Vertical distribution of mesoplankton in the open area of the Black Sea

Abstract

In April–May 1984 mesoplankton vertical distribution in the Black Sea was studied by sampling with a 150-l waterbottle, vertical plankton nets with mesh-sizes of 180 and 500 mkm and by direct counting of the jelly-fish Aurelia aurita, the ctenophore Pleurobrachia pileus, Calanus helgolandicus and the chaetognath Sagitta setosa from the manned submersible “Argus”. During daytime throughout the whole deep-water body of the sea near the lower oxycline boundary, plankton forms a layer of high concentration (from 2.5 to 38 g m-3); its thickness varies from 5 to 10–20 m and it has an unchangeable vertical structure; its upper portion is formed by the ctenophore P. pileus, its middle portion by V–VI copepodites of C. helgolandicus, and its lower portion by the population of S. setosa. The lower boundary of this layer coincides with 0.4 to 0.5 ml O2 l-1 isooxygen surface, and the depth of its location varies in different areas of the sea from 150 to 50 m, depending on the depth location of 0.5 ml O2 l-1 isooxygen surface. By night the animals, which form this layer, migrate towards the surface.