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Near the Azores Triple Junction as the Azores Plateau is approached, the ridge axis becomes shallower; its depth decreases from ca. 2400 m in the Rainbow vent field (36°13′N) to ca. 850 m in the Menez Gwen vent field (37°35′N). In this area, extensive mussel beds of the mytilid Bathymodiolus azoricus dominate the hydrothermal vent fauna, along with populations of three shrimps (Rimicaris exoculata, Mirocaris fortunata and Chorocaris chacei). The main physical and chemical characteristics of the vent habitat were studied by discrete sampling, in situ analysis and sediment trap moorings. 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-endemic species live at the border of the vent field. The variations observed in structure and composition of the communities along the depth gradient are most likely due to changes in vent fluid toxicity (metallic and sulphide content) and suspended mineral particles, which render the fluids harsher for 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.

To provide information about the export and the distribution of hydrothermal material to the deep ocean, two sediment traps with current-meters were moored on the Lucky Strike segment of the Azores Triple Junction (Mid-Atlantic Ridge). The results of a 25-day experiment with a single trap deployed 1.5 m from a chimney (2 m above the bottom) are used as a reference for the composition of particles produced by the vent. A 392-day experiment with a time-series sediment trap positioned 500 m from the Lucky Strike vents (17 m a.b.) showed seasonal variations in the particulate flux and in its composition. Particles sampled by the trap close to the chimney (264.3 mg m −2 day −1 ) were characterised by a high concentration of sulphur (10.8%) and barium (3.19%), and a low C/N ratio (5.5). From the current data recorded at the same time, we estimate that this small trap was under vent influence for about 10% of the experiment duration. The calculated mean particle flux obtained with the multisample sediment trap 500 m away (7.7 mg m −2 day −1 ) was among the lowest recorded in the Atlantic Ocean. This situation may be explained by a very low primary production in the Azores region. No significant influence of the Lucky Strike vent particles was recorded in this trap although near bottom currents were regularly favourable to transport particles towards it. No major effect was observed on the trap efficiency by variations of the hydrodynamic conditions during this experiment. Particles in the trap included bivalve larval shells. A comparison between their hinge structure and that of postlarvae of the mytilid species found on these vents suggests a non-hydrothermal origin for the larvae in the trap.

Newly discovered hydrothermal vent communities at Lucky Strike on the Mid-Atlantic Ridge (37°18'N, 32°16'W) are comprised of an invertebrate fauna sufficiently different from known vent faunas of TAG and Snake Pit to consider Lucky Strike part of a new biogeographic province. The dominant component of the fauna is a new species of mussel, and the most unusual feature of the fauna is an echinoid echinoderm, Echinus sp. An abundance of small mussels (< 5 mm) indicates a recent recruitment event at Lucky Strike, and modal analysis of length-freque ncy data indicate a discontinuous recruitment process in space and time. Copyright © 1996 Elsevier Science Ltd