Zooplankton and Micronekton Abundance in Acoustic Scattering Layers on the Nova Scotian Slope

Abstract

Zooplankton and micronekton sampling with a multiple net sampler, the BIONESS, in conjunction with a 120-kHz sounder showed significantly higher numbers of animals on the shelf water side of the shelf–slope water front than in the slope water on the Nova Scotian slope. The dominant copepod in the shelf water was Calanus finmarchicus and in the slope water it was Pleuromamma borealis. The dominant forms of micronekton were the myctophid Benthosema glaciale and the euphausiid Thysanoessa longicaudata. Calanus finmarchicus preferred a temperature of about 6 °C, B. glaciale a temperature < 12 °C. The intense acoustic scattering in the shelf water was probably due to the myctophids and euphausiids. The acoustic scattering disappeared once the warmer slope water was entered. The biomass and numbers of each species of zooplankton and micronekton∙m−2 are given for the shelf and slope water masses, for May 1977 and April 1979; these values are also given for the shelf water in June 1978. Estimates were made of the daily consumption of zooplankton by the euphausiids and myctophids in the two water masses, and it was found that both groups of animals were major consumers of the zooplankton during the spring period. There was no evidence for a food chain dominated by tunicates, chaetognaths, or gelatinous carnivores.Key words: zooplankton, micronekton, copepod, euphausiid, myctophid, acoustic scattering layer, shelf, slope