Long-termfield observation carried out in northeastern (NE) part of the Black Sea in 2005–2010, revealed strong
environmental control over phytoplankton community structure, in particular the relationship between the diatoms
and coccolithophores. We followed the changes of phytoplankton community structure in marine water
every year from May to June. An intense bloom of coccolithophores Emiliania huxleyi was observed in 2005–
2006 when the nitrogen in marine water declined and the N/P ratio dropped below the Redfield proportion. In
the 2007–2008, N/P ratio increased above the Redfield stoichiometry leading to the domination of the diatoms
in phytoplankton community. The predominance of E. huxleyi was observed again in 2009–2010 and again it
was associated with the decrease of the N/P ratio. To confirm the observation correlation between community
composition and themineral nutrient concentrationswe undertook laboratorymicrocosmstudieswith addition
of various amounts of a nitrogen and phosphorus sources tomarine water. It was found that decreased nitrogen
level and respective lower N/P stoichiometric ratiowere essential precondition for coccolithophores domination.
Under increased concentration of nitrogen and phosphorus near and above the Redfield stoichiometry, always
encouraged the development of diatoms. We conclude that at least in the NE-part of the Black Sea, the composition
of spring and summer phytoplankton is under strict environmental control by availability of mineral
nutrients.