A review of the distribution of hydrothermal vent communities along the northern Mid-Atlantic Ridge: dispersal vs. environmental controls

Until 1985, seven vent fields were described from the Mid-Atlantic Ridge (M AR). An eighth field, Mount Saldanha (36 degrees N), discovered in 1998, sh owed unusual geological and biological settings. Vent sites on the MAR exhi bit varied environmental conditions, resulting from depth variation of the axis and associated physical parameters, and different source rocks. These could be considered as first order (i.e. most dominant) factors affecting t he composition of vent communities on the MAR, in contrast to the East Paci fic Rise (EPR) where geographical isolation appears to be a major determina nt of faunal differences. In this paper, the geological setting and vent fl uid composition of the fields are considered together with their community composition to tentatively ascertain the order of a hierarchy between dispe rsal and environmental control. The deepest fields (>3000 m) are rather sta ble systems. The shallower fields, especially Rainbow and Menez Owen, prese nt some evidence of instability in time and space. The variability in fluid composition is related to phase separation processes (boiling/distillation of subsurface vent fluids) and to the nature of the basement rocks. Depend ing on depth, phase separation produces gas-enriched and metal-depleted flu ids (Menez Gwen, Lucky Strike) or metal-enriched brines (Rainbow, TAG). In addition, high methane content characterises the fluids formed in ultramafi c rocks (Rainbow, Logatchev) compared to basaltic rocks. The discrepancy in mineral particulate fluxes at Lucky Strike and Menez Gwen, on one hand, an d TAG and Rainbow, on the other, is correlated to the predominance of the v apour or brine phase. The semi-quantitative description of the faunal compo sition of the different vent fields displays a continuum from Rimicaris-dom inated to Bathymodiolus-dominated assemblages. Rather than geographic or ba thymetric zonation, this gradation appears to be related to the metal conte nt of the fluids. In addition, the penetration of non vent species into the vent environment increases with decreasing hydrostatic pressure and/or met al content in the fluids. Similarity analysis between vent communities show s that similarity is strongest between Menez Gwen and Lucky Strike (the sha llowest fields), less significant between these sites and Rainbow, and weak est for Snake Pit, The inverse relationship between filter feeding organism s and metal concentration in vent fluids could result from a hindrance of m ussel bed development by particulate or toxic metal fluxes, and has to be f urther investigated. Conversely, high metal and particulate content would l ess affect the more mobile Rimicaris populations. Considering specific simi larities of endemic fauna between the four best known hydrothermal vents, t he distance between vent fields appears to be a first order parameter. Neve rtheless, within the proximity of the Azores Triple Junction area, and in t he absence of geographical discontinuity, the similarity between fields sta ys rather low suggesting faunal islands that have distinct composition and habitat requirements.