Alternative conditions of mass appearance of the scyphozoan jellyfish, <i>Aurelia aurita</i> (Linnaeus, 1758), and the ctenophore, <i>Pleurobrachia pileus</i> (O.F. Muller, 1776), in plankton of the Black Sea

Aim. The aim of the study is to find out what external factors may be the driving force behind the growth and population dynamics of two ecologically similar species of gelatinous macroplankton (scyphomedusa Aurelia aurita and ctenophore Pleurobrachia pileus), which play an important role in the functioning of the pelagic ecosystem of the Black Sea.Material and Methods. The state of zooplankton populations was estimated by data obtained in 2000‐2014 in the outer shelf of Sevastopol Bay, where monthly quantitative samples of meso‐ and macroplankton were taken 2 miles of the coast (depth 50‐70 m). Weather and hydrology related changes were assessed using open databases.Results. It was been established that size‐specific growth rate of these species depends on weather and hydrological conditions in the winter‐spring months and varies from 0.85 to 1.02% day‐1 and from 0.27 to 0.47% day‐1 for A. aurita and P. pileus, respectively.Conclusions. External factors unequally affect the growth of these species. Sub‐ latitudinal transit of warm air masses, accompanied by increased river flow and seawater circulation, activates the growth of the ctenophore, while sub‐meridional propagation of cold and dry air increases the growth rate of the jellyfish. In both cases, somatic growth is influenced by trophic relations, differing depending on the weather in February‐May. The changes in growth of ctenophore occur in parallel with synchronous variations in biomass of crustacea, while that in jellyfish may relate to an abundance of microplankton and its mesoplanktonic consumers