Macroplankton and micronekton in the northeast tropical Atlantic: abundance, community composition and vertical distribution in relation to different trophic environments

Abstract

During the EUMELI 4 cruise in the northeast tropical Atlantic a series of oblique hauls was carried out in the 0–1000 m water column at three different sites: oligotrophic, mesotrophic and eutrophic. Based on the samples obtained, the abundance, community composition and vertical distribution of macroplanktonic and micronektonic groups are described. Data on euphausiids and thecosome pteropods were analyzed at the species or genus level. In both euphausiid and thecosome groups, the number of species decreased from oligotrophic to eutrophic sites with a few clearly dominant species at mesotrophic and eutrophic sites. Therefore, the diversity index increased from eutrophic (1.0 bits) to oligotrophic (3.5 bits) sites. Among vertically migrating species, a general decrease in migration amplitude was observed from the oligotrophic site to the mesotrophic and eutrophic ones, with the organisms often being caught in the upper layers both in the day and at night in the latter two environments. This change of behavior is discussed in relation to the characteristics of the environment (phytoplankton abundance, depth of the euphotic zone, temperature, oxygen concentration).