Variations in deep-sea hydrothermal vent communities on the Mid-Atlantic Ridge near the Azores plateau

Near the Azores Triple Junction as the Azores Plateau is approached, the ri dge axis becomes shallower; its depth decreases from ca, 2400 m in the RAIN BOW vent field (36 degrees 13'N) to ca. 850 m in the MENEZ GWEN vent held ( 37 degrees 35'N). In this area, extensive mussel beds of the mytilid Bathym odiolus azoricus dominate the hydrothermal vent fauna, along with populatio ns of three shrimps (Rimicaris exoculata, Mirocaris fortunata and Chorocari s chacei). The main physical and chemical characteristics of the vent habit at were studied by discrete sampling, in situ analysis and sediment trap mo orings. The vent fauna is distributed along a variable band where the vent fluids and seawater mix, with R. exoculata living in the most concentrated areas and Bathymodiolus azoricus in the most diluted zones. Various non-end emic species live at the border of the vent field. The variations observed in structure and composition of the communities along the depth gradient ar e most likely due to changes in vent fluid toxicity (metallic and sulphide content) and suspended mineral particles, which render the fluids harsher f or species living there. The main faunal differences observed between LUCKY STRIKE and MENEZ GWEN hydrothermal fields are due to an impoverishment in the hydrothermal endemic species and to the penetration of bathyal species. The comparison of the three studied vent fields suggests the existence of a succession of several biogeographic islands rather than a single province . (C) 2001 Published by Elsevier Science Ltd.