The results of a long-term study of the interrelated changes (IRC) in morphological and taxonomical structures of phytoplankton (PP) in the coastal zone of the Black Sea near Odessa are presented. The IRC were analyzed using the indices of specific surface S/W and the characteristics of the relative abundance of taxonomic groups forming the PP communities. The study used a methodical approach in the search for key events during the annual cycles of PP development. Key events are cases when the sequence of distribution of S/W values of taxonomic groups in the natural PP community deviated from the range established on the basis of indices generalized for each taxonomic group. In parallel with changes in morphological structure, quantitative parameters of different taxonomic groups also changed. It was revealed that, in certain periods, the most abundant taxonomic groups give way to a dominant position in the community to accompanying groups, which make up the largest contribution to the quantitative indicators of phytoplankton. The indices of S/W calculated for Dinophyta, Bacillariophyta, and Cyanophyta dominating in the PP communities of the Odessa Coast during different annual cycles showed minimal, average, and maximum values. Accompanying taxonomic groups becoming dominants demonstrate S/W values distributed in the range of S/W values of dominants. An analysis of 72 cases of replacement of the dominant taxonomic groups during the period of study, revealed three main scenarios of this process. The IRC in the morphological and taxonomical structures of the PP community are considered as a mechanism of its structural self-organization regulating the intensity of the production process in response to changes in environmental conditions.
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