High-Resolution Vertical Habitat Mapping of a Deep-Sea Cliff offshore Greenland

Recent advances in deep-sea exploration with underwater vehicles have led to the discovery of vertical environments inhabited by a diverse sessile fauna. However, despite their ecological importance, vertical habitats remain poorly characterized by conventional downward-looking survey techniques. Here we present a high-resolution 3-dimensional habitat map of a vertical cliff hosting a suspension-feeding community at the flank of an underwater glacial trough in the Greenland waters of the Labrador Sea. Using a forward-looking set-up on a Remotely Operated Vehicle (ROV), a high-resolution multibeam echosounder was used to map out the topography of the deep-sea terrain, including, for the first time, the backscatter intensity. Navigational accuracy was improved through a combination of the USBL and the DVL navigation of the ROV. Multi-scale terrain descriptors were derived and assigned to the 3D point cloud of the terrain. Following an unsupervised habitat mapping approach, the application of a K-means clustering revealed four potential habitat types, driven by geomorphology, backscatter and fine-scale features. Using groundtruthing seabed images, the ecological significance of the four habitat clusters was assessed in order to evaluate the benefit of unsupervised habitat mapping for further fine-scale ecological studies of vertical environments. This study demonstrates the importance of a priori knowledge of the terrain around habitats that are rarely explored for ecological investigations. It also emphasizes the importance of remote characterization of habitat distribution for assessing the representativeness of benthic faunal studies often constrained by time-limited sampling activities. This case study further identifies current limitations (e.g., navigation accuracy, irregular terrain acquisition difficulties) that can potentially limit the use of deep-sea terrain models for fine-scale investigations

Imaging study of the multi-scale spatio-temporal distribution of benthic communities associated with the Lucky Strike hydrothermal field

The multi-scale characterisation of ecosystem variability is fundamental to the conceptualisation of dynamic models. Since its discovery in 1993, the Lucky Strike vent field has been extensively studied, making it one of the most well-known hydrothermal area of the World’s ocean. However, remote access has limited -in space and time- the scope of the studies, preventing an integrated understanding of the ecosystem. Using new technologies, this thesis aims to unravel the relative role of processes driving the dynamics of biological communities at Lucky Strike over different spatial scales: at the assemblage, the edifice and the field scales. The monitoring of the faunal assemblages of the Eiffel Tower hydrothermal edifice were carried out at sub-annual scales through the EMSO Azores observatory and pluri-annually based on photogrammetric reconstructions. These surveys have revealed a limited intensity and number of disturbances to which the hydrothermal fauna is subjected. Decadal variability restricted to emission areas had little impact on the dominant engineer species Bathymodiolus azoricus, because of its mobility. Considering the stable distribution of species over a decade, the remarkable scale invariance (from m² to the edifice) could explain why this community has reached a climax stage maintained for at least 25 years. Finally, analysis of the factors controlling the distribution of benthic vent and peripheral communities at the scale of the field showed the importance of habitat heterogeneity over a hundred of metres. These results suggest a strong structuring and a low resilience of hydrothermal ecosystems to any potential large-scale disturbance on the Mid-Atlantic Ridge.La caractérisation multi-échelle de la variabilité d’un écosystème est fondamentale à la conceptualisation de modèles de dynamique. Depuis sa découverte en 1993, le champ hydrothermal profond Lucky Strike a fait l’objet d’un grand nombre d’études, ce qui en fait l’une des zones hydrothermales les mieux connues. La difficulté d’accès a toutefois limité la compréhension intégrée de l’écosystème dans le temps et l’espace. A l’aide de nouvelles technologies d’observation, cette thèse vise à comprendre l’influence des processus régissant la dynamique des communautés de Lucky Strike à différentes échelles spatiales : l’assemblage, l’édifice et le champ. Les suivis d’assemblages de faune de l’édifice Tour Eiffel ont été effectués aux échelles infra-annuelle grâce à l’observatoire EMSO Açores pluri-annuelle à partir de reconstructions photogrammétriques. Ils ont révélé l’intensité et le nombre limités des perturbations auxquelles est soumise la faune hydrothermale. La variabilité décimétrique restreinte aux zones d’émissions a peu d’impact sur l’espèce dominante Bathymodiolus azoricus, de par sa mobilité. La distribution des espèces n’ayant pas changé sur une décennie, cette invariance d’échelle (du m² à l’édifice) expliquerait que cette communauté ait atteint un stade de climax depuis au moins 25 ans. Enfin, à l’échelle du champ, l’analyse de la distribution préférentielle de la faune hydrothermale et périphérique a rendu compte de l’importante hétérogénéité de l’habitat sur une centaine de mètres. Ces résultats suggèrent une forte structuration et une faible résilience des écosystèmes hydrothermaux à de potentielles perturbations d’envergure sur la dorsale médio-Atlantique

Multiple nutritional strategies of hydrothermal vent shrimp (Rimicaris hybisae) assemblages at the Mid-Cayman Rise

Alvinocaridid shrimp occurring in dense assemblages close to vigorously venting orifices are characteristic of many vent fields on the Mid-Atlantic Ridge, Central Indian Ridge, and Mid-Cayman Rise. Episymbiotic bacteria of these shrimp are exposed to vent fluids enriched in inorganic nutrients (carbon dioxide, sulfide) that, together with dissolved oxygen in the surrounding seawater, sustain autotrophic growth and supply nourishment to the shrimp. Enigmatically, conspecific shrimp may also be found sparsely distributed in the periphery of a vent field, where there is little visual evidence of vent fluid flux. We tested the null hypothesis that nourishment sources were identical for the central (dense) and peripheral shrimp assemblages, using gut content and stable isotope analyses. Rimicaris hybisae were sampled from central and peripheral assemblages at the Von Damm Vent Field (Mid-Cayman Rise). Guts of centrally aggregated shrimp contained a white material inferred to be bacteria, while peripheral individuals contained this white material and/or pieces of crustacean exoskeletons. C, N, and S isotopic compositions of shrimp muscle tissues were significantly different between central and peripheral shrimp assemblages, and little overlap in isotopic space was found. A comparison of δ13C and δ34S values of gut contents and abdominal tissues revealed that abdominal tissue composition reflects the shrimp's source of nourishment. Slight but significantly elevated δ15N values in peripheral shrimp, together with crustacean exoskeleton fragments in the gut, suggest facultative carnivory in peripherally aggregated shrimp. The lower δ13C and δ34S values in individuals from the peripheral assemblages are also consistent with a mixotrophic diet. We conclude that central and peripheral assemblages of R. hybisae have different nourishment sources, with central assemblages relying on episymbionts, while peripheral assemblages have diverse nourishment sources comprising bacteria and Crustacea

Eiffel Tower: A Deep-Sea Underwater Dataset for Long-Term Visual Localization

Visual localization plays an important role in the positioning and navigation of robotics systems within previously visited environments. When visits occur over long periods of time, changes in the environment related to seasons or daynight cycles present a major challenge. Under water, the sources of variability are due to other factors such as water conditions or growth of marine organisms. Yet it remains a major obstacle and a much less studied one, partly due to the lack of data. This paper presents a new deep-sea dataset to benchmark underwater long-term visual localization. The dataset is composed of images from four visits to the same hydrothermal vent edifice over the course of five years. Camera poses and a common geometry of the scene were estimated using navigation data and Structure-from-Motion. This serves as a reference when evaluating visual localization techniques. An analysis of the data provides insights about the major changes observed throughout the years. Furthermore, several well-established visual localization methods are evaluated on the dataset, showing there is still room for improvement in underwater long-term visual localization. The data is made publicly available at seanoe.org/data/00810/92226/

Workshop on 3D mapping of habitats and biological communities with underwater photogrammetry

For the past decades, photogrammetry has been increasingly used for monitoring spatial arrangement or temporal dynamics of submerged man-made structures and natural systems. As photogrammetry remains a nascent technique for data collection in the underwater environment, acquisition workflows have evolved constrained by specific methodological practicalities (e.g. euphotic environments vs. deep-sea waters). The annual GeoHab conference gathers a world-wide range of scientists interested in mapping and is, therefore, an adequate event to set up a state-of-the-art workshop on (underwater) photogrammetry. More specifically, a preliminary survey identified the overall lack of photogrammetry knowledge from the audience. A programme was conceptualised to explore within a day theoretical concepts, sampling design and practicalities and a wide range of case studies in various underwater environments. Furthermore, we provided manual training on data acquisition and processing. In overall, a post-survey demonstrated the audience’s satisfaction despite a remaining lack of confidence for implementing their own photogrammetry studies. As this workshop gathers a diversity of materials and a training relevant for a scientific audience, it sets the stage for a reproducible event and leaves room for future improvements. Finally, it provided relevant materials and discussions that enabled us to identify the aspects limiting photogrammetry methodology across scientific applications and institutes, in order to work towards standardisation