The deep scattering layer associated with the Endeavour Ridge hydrothermal plume

Abstract

We analyse surface-to-bottom profiles of temperature, salinity, light attenuation coefficient and acoustic (150 kHz) backscatter intensity collected in June 1990 in the vicinity of the central hydrothermal vent field on Endeavour Ridge in the northeast Pacific. Data from coincident deep plankton net tows corroborate earlier speculation (Thomson et al., 1991, Journal of Geophysical Research, 96, 4839-4844) that the 100 m thick acoustic scattering layer found near 1.9 km depth in this region consists of a dense concentration of macrozooplankton living near the top of the plume-contaminated bottom waters. Peak (≈10 dB) acoustic anomalies in the June 1990 backscatter layer were located near the top of the plume at depths of 1800–2000 m throughout the principal 60 km 2 study area. The scattering layer contained a high Zooplankton biomass of 21 mg m −3 and maximum species richness of 83 taxa. In all profiles, the backscatter intensity decreased to anomalously low values beneath the core of the plume. The presence of the backscatter layer 12 km to the east of the vent site is evidence for Zooplankton layering beyond the immediate confines of the ridge. We conclude that pelagic and deep-sea Zooplankton congregate near the top of the plumecontaminated bottom waters in the vicinity of the ridge to take advantage of increased concentrations of chemosynthetic bacteria, fine-grained particles and other nutrients carried vertically upward by the buoyant portions of the plume. The Zooplankton depletion below the central core of the spreading hydrothermal plume indicates that Zooplankton avoid elevated concentrations of hydrothermally-derived minerals and other chemicals inherent to the main body of the plume.