Structure et dynamique temporelle des communautés hydrothermales inféodées à la dorsale Juan de Fuca : utilisation d’une approche observatoire fond de mer

Hydrothermal vents are distributed along mid-ocean ridges and back-arc basins. These ecosystems harbour a specialized fauna supported by a local chemosynthetic microbial production, made possible through the oxidation of reduced elements present in the hydrothermal fluid. While the relationships between faunal assemblages and habitats have been relatively well described around the globe, our knowledge on the temporal dynamics of these environments is still limited, particularly at time scale relevant to organisms (from few minutes to several years). Based on hydrothermal vent communities occurring on the Juan de Fuca Ridge, located in the northeast Pacific Ocean, this thesis aims at understanding the functioning and temporal dynamics of macrofaunal communities associated with the Siboglinidae Ridgeia piscesae tubeworm assemblages through the coupling of sampling and seafloor observatory approach.  This study first allowed the quantitative description (composition, diversity and biomass) of six siboglinid species assemblages on the Grotto hydrothermal edifice (Main Endeavour, Juan de Fuca Ridge) and the assessment of the trophic structure of these communities using stable isotopes (carbon 13C and nitrogen 15N). The results enabled the identification of 36 taxa. An elevated polychaete diversities and high densities of gastropods characterized all assemblages. The most developed R. piscesae assemblages (i.e. longer tube lengths) exhibited greater diversity and more complex food webs. The species coexistence within the biogenic structures created by R. piscesae could be explained by a strong partitioning of ecological niches and nutritional resources.  The analysis of video images gathered by the ecological observatory module TEMPO-mini, deployed on the Grotto edifice on the cabled observatory Ocean Networks Canada, provided information on the temporal dynamics of the assemblage. The abundance variations of four macrofaunal species were tracked at a daily frequency during one year and every 4 hours during one summer and two winter months. Our results highlighted a significant influence of the semi-diurnal tidal cycle and winter surface storms on the distribution of two mobile and non-symbiotic taxa (Sericosura sp. pycnogonids and Polynoidae polychaetes). The local ocean dynamics seemed to periodically affect the mixing between hydrothermal fluid inputs and surrounding seawater, therefore modifying environmental conditions in vent habitats. This study suggests that hydrothermal species respond to these habitat modifications by adjusting their behaviour.  Finally, considering the increased importance of observational and imagery approaches in the study of deep-sea ecosystems, this thesis proposes a guideline and methodology in order to optimise video imagery methods both for the extraction of ecological data and the search of natural cycles. Overall, these results bring novel and valuable information to help understand the functioning and dynamics of deep-sea hydrothermal communities and opens new research avenues for deep-sea ecology in general.

Keyphrases: Deep-sea ecology, Juan de Fuca Ridge, Hydrothermal vents, Seafloor observatories, Communities, Diversity, Food webs, Temporal dynamics, Imagery, Methodologies