Emiliania huxleyi blooms in the Black Sea
 

The SeaWIFS mean normalized water-leaving radiances data for 1998-2002 suggest that the Black Sea consistently experiences high reflectance patches of coccolith platelets throughout the basin each year during the May-July period. For all these years, the major bloom activity attains its most intense and widest coverage during June, and diminishes gradually within the first half of July. Although the Black Sea is masked by clouds, the data also suggest enhanced coccolith activity for some period during autumn and early winter. The in situ data collected during the summer-autumn 1998 and early winter 1999 periods within the interior part of the western basin provide an independent support for the bloom occurence, and more importantly establish validity and reliability of the SeaWIFS coccolith detection algorithm.

SeaWIFS true color images of the Black Sea during (a) 13 June 2000 (upper), (b) 15 June 2002 (middle), (c) 4 June 2001 (lower). Coccoliths are marked by the turquoise color in the upper and middle images. The distinct color change in the lower image corresponds to the conditions of no dense coccolith development.

Distribution of the monthly composite coccolith coverages for June of (a) 1997, (b) 1998, (c) 1999, (d) 2000, (e) 2001 and (f) 2002 are shown in the figure on the right. The regions in white depict absence of coccoliths. Black represents the cloud coverage. Except June 2001, coccolithophore bloom events emerge as a robust signature of the annual phytoplankton structure each summer during the 1997-2002 period.

A one-dimensional, depth-resolving, coupled physical-ecosystem model is used to identify factors causing blooms of the coccolithophore Emilania huxleyi in the Black Sea, regularly observed during the early summer periods. The model specifically applies for the interior basin away from the coastal zone, where a more idealized ecosystem structure is represented in the form of four groups of phytoplankton (diatoms, dinoflagellates, E. huxleyi, and a small phytoplankton group with size <5µm), two groups of zooplankton (microzooplankton and  mesozooplankton), further accompanied by simplified nitrogen and phosphorus cycles. The model parameters are optimized through a set of sensitivity experiments in order to generate a reasonably realistic simulation, predicting the observed annual plankton structure.

The simulations show that the internal trophodynamic conditions in the Black Sea support E. huxleyi bloom development during May-July period as a part of the seasonal phytoplankton succession (shown by the contours red color in the figure). It forms after a diatom-dominated bloom in March and dinoflagellate-dominated bloom in April under nitrogen depleted conditions by making use of regenerated nutrients available in the surface mixed layer. E. huxleyi growth occurs at the temperature and salinity ranges of 25^oC, 18psu at the surface to 8^oC, 19psu at the base of the seasonal thermocline at about 20 m. Picophytoplankton accompanies them below the mixed layer throught the summer and constitutes later in November the autumn bloom.

The top-down grazing pressure is found to impose the most delicate control on timing and intensity of E. huxleyi bloom.  Small changes in zooplankton grazing preferences on E. huxleyi and picophytoplankton can introduce considerable modifications on relative proportions of their biomass.  Their ability to grow at high light conditions does not give E. huxleyi a competitive advantage in the Black Sea, since they start flourishing at moderate light conditions in May due to their higher growth rate as compared to picophytoplankton, and the other groups (diatoms and dinoflagellates) have already bloomed and their further growth is suppressed by their mortality and grazing pressure from zooplankton.