Specific Composition and Distribution of Euphausiids in the Zone of the North Atlantic Subtropical Convergence

Abstract

The euphausiid community was studied between the surface and upper bathypelagics northeast of the Azores Islands (46°–37° N, 14°–29° W) during the hydrological summer (August to September) of 1984 using a midwater trawl. In total, 11 species of euphausiids were identified. The two most common species, Meganyctiphanes norvegica and Thysanopoda acutifrons, were found in 43.8 and 40.8% of the samples, respectively. In terms of biomass, M. norvegica dominated the catches, its densities reaching up to 160 kg per trawling hour. The region is dominated by North Atlantic arctic–boreal and boreal species, followed by pan-oceanic meso- and bathypelagic, as well as tropical–subtropical and widespread tropical euphausiid species. The study area, which is situated in the zone of the Atlantic subtropical convergence, is subjected to seasonal migrations of the Subpolar Frontal system. Owing to this, a quasi-stationary ecotone is established, where arctic–boreal species are spatially replaced first by tropical–subtropical and then by widespread tropical elements. There are grounds to believe that the quasi-stationary ecotone lasts both for many years and from season to season. In the latter case, seasonal succession of species takes place, but the ecological structure of the ecotone remains almost constant.