SPATIAL AND INTERANNUAL VARIATION IN THE FAUNAL DISTRIBUTION AT BROKEN-SPUR VENT FIELD (29-DEGREES-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 m o at Broken Spur vents appears to refute the hypothesis that the commu nity is expanding through reproduction and immigration to occupy habit at vacated during a recent period of hydrothermal quiescence or catast rophe. Although the absence of ''swarms'' of shrimp similar to those f ound at the hydrothermal mounds of TAG (26 degrees N) and Snake Pit (2 3 degrees N) is a visually striking feature of the majority of venting structures known at Broken Spur, the biomass of fauna other than shri mp 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 expos ed to the flow of hydrothermal fluids, which is a function of the topo graphy of venting structures, may be a prerequisite for the developmen t of these dense aggregations. Two testable predictions arise from thi s hypothesis. Firstly, dense aggregations of Rimicaris exoculata shoul d 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. Seco ndly, ''swarms'' of shrimp should appear at any Broken Spur chimneys t hat develop into structures with a morphology more similar to that of Bogdanov Site in future.