4.4. Bio-optical characteristics of the Black Sea in June

Field and satellite studies, carried out in the north-eastern part of the Black Sea in 2004-2010, showed that the Chl values and spatial distributions of bio-optical parameters in June strongly differed from other seasons. There are two hypotheses to explain this phenomenon: (i) the particulate matter from the river runoff spreading to the whole basin across the boundary Rim Current by mesoscale eddies and via turbulent exchange; (ii) coccolithophore bloom covering the whole basin [34-35]. A lot of concurrent field measurements of water-leaving radiances LWN and seawater constituents (total suspended matter TSM, chlorophyll-a Chl and coccolithophoride concentrations Ncoc) were made. That allowed to develop regional algorithms for June (using data of ocean color scanner SeaWiFS). Following empirical relationships were obtained and used for computations in our previous CD [6]:

Chl = 0.73 [LWN (510)/LWN (555)]-2.73, n=81, r2=0.54, (11)

TSM = 72.4 bbp +0.11, n=48, r2=0.25, (12)

Ncoc = 768 bbp 1.55 , n=48, r2=0.54, (13)

where n is number of subsatellite points, r2 is coefficient of determination, Ncoc is coccolithophoride concentrations in 106 cells/l, bbp(550) is measured in m-1 and TSM is presented in mg/l.

Note that Chl algorithm for June differs from the algorithm described in 4.2. This can be associated with difference in phytoplankton composition in June and autumn.

The algorithm to derive chlorophyll concentration in June from MODIS-Aqua data uses the ratio of Rrs(531)/Rrs(547):

Chl = 0.89 [0.996Rrs(531)/Rrs(547)]–6.0. (14)

Below we present data for all area of the Black Sea, even though the algorithms were developed with data for the north-eastern part only. It should be noted that for other sub-regions the data presented are not reliable.

Fig. 20, 21, 22 show a variability of the June mean concentrations of Chl, TSM, and Ncoc in different regions from 1998 to 2010. One can see that correlation between inter-annual variability of Chl and TSM is not so strong whereas the TSM and Ncoc changes are very close to each other. This suggests that the June increasing TSM is mainly associated with coccolithophore bloom. But influence of the river run-off on the TSM values has been also found. The river run-off is displayed best by lowered values of salinity S, and essential anti-correlation between changes of surface salinity S and of the particle backscattering coefficient bbp was observed in 2004. In the next expeditions (2005-2006) correspondence between bbp and S changes was not so close. Essential differences in spatial distribution of suspended matter concentration were found in different years [36]. Recent results relating to the June coccolithophore blooms in the Black Sea are given in [8,19].