Relationship between the abundance of small pelagic fishes and the phytoplankton biomass as an indicator of the state of the pelagic ecosystem of the Black Sea

Abstract

Since early 1970s, the Black Sea pelagic ecosystem has been experienced severe impact caused by numer� ous anthropogenic factors, which have significantly changed its state [1–3]. Among the latter, eutrophica� tion was of great importance. It has altered the tropho� dynamics of the pelagic zone owing to an increase in the phytoplankton abundance and biomass and a sig� nificant change in its species composition. For exam� ple, the proportion of diatoms, which account for over 90% of the total phytoplankton abundance on the Black Sea shelf, has significantly reduced. As a result, dynoflagellates and small species, primarily coccoli� thophorids and small flagellates became dominants [4]. Such changes in the Black Sea pelagic zone affected both the entire structure of its food web, including the upper trophic layers of the ecosystem (primarily the stock of small pelagic fishes), and its state in general. On the one hand, the primary produc� tive base in the ecosystem has increased; on the other hand, qualitative changes in the primary producers of the trophic web resulted in a decrease in the ecological efficiency of utilization of matter and energy in the Black Sea ecosystem on the whole. This, in turn, has disturbed its resistance to adverse impacts [1, 5]. To clarify the relationship between the primary and ter� minal trophic components in the Black Sea pelagic