Environmental control on phytoplankton community structure in the NE Black Sea

Long-term field 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 the mineral nutrient concentrations we undertook laboratory microcosm studies with addition of various amounts of a nitrogen and phosphorus sources to marine water. It was found that decreased nitrogen level and respective lower N/P stoichiometric ratio were 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.