In the last two decades there has been a significant shift in the
hydrochemical regime of the water masses in Black Sea (Oguz and
Gilbert, 2007; Oguz and Velikova, 2010; Pakhomova et al., 2014;
Yakushev et al., 2010). In particular, the trophic status of the sea surface
in the NE Black Sea has significantly changed during the last two decades,
with a relatively stable decrease in inorganic phosphorus since
1995, and in dissolved inorganic nitrogen since 2001; however no
trend for silica has been observed since 2001.
Therefore, the concentration of nitrogen and phosphorus can be limiting
factors for the growth of different phytoplankton species in the
spring and early summer in the NE Black Sea, coinciding with the observed
maximum abundance of coccolithophores. We assume that the
concentrations of nitrogen and phosphorus, as well as their ratio are
key regulators of the diatom–coccolithophores system. In this study,
we combined the field surveys of biological and hydrochemical parameters
and laboratory experiments with nutrient addition to the natural
phytoplankton populations, to reveal the mechanisms of community
structure regulation in the NE Black Sea.