Spatial and interannual variation in the faunal distribution at Broken Spur vent field (29°N, Mid-Atlantic Ridge)

Initial ecological observations at Broken Spur in 1993 suggested a low biomass relative to other deep-water vent communities known along the Mid-Atlantic Ridge. The persistence of a low shrimp biomass over 15 mo at Broken Spur vents appears to refute the hypothesis that the community is expanding through reproduction and immigration to occupy habitat vacated during a recent period of hydrothermal quiescence or catastrophe. Although the absence of “swarms” of shrimp similar to those found at the hydrothermal mounds of TAG (26°N) and Snake Pit (23°N) is a visually striking feature of the majority of venting structures known at Broken Spur, the biomass of fauna other than shrimp may not be significantly less than that of other Mid-Atlantic sites. The discovery of “swarms” of shrimp at Bogdanov Site, visited for the first time in 1994, suggests that availability of substratum exposed to the flow of hydrothermal fluids, which is a function of the topography of venting structures, may be a prerequisite for the development of these dense aggregations. Two testable predictions arise from this hypothesis. Firstly, dense aggregations of Rimicaris exoculata should occur at any other structures with a morphology similar to Bogdanov Site that may be discovered in the Broken Spur vent field, and should not occur at other isolated chimney structures that may be found. Secondly, “swarms” of shrimp should appear at any Broken Spur chimneys that develop into structures with a morphology more similar to that of Bogdanov Site in future. Received: 2 July 1997 / Accepted: 9 July 1997