Spatial variability and trophic characteristics of jellyfish populations in the Crimea marine area in summer

The existence of two alternative points of view on the long-term dynamics of gelatinous macroplankton populations in the World Ocean determines the need for long-term monitoring as a basis for assessing their condition. The accumulation of long-term series of data on the development of gelatinous predators in the Black Sea makes it possible to assess their role in the functioning of the pelagic ecosystem in connection with climatic variability. The abundance, biomass, the size structure of the gelatinous predators (jellyfish Aurelia aurita; ctenophores Mnemiopsis leidyi and Pleurobrachia pileus) and the feeding intensity of the two species (A. aurita and M. leidyi) were investigated in the early summer period (June) of 2016 at 45 stations covering the shelf zone off the coast of Crimea from Cape Tarkhankut to Kerch, as well as at 3 stations of the deepwater area with the coordinates 44°23′N–45°5′N and 32°22′E–36°36′E. The material was collected from the bottom (10–100 m) up to the surface at the inshore stations and from the depth with σt = 16.2 to the surface – in deepwater by vertical trawls with modified Bogorov – Russ net with an inlet diameter of 80 cm, a mesh of 300 μm. The nutritional spectrum and its quantitative composition were determined under a microscope in the laboratory immediately after the catch of animals, followed by the calculation of diurnal rations and the rate of ingestion of zooplankton. Spatial differences in abundance of gelatinous species were observed: the maximum biomass of A. aurita, prevailing in all regions, was observed in the Yevpatoriya – Sevastopol region and was due to large accumulations of middle-sized specimens of new generation. Ctenophora P. pileus dominated in the abundance in the halistatic zone. The food spectrum of jellyfish was very diverse and included various stages of Copepoda, Tunicata, Cladocera, and also the meroplankton – the Bivalvia and Gastropoda veligers. The halistatic zone with great depths was characterized by the widest species composition of planktonic Copepoda in A. aurita food. The average daily rations of jellyfish varied from one region to another, being the lowest in the Karkinitsky Bay [(0.113 ± 0.10) mg C−1·day−1, or (1.6 ± 2.18) % of C body] with most of the food being algae C. granii. The most intensive feeding was in the halistatic zone [(56.2 ± 23.7) % of C body] with a predominance of Calanus euxinus and Pseudocalanus elongatus in the diet. In no region A. aurita can compensate the respiration requirements by the consumption of mesozooplankton. The highest impact on zooplankton was provided by jellyfish population in the Karkinitsky Bay and in the Yevpatoriya – Sevastopol region, although along all the shelf its impact was very low and did not lead to a dramatic reduction in the zooplankton community abundance.