State and Dynamics of Ctenophores Mnemiopsis leidyi and Beroe ovata Populations on the Crimean Shelf of the Black Sea in 2013–2021

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.