement of juveniles and larvae. Sponge substrata may, therefore, help sample a wider range of organisms than other substrata, and, thereby, provide a more complete picture of vent biodiversity. The results provided in this study might improve our understanding of mechanisms that govern faunal colonization processes at vents.

Biological and environmental rhythms in (dark) deep-sea hydrothermal ecosystems

Abstract. During 2011, two deep-sea observatories focusing on hydrothermal vent ecology were up and running in the Atlantic (Eiffel Tower, Lucky Strike vent field) and the Northeast Pacific Ocean (NEP) (Grotto, Main Endeavour Field). Both ecological modules recorded imagery and environmental variables jointly for a time span of 23 days (7–30 October 2011) and environmental variables for up to 9 months (October 2011–June 2012). Community dynamics were assessed based on imagery analysis and rhythms in temporal variation for both fauna and environment were revealed. Tidal rhythms were found to be at play in the two settings and were most visible in temperature and tubeworm appearances (at NEP). A ∼ 6 h lag in tidal rhythm occurrence was observed between Pacific and Atlantic hydrothermal vents, which corresponds to the geographical distance and time delay between the two sites.

Copepod colonization of organic and inorganic substrata at a deep-sea hydrothermal vent site on the Mid-Atlantic Ridge

The few existing studies on deep-sea hydrothermal vent copepods indicate low connectivity with surrounding environments and reveal high endemism among vents. However, the finding of non-endemic copepod species in association with engineer species at different reduced ecosystems poses questions about the dispersal of copepods and the colonization of hydrothermal vents as well as their ecological connectivity.The objective of this study is to understand copepod colonization patterns at a hydrothermal vent site in response to environmental factors such as temperature and fluid flow as well as the presence of different types of substrata. To address this objective, an in situ experiment was deployed using both organic (woods, pig bones) and inorganic (slates) substrata along a gradient of hydrothermal activity at the Lucky Strike vent field (Eiffel Tower, Mid-Atlantic Ridge). The substrata were deployed in 2011 during the MoMARSAT cruise and were recovered after two years in 2013.Overall, copepod density showed significant differences between substrata types, but was similar among different hydrothermal activity regimes. Highest densities were observed on woods at sites with moderate or low fluid input, whereas bones were the most densely colonized substrata at the 2 sites with higher hydrothermal influence. Although differences in copepod diversity were not significant, the observed trends revealed overall increasing diversity with decreasing temperature and fluid input. Slates showed highest diversity compared to the organic substrata. Temperature and fluid input had a significant influence on copepod community composition, resulting in higher similarity among stations with relatively high and low fluid inputs, respectively. While vent-specialists such as dirivultids and the tegastid Smacigastes micheli dominated substrata at high vent activity, the experiment demonstrated increasing abundance and dominance of non-vent taxa with decreasing temperature and fluid input. Effects of the substratum type on community composition were not significant, although at sites with moderate or low fluid input, woods exhibited distinctive communities with high densities and relative abundance of the taxon Nitocrella sp. In conclusion, copepod colonization and species composition were mainly influenced by hydrothermal fluid input and temperature rather than the type of substratum. The outcome of this study provides fundamental knowledge to better understand copepod colonization at hydrothermal vents.

Picturing thermal niches and biomass of hydrothermal vent species

In community ecology, niche analysis is a classic tool for investigating species׳ distribution and dynamics. Components of a species׳ niche include biotic and abiotic factors. In the hydrothermal vent ecosystem, although composition and temporal variation have been investigated since these deep-sea habitats were discovered nearly 40 years ago, the roles and the factors behind the success of the dominant species of these ecosystems have yet to be fully elucidated. In the Lucky Strike vent field on the Mid-Atlantic Ridge (MAR), the dominant species is the mussel Bathymodiolus azoricus. Data on this species and its associated community were collected during four oceanographic cruises on the Eiffel Tower edifice and integrated in a novel statistical framework for niche analysis. We assessed the thermal range, density, biomass and niche similarities of B. azoricus and its associated fauna.Habitat similarities grouped mussels into three size categories: mussels with lengths ranging from 0.5 to 1.5cm, from 1.5 to 6cm, and mussels longer than 6cm. These size categories were consistent with those found in previous studies based on video imagery. The three size categories featured different associated fauna. The thermal range of mussels was shown to change with organism size, with intermediate sizes having a broader thermal niche than small or large mussels. Temperature maxima seem to drive their distribution along the mixing gradient between warm hydrothermal fluids and cold seawater. B. azoricus constitutes nearly 90% of the biomass (in g dry weight /m2) of the ecosystem. Mean individual weights were calculated for 39 of the 79 known taxa on Eiffel Tower and thermal ranges were obtained for all the inventoried species of this edifice. The analysis showed that temperature is a suitable variable to describe density variations among samples for 71 taxa. However, thermal conditions do not suffice to explain biomass variability. Our results provide valuable insights into mussel ecology, biotic interactions and the role of B. azoricus in the community.