Alternation of diatoms and coccolithophores in the north-eastern Black Sea: a response to nutrient changes

In many regions phosphorus limits coccolithophore growth, whereas nitrogen generally controls development of diatoms. We tested the hypothesis that a change in nutrient composition defines the alternation of these algae. Data on phytoplankton, nutrients, chlorophyll and primary production were obtained in May-June from 2002 to 2012. Coccolithophore bloom dynamics were analysed using satellite images of particular inorganic carbon (PIC). In some years coccolithophore bloom occupied the sea interior and has spread on shelf areas. Most frequently blooms occurred in the coastal waters and were absent in the deep basin. Diatoms and coccolithophores interchangeably prevailed in phytoplankton. In the coastal waters, high biomass of diatoms corresponded to the increased ammonium. In the deep basin, PIC was positively correlated with phosphate and negatively with the sea air temperature in February indicating that the bottom-up flux of pycnocline waters during winter convection is the main driver of coccolithophore bloom. In the coastal zone, high concentrations of phosphate and PIC corresponded to high precipitation periods. Thus, independent from origin, phosphorus might have determined the development of coccolithophores both in the coastal and deep waters. In general, diatoms predominated in phytoplankton at high nitrogen:phosphorus ratio, whereas coccolithophores at low.