An intense acoustic scattering layer at the top of a mid‐ocean ridge hydrothermal plume

We have used a tethered oceanic profiling package consisting of a conductivity‐temperature‐depth (CTD) sensor, transmissometer and acoustic Doppler current profiler (ADCP) to investigate the plume emanating from a known hydrothermal vent site at a depth of 2200 m near the crest of Endeavour Ridge in the northeast Pacific Ocean. Acoustic profiles obtained in August 1988 about 3 km to the southwest of the vent site revealed the presence of an intense scattering layer centered at approximately 1870 m depth near the top of the neutrally buoyant portion of the plume. Peak intensity anomalies in the 100‐m‐thick scattering layer ranged from 10 to 15 dB and were of comparable magnitude to upper ocean values observed in the euphotic zone. Below the top of the plume, backscatter intensities were significantly below background levels. We speculate that the strong backscatter layer consists of highly mobile macrozooplankton which congregate near the top of the plume to take advantage of the increased concentrations of nutrients originating with vertical buoyant transport of chemosynthetic bacteria and fine‐grain particles from the vent site. At the same time, the animals are able to avoid toxic levels of hydrothermally derived metals and sulfur inherent to the main body of the plume.