The Black Sea autumn bloom of the diatom Hemiaulus hauckii Grunow ex Van Heurck: peculiarities of the phytoplankton’s structural and functional parameters

Context The fixation of atmospheric nitrogen in the sea is an important source of new nitrogen to the ocean surface waters, stimulating phytoplankton productivity. Diazotrophic cyanobacterium R. intracellularis in association with the large diatom Hemiaulus hauckii Grunow ex Van Heurck is often responsible for nitrogen fixation. Aims To examine the peculiarities of the phytoplankton structural and functional traits during the autumn diatom–diazotroph association (DDA) Hemiaulus–Richellii bloom in the Black Sea. Methods A cruise was conducted aboard the Research Vessel ‘Professor Vodyanitsky’ in October 2022. The process involved the use of light microscopy, hydrochemical and hydrobiological methods, and model calculations. Key results In the Black Sea, we recorded the diatom–diazotrophic association Hemiaulus–Richellii bloom for the first time. Conclusion The DDA probably triggered the unusually high values of primary production in the autumn period. Implication The obtained results are important for the development of a modern model of the functioning of the Black Sea ecosystem.