Development of assemblages associated with alvinellid colonies on the walls of high-temperature vents at the East Pacific Rise

Symposium on Marine Benthic Ecology and Biodiversity - A Compilation of Recent Advances held in Honor of J Frederick Grassle, Rutgers Univ, Inst Marine Coastal Sci, New Brunswick, NJ, NOV 20-21, 2008International audienceSeveral species of the polychaete family Alvinellidae may be considered as `ecosystem engineer' because, by building their tubes, they modify the architecture of the hydrothermal fluid-seawater interface on the walls of vent chimneys. This affects the thermal and chemical gradients, and creates a mosaic of micro-niches, which could enable colonization by a variety of less-tolerant species. On high temperature vents of the Juan de Fuca Ridge, Alvinellid-dominated communities colonizing first mineral surfaces are followed by a succession of communities with different species composition. On the East Pacific Rise (EPR), tubes of Alvinella spp, may seal the mineral surface on which they grow and decrease fluid seepage, or tubes may become encrusted in mineral precipitations. An alvinellid colony may therefore persist for only a restricted time period at a given place. Here we investigated the development of alvinellid colonies on the EPR vent sites in order to detect whether a succession of new species less tolerant would follow colonization by Alvinella spp. or if different assemblages are forming depending on local conditions. Using a specially designed device called TRAC (titanium ring for alvinellid colonization), we described the evolution of newly formed colonies. Fifteen experiments were conducted on several chimneys of the 9 degrees N and 13 degrees N vent fields of the EPR, over durations ranging from 5 days up to 5 months. Through video analysis, different types of colonies were identified, characterized by increasing thickness of the Alvinella coverage, decreasing fluid flow bathing the colony, and decreasing surface temperatures. We showed that the assemblage formed by minerals, tubes, and organisms is produced at a very high rate. While animals may colonize the new surface in less than a week, and tubes are also quickly produced (Alvinella species may grow their tube up to 1 cm day(-1) during the early stages of colonization), mineral precipitation progressively increases the mineral content of the assemblage. Active colonies do not seem to persist longer than 5 months, highlighting the rapid turnover of this habitat. Colonies collected had different species compositions, but community structures exhibited no significant variations with vent field (9 degrees N/13 degrees N), deployment duration ( 1 month), or type of colony. However, multivariate analysis revealed that mineral content would best explain community structure variations. Biodiversity indexes increased quickly within the first weeks of TRAC deployment time, as well as with the amount of Alvinella tube. The development of a complex architecture may thus promote the installation of species lacking adaptations to temperature. However, our results do not support a succession pattern but rather the development of communities with slight differences in species compositions that may reflect local environmental conditions. (C) 2009 Elsevier Ltd. All rights reserved