Winter jellyfish macroplankton off the coastal of Crimea

Abstract

Predatory macroplankton of jellyfish species is an important component of the pelagic ecosystem. It is capable of mass development, and may influence pelagic communities. Surveillance monitoring of jellyfish populations can detect the seasonal and long-term trends of their variability and assess the role of jellyfish in the trophic structure of the pelagic zone. Monitoring of plankton community (jellyfish macroplankton) off the coastal of Crimea from the Cape Tarkhankut to the Kerch coast (including the Azov Sea) in winter period is presented. 22 comprehensive oceanographic stations from Cape Tarkhankut to the Kerch Strait and in Azov Sea at the depths of up to 100 m were made. We used a Bogorov — Russ plankton net (entrance area is 0.5 square meters) for collecting plankton in the upper 100-m water layer. Jellyfish were measured immediately, we used a standard method of measuring and calculated the mass of the individual instances by the formulas. The biomass is calculated as the product of the abundance and the average wet weight of organisms in the population. Abundance and biomass macroplankton were counted per square meter of water surface or per the volume of filtered water. The species composition of jellyfish macroplankton was represented by 1 species of jellyfish and 3 species of ctenophores. Aurelia aurita dominated in biomass (98 % of the total biomass of gelatinous macroplankton)  everywhere; Pleurobrachia and Aurelia dominated in abundance (67 and 25 % respectively). The share of both species of ctenophores-invaders did not exceed 8 %. The average relative abundance of jellyfish along the coast of Crimea was fluctuated between 9 and 43 %, comb jellies — from 32 to 77 %. Quantitative distribution of gelatinous macroplankton was non-uniform — from complete absence to 83.3 g·m^-3 (average of 12.6 g·m^-3). The highest biomass was registered in the area of Sevastopol and Eupatoria, the minimum — off the southern coast of Crimea. Jellyfish were noted at almost all stations. Their biomass reached a value of 18 g·m^-3. Distribution of jellyfish is characterized by marked heterogeneity with higher values of abundance and biomass in the area of the southern coast of Crimea and lower — in the stations in the western part of the Black Sea. Aurelia was presented with large dome diameter of 21–220 mm (average diameter — 93 mm). The bulk of the population was accounted for last year’s individuals generation. Biomass of the ctenophore M. leidyi varied from 86 to 4788 mg·m^-3 (average of 1595 mg·m^-3). The structure of its population was represented by wide size range of animals — 10–65 mm, while the share of larger animals of 60–100 % was observed in the western and south-eastern parts of the Black Sea. On the southern coast of Crimea 45 % of population of ctenophore were animals of 15–25 mm. Biomass of the ctenophore B. ovata was within 34.8–1316.6 mg·m^-3 (average of 371.4 mg·m^-3). Its abundance varied within a small range, except the area of Feodosia stations, where the maximum value was observed (it was more than 1 g·m^-3). B. ovata population consisted of individuals of 20–40 mm long, while it formed the basis of immature specimens of 20–30 mm (50–70 % of the total). In Kerch area their share has reached 100 %. There were hibernating animals of last year’s generation. The coldwater ctenophore Pleurobrachia pileus was present almost everywhere, with biomass from 0.1 to 6.0 mg·m^-3. Maximum values of abundance were observed in Alushta region. The stations of Crimean southern coast were characterized by intermediate, relatively flat values. South-eastern and western parts of the Black Sea were marked by similar values of abundance and biomass. In all studied regions, the prevailing size group in the structure of P. pileus populations were individuals with a  diameter of 10 mm. In the deep-sea area of the southern part of Crimea population was represented in the larger size range, and the proportion of animals ranging in size from 11 to 20 mm was low (5 %). In general, “winter” composition, size structure and abundance of jellyfish off the coast of the Crimea are the same of indicators in the coastal waters of Sevastopol. But the results show the existence of certain differences in the structure and quantitative development of jellyfish macroplankton in southern and  western coasts of Crimea.

Keywords: macroplankton, jellyfish, ctenophores, abundance, Black Sea