THE QUANTITATIVE VERTICAL DISTRIBUTION OF MACROZOÖPLANKTON IN THE ATLANTIC OCEAN BASIN

Abstract

The volume of material collected in 2 m. closing nets in 117 catches from different depths at 9 "Atlantis" stations was measured and sorted into taxonomic groups. A definite minimum quantity of animal life was found at some depth between 1200 and 1800 m. In every case the water at 1200 m. contained less than any level above this and at all stations there was at least one place below this minimum which was considerably richer in material. The position of the deep increase varied with the minimum layer. The vertical distribution of the separate groups of animals paralleled that of the total volumes; a balanced plankton community occupied different depths, no one group dominated any level. Euphausiids comprised 37 spp. in 8 genera. Decrease in number of individuals with increasing depth was accompanied by a corresponding decrease in the specific diversity of the group. No direct correlation between the quantity of animals and the variations in temp., salinity, O2, or density of the water at different depths was observed. The volumes found at great depths were from 4 sources; animals sifting down from higher levels; an occasional large specimen of some well recognized deep sea sp.; a fauna which for some undetermined reason finds levels above and below unsuitable; cold water northern spp. which may have sunk with a mass of water from more northern latitudes.