DISTRIBUTION OF ZOOPLANKTON ASSOCIATED WITH THE ENDEAVOR RIDGE HYDROTHERMAL PLUME

Enhanced zooplankton biomass was found in shallow (<900 m) and deep (> 900 m) net samples collected over the Endeavour Ridge hydrothermal plu me in 1991 and 1992. This enhanced biomass was manifest in considerabl y higher abundance values for most species in proximity to the vent fi eld 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 4 00-900 m depth. These mid-depth animals were also the dominant fauna i n the deep scattering layers that overlie the hydrothermal plume at de pths of 1200-1900 m near the main vent field. The abundance and biomas s dominants were species with Pronounced ontogenetic migration pattern s, and their associated predators. The abundance of many typically dee p species was also enhanced over the main vent field. Faunal compositi ons of net samples were compared using a similarity measure and averag e linkage rule. Deep fauna in proximity to the vent field but not asso ciated 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 similar to surface and mid-depth scattering l ayer 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 sign ificantly distinct in 1992, but not in 1991. We conclude that there wa s considerable infiltration of shallow fauna into the deep scattering layers within 2-3 km of the main vent field, less extensive infiltrati on 10-15 km to the north and south of the vent field in 1991, and insi gnificant infiltration at stations 50 km to the west of the vent field in 1992. A bootstrap analysis comparing the faunal composition of net s towed above 900 m depth showed that shallow fauna were not significa ntly distinct between the two sampling years or up to 50 km away from the vent field.