Characteristics of a hydrothermal vent assemblage on a volcanically activesegment of Juan de Fuca Ridge, northeast Pacific

An eruption on Cleft segment, Juan de Fuca Ridge, in 1986 provided an oppor tunity to observe potential successional patterns in vent-animal colonisati on. Other objectives were to describe the Cleft fauna, examine the distribu tion and abundance of selected taxa, and determine if the fauna reflected c hanges in water chemistry. Biological samples were tubeworm grabs collected by submersible, and visual data were still photographs and videos. Two yea rs post eruption, there were extensive diffuse vents and 23 of the 44 speci es constituting the Cleft species pool were present. Five years post erupti on, most low-temperature vents were extinct. High-temperature venting was m aintained, and biological communities were reduced in visual extent. Four o f the 44 species in samples from 1988-1994 accounted for over 90% of the in dividuals. Cluster analyses of species collected on tubes did not distingui sh year or substratum differences, suggesting that a study of less than a y ear is necessary to document successional patterns at new vents. The Cleft subset of the Juan de Fuca Ridge species pool is likely adapted to episodic eruptive events on the decadal scale. Major changes in fluid chemistry did not result in detectable community changes other than habitat loss due to a decrease in dissolved sulphide availability.