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¹ Southern Seas (IBSS) National Academy of Sciences of Ukraine, A.O. Kovalevsky Institute of Biology, Sevastopol, Nakhimov Av., 2, Ukraine DOI : 10.4194/1303-2712-v14_4_15 Viewed : 5141 - Downloaded : 3903 For the first time microzooplankton grazing impact on phytoplankton in the Black Sea was investigated in relation to seasonal and spatial variability. In this work we used scientific evidence obtained in the experiments conducted in the Sevastopol Bay (2006-2010) and in field during the Black Sea expeditions of the R/V "Vladimir Parshin" (September-October 2005) and the R/V "Professor Vodyanitsky" (May 2013). In the Sevastopol Bay the ratio between microzooplankton grazing impact and specific rate of phytoplankton growth (g/μ) yielded the annual average of 65% that agrees with the annual average for phytoplankton primary production consumed by microzooplankton. It was found that, irrespective of the location, with sufficient quantity of nutrients in the stratified sea water, phytoplankton blooms began to form as the ratio between microzooplankton grazing and phytoplankton growth rates (g/μ) was ≤75%; as the bloom was ceasing, the ratio was evaluated >100%. Therefore, we suppose, that initiation of phytoplankton blooms is possible when conditions are favorable for rapid phytoplankton growth and the ratio of grazing to growth (g/μ) will be <75%. Keywords : Phytoplankton bloom, growth rate, microzooplankton grazing, Black Sea