Visual observations of the vertical distribution of plankton throughout the water column above Broken Spur vent field, Mid-Atlantic Ridge

Visual observations were made in September 1997 during the 39 cruise of R/V “Akademik Mstislav Keldysh” with 2 deep-sea manned submersibles “Mir” aboard. During 4 dives the following plankton countings were made: 3 vertical throughout the water column during the day, 2 vertical in the upper 1000 m at night, and 1 oblique in the plume area during the day. Biomass profiles are represented for each dive for all abundant animal groups: copepods, euphausiids+decapods+mysids, chaetognaths, medusae, ctenophores, siphonophores, cyclothones, myctophides, radiolarians, and the total zooplankton. Plankton distribution shows 2 aggregations, one within the main pycnocline and the other near the plume; Gelatinous animals and radiolarians dominate in both aggregations by biomass and make a significant contribution to the plankton biomass throughout the water column. Oblique counting indicates the presence of aggregations of animals near the upper and lower borders of the plume and biomass depletion within the plume core.