Zooplankton constitute a key trophic link between primary producers and higher trophic levels in marine food webs, making them important indicators of environmental and climatic change. However, in many marine systems, long-term data are still limited, and major interannual fluctuations in zooplankton biomass and composition remain poorly understood. The article presents data assessing the impact of climate and warming in the Black Sea region on the interannual dynamics of macro- and mesozooplankton biomass in the outer shelf of Sevastopol Bay from 2002 to 2021. Over this period, the biomass of organic matter accumulated by gelatinous species has decreased, while that of mesozooplankton has increased by 15-20 %. Winter warming, which minimises convective mixing and nutrient transport into the zone of active photosynthesis, may have negatively affected the populations of the jellyfish Aurelia aurita, the dinoflagellate Noctiluca scintillans, and the chaetognath Parasagitta setosa. The increase in sea surface temperature (SST) during January-March appears to have a positive effect on microphages such as the copepod Paracalanus parvus and the appendicularian Oikopleura dioica. Early increases in SST during April-May, along with a synchronous improvement in the biogenic regime of the upper pelagic zone, suggest an increase in the biomass of large copepods (Calanus euxinus, Paracalanus elongatus, Acartia clausi and Centropages ponticus), regardless of their temperature tolerances. The extreme increase in SST during July-August was accompanied by the early development of ctenophores Beroe ovata and Mnemiopsis leidyi, as well as small crustaceans like Penilia avirostris and Oithona davisae. Overall, outer shelf macro- and mesozooplankton communities have remained relatively stable in species composition and total biomass of organic carbon. This study highlights the complex response of Black Sea macro- and mesozooplankton communities to regional warming, with species-specific seasonal sensitivities to changing environmental conditions.

The study of the seasonal and interannual dynamics of populations of ctenophore invaders Mnemiopsis leidyi A. Agassiz, 1875 and Beroe ovata Bruguière, 1789 on the shelf of Sevastopol Bay in 2013–2021 established that gradual warming accompanied by a change in the hydrologic regime of the Black Sea has led to a significant transformation of the phenology, structure, and intensity of development of their populations in recent years. The early seasonal emergence of the warm-water ctenophore B. ovata in the pelagic zone expanded the time span of its presence in plankton from two or three months in the first years of introduction (1999–2000) to eight months in 2019–2020. At the same time, the maximum size of individuals decreased from 50 (2000–2001) to 35 mm (2019–2020). Significant decrease in the abundance of M. leidyi and weakening of its trophic pressure on the forage zooplankton in the coastal areas of the Black Sea were the consequence of the longer and active predation, as well as structural changes in the population of B. ovata expressed in the reduction of body weight. Increased warming of the upper mixed layer of the sea in the summer months resulted in a decrease in the size of M. leidyi, the intensity of its reproduction, and the proportion of reproductive individuals in the population.

The abundance, biomass, distribution and feeding patterns of the ctenophoreMnemiopsisleidyiA. Agassiz 1865 were studied in the deep-water Black Sea in October 2019. The biomass of the ctenophore in the open areas of the sea was 100–200 g m–2, the abundance was 16–38 ind. m–2. With the dominance of large individuals (≥30 mm) in the population, the activity of their reproduction remained extremely weak. The specific daily ration varied from ~4% to 13% of body carbon in adults and juveniles, respectively, and exceeded the minimum food requirements of the ctenophore. The daily consumption of three species of copepods (Acartiaspp.,Calanus euxinus,Oithona davisae) byM. leidyireached 4.5–11% of their biomass, while that of appendiculariansOikopleura(Vexillaria)dioica– 1.6%. The predatory pressure ofM. leidyion the entire prey zooplankton attained 2–4% of the biomass of organisms per day.

The article presents data assessing the impact of climate and warming in the Black Sea region on the interannual dynamics of the biomass of macro- and mesoplankton at the outer shelf of the Sevastopol Bay of the Black Sea in 2002-2021. Over these years, the biomass of organic matter accumulated by gelatinous species has decreased, while that of mesozooplankton has increased by 15–20%. Four significant seasonal periods are identified for the development of mass macro- and mesoplankton in the sea. Winter warming, which minimizes convective mixing of waters and the transport of nutrients into the zone of active photosynthesis, negatively affected the populations of the jellyfish Aurelia aurita, the dinoflagellate Noctiluca scintillans, and the chaetognath Parasagitta setosa. The increase in sea surface temperature (SST) in January-March was perceived positively by microphages such as the copepods Paracalanus parvus and Oithona similis and the appendicularian Oikopleura dioica. An early warm spring, with an increase in SST in April-May and a synchronous improvement in the biogenic regime of the upper pelagic zone, led to an increase in the biomass of all the largest copepods (Calanus euxinus, Paracalanus elongatus, Acartia clausi and Centropages ponticus), albeit with temperature tolerances. The extreme increase in SST in July–August was accompanied by the early development of ctenophores Beroe ovata and Mnemiopsis leidyi, as well as small crustaceans, Penilia avirostris and Oithona davisae. In general, while indirect impacts of warming are evident, the direct influence of climate on macro- and mesoplankton communities on the outer shelf has maintained a relatively stable pattern over the last two decades. This resilience is attributed to the organisms' capacity to endure conditions in the deep tolerant zone. This study highlights the intricate response of Black Sea macro- and mesoplankton communities to regional warming, with notable shifts in biomass dynamics, seasonal patterns, and species-specific sensitivities, underscoring the complex interplay between climatic factors in the Black Sea.

On the basis of the results of cruises and long-term monitoring at the outer shelf of Sevastopol Bay, the interannual dynamics of the state of the population of the alien ctenophore Beroe ovata in the Black Sea for 2000–2021 was analyzed. It was established that, in recent years, the ctenophore was found in plankton earlier than usual, the period of its active pelagic life increased to 7–8 months, and the body length in juveniles decreased from 40 to 20–25 mm. Three seasonal periods are distinguished, which, owing to their temperature conditions, were especially significant for the species: January–February, April–May, and July–August. The high temperature of seawater in July–August usually contributes to an increase in abundance and biomass of the ctenophore, as well as a decrease in the average size. The period of active life of B. ovata depends on the warming of the sea in May, as well as on the biomass of Pleurobrachia pileus. The early appearance of B. ovata leads to faster suppression of Mnemiopsis leidyi, preventing it from reaching a high abundance and biomass in summer.

The state of the population of the invader Mnemiopsis leidyi (A. Agassiz, 1865), the concentration and composition of mesozooplankton, and the trophic relationships and predatory impact of M. leidyi on the prey populations in the western and eastern sectors of the open Black Sea were studied in July–August 2017. The average biomass of M. leidyi at the peak of its seasonal development in these areas was 92–258 g m–2. The food spectrum of M. leidyi was dominated by copepods Acartia spp. and Calanus euxinus (Hulsemann, 1984). Spatial differences in the quantitative development of M. leidyi and zooplankton, the intensity of food consumption, and prey grazing were found. A high rate of food consumption (specific daily ration of 45.50 ± 8.31% C of body) and intensive grazing of M. leidyi (up to ~50% of the biomass of Acartia spp. per day) in the western offshore part of the sea should be accompanied by a decrease in mesoplankton biomass. On the contrary, predation by M. leidyi did not significantly affect the plankton community in the east, where M. leidyi consumed about 6% of the biomass of Acartia spp.

The seasonal and spatial dynamics of the key trophic characteristics were studied (food spectrum, feeding rate, and predatory impact on mesozooplankton) for populations of the jellyfish Aurelia aurita (Linnaeus, 1758) and ctenophore Mnemiopsis leidyi A. Agassiz, 1865. The investigation was carried out during four cruises of the RV “Professor Vodyanitsky” in the shelf areas of Crimean Peninsula in January to October 2016. The area was divided into inner (depth of < 50 m) and outer (51–150 m) shelves. To study the food spectrum and feeding rate of gelatinous predators, the composition of food items in the gastric cavity was analyzed under a binocular microscope. Daily ration (R, mg C·ind.−1·day−1) was calculated by the formula: R = Bz × DT−1 × 24, where Bz is zooplankton biomass in the predator gastric cavity (mg), and DT is zooplankton digestion time (h). Predatory impact of gelatinous zooplankton was estimated by the values of daily ration and mesozooplankton biomass. Zooplankton was sampled with a Juday plankton net with mouth diameter of 38 cm and mesh size of 140 µm. Vertical net hauls were performed: at the inner shelf stations, from the sea surface down to the bottom; at the outer shelf stations, down to the boundary of the hydrogen sulfide zone (δt = 16.2 conventional units according to a Sea-Bird probe). In the samples fixed with 4 % formalin solution, zooplankton abundance, its taxonomic composition, and size–age structure were quantified by standard method. In the food spectrum of the jellyfish, seasonal differences were revealed: predominance of Bivalvia veligers in winter and spring and wide species composition of Crustacea and other groups of prey in summer. The feeding rates of the studied species were similar: specific daily rations in winter, spring, and autumn did not exceed tenth of a percent of the carbon content in the body. Both species fed at a maximum rate in summer on the outer shelf: the specific rations reached 12.9 and 5.1 % C of the body for the jellyfish and ctenophore, respectively. A. aurita and M. leidyi populations consumed 0.2 to 5 % of the fodder zooplankton biomass per day; it did not result in a drastic reduction in zooplankton abundance and provided favorable feeding conditions for small planktivorous pelagic fish.

Species composition, abundance, and distribution of gelatinous macroplankton in coastal and deep-sea areas of the Black Sea in November 2017 are discussed. Despite the fact that species composition of gelatinous macroplankton (medusa Aurelia aurita, ctenophores Mnemiopsis leidyi, Pleurobrachia pileus, and Beroe ovata) in 2017 did not differ from that in 2016, structural changes occured. In autumn 2017 A. aurita and B. ovata biomass was higher than in 2016 whereas the inverse difference was observed in their abundance values. Namely, A. aurita biomass was on average 514 g m(-2) and 634 g m(-2) in the outer shelf and deep-sea areas, respectively. Both parameters of M. leidyi and P. pileus declined in 2017 compared to the previous year. In addition, the share of large-sized individuals increased in 2017, while that of small-sized animals decreased, in comparison with 2016. The number of small individuals increased in P. pileus and decreased in M. leidyi, although their biomass before the seasonal appearance of B. ovata in plankton was close to that in 2016. The unprecedented increase in the medusae biomass in November 2017 could not be explained only by the competition with planktivorous ctenophores and may indicate alternative drivers controlling population dynamics of A. aurita in the Black Sea. A negative correlation was depicted between the Danube discharge in February-March and the rate of somatic growth of A. aurita.

The study reports the data on the distribution and predatory impact of the gelatinous macroplankton on mesozooplankton in the inshore waters of Crimea in April 2016. In the study areas, gelatinous macroplankton comprised Scyphomedusae Aurelia aurita (Linnaeus, 1758) and three ctenophore species (Mnemiopsis leidyi A. Agassiz, 1865, Beroe ovata Bruguire, 1789, and Pleurobrachia pileus (O. F. Müller, 1776). The biomass of A. aurita dominated everywhere and varied from ~ 62 to 330 g·m‑2 in different areas. The largest A. aurita biomass was observed in the South Coast of Crimea and the smallest in Karkinitsky Bay, where the number of jellyfish was high, suggesting the predominance of small individuals in the population. The size structure of A. aurita population differed by region: the proportion of large animals increased from the West to the East. The abundance and biomass of M. leidyi were rather low: 0.33–1.45 ind·m‑2 and 1–51.7 g·m‑2, respectively (the exception was the position in the Kerch Strait, where M. leidyi abundance reached 2 ind. m‑2 and biomass 126.3 g·m‑2); that species occurred only at 33–45 % of the stations (the exception was Karkinitsky Bay – 17 %), with the maximum values in the South Coast of Crimea. Large, 55–70-mm, adult individuals predominated. P. pileus biomass was lower than the biomass of A. aurita and M. leidyi in all areas, but its abundance was much higher. The daily rations of A. aurita varied widely both in the entire area and from one region to another. The daily ration values correlated with the carbon content in A. aurita body. The predation pressure of A. aurita on zooplankton in the inshore waters of Crimea was very low and did not result in a crucial decrease in mesozooplankton abundance. The effect of M. leidyi, even with its low abundance and biomass, was much stronger

Experiments on the feeding of the Moon jellyfish Aurelia aurita have shown that digestion time of zooplankton typically varies between 2.2 and 5.1 h depending on the body weight of the predator, diet composition, amount of food, and seawater temperature. The daily ration of a Moon jellyfish with a wet body mass of 1 g reached 0.025-2.845 mg zooplankton.ind-1.day-1 corresponding to a carbon-specific food uptake of 0.2-9.1% C day-1 with a mean value around 0.5% C day-1. Such mesozooplankton consumption rates are consistent with the ration values (around 0.9% C day-1) calculated from the feeding rate experiments of jellyfish at natural food concentrations. However, in both cases the amount of mesozooplankton was insufficient to compensate for the minimum food requirements, calculated from oxygen consumption (6.7 ± 0.6% C day-1) of the jellyfish. On average, the metabolic expenses of A. aurita were about one order of magnitude greater than its ration supplied by mesozooplankton, indicating the important role of visually overlooked food components in the jellyfish diet. Résumé : Écologie trophique et estimation de l’impact lié à la prédation par la méduse commune Aurelia aurita (Linnaeus, 1758) sur le zooplancton de la Mer Noire. Des expériences de nutrition menées sur la méduse Aurelia aurita ont montré que la durée de digestion du zooplancton varie entre 2,2 et 5,1 h, selon la masse du prédateur, la nature et la quantité de la nourriture, et la température de l’eau de mer. La ration journalière d’une méduse de masse 1 g est de 0,025-2,845 mg zooplancton.ind-1.jour-1, ce qui correspond à une absorption de carbone de 0,2-9,1% C jour-1, avec une valeur moyenne de 0,5% C jour-1. Ces taux de consommation du mésozooplancton sont en adéquation avec ceux (environ 0,9% C jour-1) calculés à partir d’expériences de nutrition de méduses menées à des concentrations naturelles de nourriture. Néanmoins, dans ces deux cas, la quantité de mésozooplancton est insuffisante pour compenser les besoins nutritionnels calculés à partir de la consommation d’oxygène (6,7 ± 0,6 % C jour-1) de la méduse. En moyenne, les dépenses métaboliques d’A. aurita sont supérieures d’environ un ordre de grandeur à la ration fournie par le mésozooplancton, ce qui suggère que d’autres sources de nourriture, négligées car moins visibles, pourraient jouer un rôle important dans le régime alimentaire de cette méduse.

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.

Сообщество желетелого макропланктона Чёрного моря обновляется ежегодно и функционирует по-разному в зависимости от сложной системы трофических отношений и межгодовых колебаний биотических и абиотических факторов. Мониторинг этого сообщества необходим для оценки современного состояния и векторов возможной эволюции всей пелагической экосистемы. С этой целью в ходе планктонных работ по программе 89-го рейса НИС «Профессор Водяницкий» (сентябрь ― октябрь 2016 г.) в прибрежных и открытых районах моря к югу и юго-западу от полуострова Крым на 62 станциях собраны данные по составу, численности, биомассе и распределению массовых видов желетелых. Пробы отбирали сетью Богорова ― Расса (площадь входного отверстия ― 0,5 м², ячея ― 300 мкм) методом тотальных вертикальных ловов от дна до поверхности моря в области мелководного шельфа и от нижней границы кислородной зоны [по данным зонда CTD SBE plus (Sea Bird)] до поверхности моря ― в его глубоководной части. Сцифомедуза Aurelia aurita (Linnaeus, 1758) и три вида гребневиков (Mnemiopsis leidyi A. Agassiz, 1865; Pleurobrachia pileus (O. F. Müller, 1776); Beroe ovata Bruguière, 1789) присутствовали совместно на 56 станциях. В глубоководных районах моря отмечено увеличение средней биомассы каждого из видов (A. aurita ― до 260 г·м−2). Достоверных различий между показателями 2016 и 2010 г. не обнаружено (p > 0,05), при этом численность медузы возросла почти на порядок (p < 0,001), P. pileus ― в 2–5 раз (p < 0,001), B. ovata ― в 3–15 раз (p < 0,01). Более высокая численность M. leidyi наблюдалась лишь на внешнем шельфе (p < 0,01); в других районах моря её увеличению могло помешать хищничество B. ovata. Скорость соматического роста медузы в 2016 г. была одной из самых низких за последние 15 лет. Опережающий рост численности желетелых по отношению к их биомассе свидетельствует о тотальном уменьшении размеров особей как в популяциях гребневиков, так и в популяции медузы. Низкая скорость соматического роста медузы указывает на неблагоприятные условия существования этого вида в 2016 г. Тем не менее из-за высокой численности особей весеннего поколения биомасса медузы была выше, чем в прежние годы. Вследствие ослабления конкуренции со стороны гребневиков-планктофагов, в перспективе на ближайшие годы следует ожидать ещё большего увеличения экологической валентности A. aurita среди желетелых хищников.

Сообщество желетелого макропланктона Чёрного моря обновляется ежегодно и функционирует по-разному в зависимости от сложной системы трофических отношений и межгодовых колебаний биотических и абиотических факторов. Мониторинг этого сообщества необходим для оценки современного состояния и векторов возможной эволюции всей пелагической экосистемы. С этой целью в ходе планктонных работ по программе 89-го рейса НИС «Профессор Водяницкий» (сентябрь ― октябрь 2016 г.) в прибрежных и открытых районах моря к югу и юго-западу от полуострова Крым на 62 станциях собраны данные по составу, численности, биомассе и распределению массовых видов желетелых. Пробы отбирали сетью Богорова― Расса (площадь входного отверстия ― 0,5 м2, ячея ― 300мкм) методом тотальных вертикальных ловов от дна до поверхности моря в области мелководного шельфа и от нижней границы кислородной зоны [по данным зонда CTD SBE plus (Sea Bird)] до поверхности моря ― в его глубоководной части. Сцифомедуза Aurelia aurita (Linnaeus, 1758) и три вида гребневиков (Mnemiopsis leidyi A.Agassiz, 1865; Pleurobrachia pileus (O. F. Müller, 1776); Beroe ovata Bruguière, 1789) присутствовали совместно на 56 станциях. В глубоководных районах моря отмечено увеличение средней биомассы каждого из видов (A. aurita― до 260 г·м−2). Достоверных различий между показателями 2016 и 2010 г. не обнаружено (p > 0,05), при этом численность медузы возросла почти на порядок (p < 0,001), P. pileus ― в 2–5 раз (p < 0,001), B. ovata ― в 3–15 раз (p < 0,01). Более высокая численность M. leidyi наблюдалась лишь на внешнем шельфе (p < 0,01); в других районах моря её увеличению могло помешать хищничество B. ovata. Скорость соматического роста медузы в 2016 г. была одной из самых низких за последние 15 лет. Опережающий рост численности желетелых по отношению к их биомассе свидетельствует о тотальном уменьшении размеров особей как в популяциях гребневиков, так и в популяции медузы. Низкая скорость соматического роста медузы указывает на неблагоприятные условия существования этого вида в 2016 г. Тем не менее из-за высокой численности особей весеннего поколения биомасса медузы была выше, чем в прежние годы. Вследствие ослабления конкуренции со стороны гребневиков-планктофагов, в перспективе на ближайшие годы следует ожидать ещё большего увеличения экологической валентности A. aurita среди желетелых хищников. Ключевые слова: желетелый макропланктон, гребневик, медуза, биомасса, пространственное распределение, межгодовые изменения, Чёрное море

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.

В прибрежной акватории Юго-Западного Крыма летом 2013 г. изучены видовой состав и количественные показатели мезо-, макро- и ихтиопланктона, а также питание личинок рыб. В ихтиопланктоне обнаружены икра и личинки 33 видов рыб из 24 семейств, что вдвое превысило показатели 1990-х гг. В мае 2013 г. в прибрежной акватории Севастополя наблюдался результативный нерест хамсы, которому способствовали благоприятная температура воды в море (+21 °C) и хорошие кормовые условия. Наибольшее количество летненерестующих видов зарегистрировано в июне при температуре поверхностной воды +22,7 °C. Численность икры рыб в 2013 г. увеличилась по сравнению с таковой в 2000 г. в 3,4 раза, личинок ― в 2,3 раза, что связано с их большей выживаемостью при улучшившихся условиях питания в результате относительно высокой численности зоопланктона с преобладанием мелкоразмерной фракции и доминированием в ней ювенильных стадий копепод. Другими факторами, улучшившими состояние кормовой базы личинок, были низкая численность гребневика Mnemiopsis leidyi A. Agassiz, 1865 и его слабое воздействие

The gelatinous macroplankton community of the Black Sea renews annually and functionates differently depending on the complicated system of trophic relations and interannual fluctuations of biotic and abiotic factors. Its monitoring is necessary for the evaluation of the current state and possible vectors of evolution of the whole pelagic ecosystem. For this purpose the data on the composition and distribution of gelatinous organisms were collected on the 89th cruise of the RV “Professor Vodyanitsky” (September – October of 2016) at 62 stations located in coastal areas and in the open sea to the south and southwest of Crimea. Samples were taken with the Bogorov – Rass net (inlet area of 0.5 m², mesh of 300 µm) using vertical net hauls from the bottom to the sea surface in the shallow shelf and from the lower boundary of the oxygen zone [according to the dissolved oxygen sensor CTD SBE plus (Sea Bird)] to the sea surface – in deepwater areas. Aurelia aurita and 3 ctenophore species (Mnemiopsis leidyi, Pleurobrachia pileus, and Beroe ovata) were present together in the 90 % of the samples (on 56 stations). In spite of some increase in average biomass of every species in deep sea areas, and, particularly, of A. aurita up to 260 g per m², there was no difference between the data of 2016 and 2010 (p > 0.05). The abundance of A. aurita increased by about one order of magnitude (p < 0.001); the abundance of P. pileus increased 2–5 times (p < 0.001); of B. ovata – 3–15 times (p < 0.01). M. leidyi abundance increased only at the outer Black Sea shelf; in other regions it could have been restricted by B. ovata predation. A. aurita linear specific growth rate in 2016 was one of the lowest for the last 15 years. Slow somatic growth of the jellyfish indicates unfavorable conditions for this species in 2016. However, the jellyfish biomass was higher than that of previous years because of the huge abundance of the spring generation. The weakening competition with planktivorous ctenophores may lead to even greater increase in the ecological valence of A. aurita among other gelatinous predators in the nearest future.

Species composition and quantitative indicators of zoo- and ichthyoplankton feeding of fish larvae were studied inshore of the South-Western Crimea in the summer of 2013. There were 33 species of fish eggs and larvae from 24 families identified in ichthyoplankton. It was twice as large as it was in the 1990s. Inshore of Sevastopol in May 2013 the anchovy spawning was efficiently spawned due to favorable temperature of the sea water (+21 °C) and good food conditions. The largest number of summer-spawning species was recorded in June at the surface water temperature of +22.7 °C. Abundance of fish larvae in inshore region increased too, compared to this of the 2000s, due to their high survival rate resulting from improved food conditions. There was relatively high abundance of zooplankton and the predominance of a small-sized fraction mainly by juvenile stages of copepods. Low abundance of ctenophore Mnemiopsis leidyi and its slight impact on zooplankton were other factors that improved the food base of fish larvae.