Distribution of zooplankton associated with the Endeavour Ridge Hydrothermal Plume

Abstract

Enhanced zooplankton biomass was found in shallow ( 900 m) net samples collected over the Endeavour Ridge hydrothermal plume in 1991 and 1992. This enhanced biomass was manifest in considerably higher abundance values for most species in proximity to the vent field compared with abundance values from net samples collected 10-50 km off-axis. The species most enriched in abundance by the hydrothermal plume were those normally found in the mid-depth scattering layer at 400- 900 m depth. These mid-depth animals were also the dominant fauna in the deep scattering layers that overlie the hydrothermal plume at depths of 1200-1900 m near the main vent field. The abundance and biomass dominants were species with pronounced ontogenetic migration patterns, and their associated predators. The abundance of many typically deep species was also enhanced over the main vent field. Faunal compositions of net samples were compared using a similarity measure and average linkage rule. Deep fauna in proximity to the vent field but not associated with scattering layers (Group 1), were similar to deep fauna 10-50 km off- axis. The fauna of the deep scattering layer over the vent field (Group 2) was most similaT to surface and mid- depth scattering layer fauna found within a 50 km radius of the vent field. Statistical tests of linkages obtained using a bootstrap method indicate that the abundance and taxonomic composition of the two faunal groups were significantly distinct in 1992, but not in 1991. We conclude that there was considerable infiltration of shallow fauna into the deep scattering layers within 2-3 km of the main vent field, less extensive infiltration 10-15 km to the north and south of the vent field in 1991, and insignificant infiltration at stations 50 km to the west of the vent field in 1992. A bootstrap analysis comparing the faunal composition of nets towed above 900 m depth showed that shallow fauna were not significantly distinct between the two sampling years or up to 50 km away from the vent field.