On the quantitative distribution of zooplankton in deep scattering layers

Abstract

The plankton samples described have been collected in oblique hauls with closing nets from the depth of the deep scattering layer (DSL), as recorded by an ELAC echo-sounder at a frequency of 15 kc/s. In the Eastern Atlantic, the plankton biomass (wet volume) of the surface layer (0–150 m) was in general only twice as rich as the zooplankton collected at scattering layer depth (450–600 m). The plankton of the DSL was for most part significantly richer in volume and in number of organisms than those from samples below the DSL (600–700 m) and above (150–450 m). The upper of the double component scattering layer was usually somewhat richer in volume and particularly in number of copepods and euphausiids. In the eastern Arabian Sea plankton concentrations found in the O 2-deficient DSL (0·10-·04 ml O 2/l.) averaged about two-thirds less than those obtained in surface hauls. As in the Atlantic, copepods far outnumbered (up to 90%) all other planktonic organisms within the DSL. Stomach contents of myctophid fishes indicated that large populations of Benthosema pterota collected in the DSL feed on copepods.