HYDROTHERMAL VENTING AT ENDEAVOR RIDGE - EFFECT ON ZOOPLANKTON BIOMASS THROUGHOUT THE WATER COLUMN

Bio-acoustical data reveal that the hydrothermal plume emanating from the main vent field near 2100 m depth on Endeavour Ridge (47-degrees57 'N, 129-degrees06'W) affects the distribution and migration of zooplan kton throughout the entire water column. Net samples taken in July of 1991 and 1992 show that the standing stock of macrozooplankton integra ted over the water column was considerably higher within several kilom eters of the main vent site than at locations tens of kilometers from the vent site. Community analysis reveals that there were distinct sha llow (<800 m depth) and deep (> 800 m depth) faunal assemblages in the vent region. Shallow fauna infiltrated the deep zooplankton acoustic scattering layers in the immediate vicinity of Endeavour Ridge, produc ing a mixed assemblage of animals, including large numbers of juvenile filter-feeding copepods and their predators that normally inhabit the shallow layer. In contrast, the deep acoustic scattering layers found 11 km to the southeast and 15 km to the north of the central vent fie ld in 1991, and 50 km to the west of the central vent field in 1992, w ere composed of distinctly deep-sea fauna. The enhanced, vertically-in tegrated biomass over the vent region appears to result from vertical migration of zooplankton. A simple circulation model indicates that fa una can make the round-trip journey between the top of the plume and t he upper ocean without being advected beyond the range of the detectab le hydrothermal effluent.