Reasons for the appearance and preservation of boreal pelagic biota in the Black Sea

The work is devoted to the question of describing the uniqueness of the ecosystem of the Black Sea, which has preserved in its depths a complex of boreal pelagic relics preserved since the last glacial epoch. The author of this paper suggests that the processes of the emergence of boreal biota in the Black Sea should be considered on the basis of the theory of the change of biocenoses of the Mediterranean Sea in glacial and interglacial periods due to changes in the directions of currents and the structure of water masses (Pérès, 1985). Currently, many generalizing works on the history of the Mediterranean biota have been published (Bianchi et al., 2011), which convincingly showed that the patterns of changes in the biodiversity of the entire Mediterranean basin are associated with the processes of periodic climate changes (glaciations and warming), sea level fluctuations, changes in currents and stratification of water masses, paleogeographic and ecological causes. Based on this, the author identifies the main stages of the transformation of the freshwater lake ecosystem into a boreal marine one, and then, about 3.3 thousand. years ago, into the modern ecosystem of the Black Sea, consisting of warm-water intruders living in the upper Black Sea water mass and relics preserved since the last glaciation in the Cold Intermediate Layer (CIL). The latter is a unique feature of the Black Sea, which allows it to preserve boreal species in this natural “refrigerator”. The author notes that the processes of global warming have already led to a gradual heating of the CIL core (Belokopytov, 2017), which in the future may lead to the gradual death of cold-water species, many of which are important in the trophic chains of the ecosystem.