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© 2016, Springer Science+Business Media Dordrecht. In the north-eastern part of the Black Sea, the seasonal complexes of dominant species of phytoplankton were fixed: small-celled diatom (spring), coccolithophores (late spring, early summer) and large diatoms (summer and autumn). In May–June 2005 and 2006, two invasive species of marine diatoms Chaetoceros throndsenii (maximal abundance 1.92 × 105 cells/l) and Chaetoceros minimus (1.6 × 105 cells/l) were recorded. These species have been incorporated in the complex of the late spring and early summer and grew simultaneously with the coccolithophore Emiliania huxleyi. The coccolithophore was dominant species, whose abundance had reached the level of a bloom. C. throndsenii was observed over the entire area from the coast to the centre of the sea; C. minimus were recorded at coastal stations and only at two stations of the open sea. Stratification of the water mass and the low (below the Redfield) ratio of nitrogen to phosphorus were observed at this time. Then, C. throndsenii was not marked even once, and C. minimus has been registered once on a shelf in June 2011. Experimental studies (2005) have shown that intensive growth C. throndsenii requires the simultaneous addition of nitrogen and phosphorus in a ratio close to the Redfield ratio. C. minimus shows the intensive growth only at high concentrations of phosphorus and at low nitrogen-to-phosphorus ratio (the experiment was carried out in June 2011). Mathematical modelling shows that C. throndsenii and Emiliania huxleyi form a stable couple whose growth is limited by different factors: the diatoms by nitrogen and the coccolithophores by phosphorus. C. minimus might not be able to form a stable couple with coccolithophores because they have the same limiting factor, i.e. phosphorus. However, this species could become the dominant one, if low (0.5–1) nitrogen-to-phosphorus ratio conditions were stable for more than 2 months. However, this scenario is improbable in natural circumstances since the existence of this complex seasonal rarely exceeds 1.5 months.

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Aquatic Ecology

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