The original results of studies of metazoan microzooplankton (MM) in different locations of the Sevastopol Bay and the nearest open seashore at the end of the spring hydrological season are presented. Using the Primer 5 software package, changes in the abundance, species composition and structure of the MM community were analyzed. Samples were taken in 3 areas of Sevastopol Bay, differing in the degree of pollution and trophicity of the water. MM sampling, temperature and salinity measurements were carried out in three layers of the water column: from the surface to a 10-meter depth. The abundance of organisms, species diversity indices (Margalef, Pielou, Simpson, Shannon), the degree of similarity and difference of communities were calculated for successive layers: surface, middle (0-5 m) and bottom (5-10 m), as well as on average for the whole column of water. The species diversity of the MM community during the study period was generally moderately high. The middle layer of water at all stations was characterized by the highest species richness. In spatial terms, a decrease in species diversity was observed in the direction from the open seashore into the bay. Against the backdrop of an increase in water temperature from the end of May to the first ten days of June, a significant increase in the abundance of MM was recorded. In May, the average abundance in the 10-meter layer in different water areas ranged from 10 to 40 thousand ind./m³, and at the end of the first ten days of June it increased by 1.5-3.6 times. In both periods there was a trend of increasing abundance from open waters deeper into the bay, coinciding with the temperature trend of local water areas. The absolute maximum of abundance was noted in June in the surface layer in the inner part of the bay – more than 720×103 ind./m³. Quantitative changes in abundance were accompanied and largely determined by changes in the qualitative composition of MM. At the beginning of the period, the main role in the abundance and determination of similarities/differences in the community structure was played by native species of copepods, rotifers, and mollusk veligers. When the water warmed above 18-19 ºС, the role of the heat-loving invader, the copepod Oithona davisae Ferari and Orsi, 1984, sharply increased. The maximum abundance of the non-native species in the local water areas from open waters to the internal waters of the bay increased by more than an order of magnitude: from 23 to 550 ×103 ind./m³. The direction of changes in the abundance of O. davisae in the local water areas studied coincided with an increase in the level of their pollution and trophicity: from the open seashore towards the depths of the bay, both the absolute and relative abundance of the invader and its role in the community structure increased. This testifies to the indicator properties of this species. In general, the changes in abundance, species diversity and community structure of MM in local water areas reflect both seasonal and spatial changes in their trophic status in the natural gradient of the open seaside – the internal space of the Sevastopol Bay and can be used for its indirect assessment.

Seregin Sergei Alexandrovich Candidate of Sciences (Biology), Senior Research Scientist, Institute of Biology of the Southern Seas named after A.O. Kovalevsky RAS, 2, Nakhimov ave., Sevastopol, 299011, Russian Federation, e-mail: serg-seryogin@yandex.ru

Popova Elena Viktorovna, Leading Engineer, Institute of Biology of the Southern Seas named after A.O. Kovalevsky RAS, 2, Nakhimov ave., Sevastopol, 299011, Russian Federation, e-mail: el-popova@yandex.ru

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