4.3. Results and discussion

The mean monthly distributions of chlorophyll concentration and the particle backscattering coefficient bbp in the Black Sea from January 1998 to December 2010 are presented on the color maps; the mean monthly distributions of the yellow substance absorption coefficient ag for these years are given on these maps only for warm season (May-September). As seen, the northwestern part of the sea exposed to the influence of the runoff of the Dnieper, Dniester, Bug, and Danube rivers (regions 1 and 2) is distinguished by enhanced values of all of the considered characteristics. A clearly manifested impact of the Caucasian and Turkish rivers is restricted by a narrow near-shore band along the eastern and southern coasts of the sea.

The influence of the river runoff from the northwestern coast mostly extends southward along the coast; the transverse propagation is significantly weaker. The latter is mainly implemented by the mesoscale eddies permanently observed in the Black Sea.

Fig. 16, 17, 18 show a variability of the monthly means of the bio-optical characteristics in different regions. A similarity between seasonal changes of all characteristics is observed in the regions #1-3 being under strong influence of the river run-off. The result is easy to understand: rivers bring to a coastal zone both particulate and dissolved matter as well as nutrients which stimulate primary production and increasing chlorophyll concentration.

The variations of Chl values in the regions #6, 7 are weakly connected with the ones in the western regions; they correlate with each other (#6 and #7), and Chl in the region #7 correlate with Chl in the region #8. The latter can be explained by similarity of natural conditions governing by variability of primary production and chlorophyll concentration in the central and eastern regions of the Black Sea.

According to Demidov [33], who analyzed seasonal variability of chlorophyll concentration in the Black Sea since 1960 to 1997, there were three maxima observed in its seasonal changes: in winter-spring (January-March), summer (June-August) and autumn (October-November). The satellite data presented in Fig. 16, are in agreement with that, but they show that number of seasonal maxima and the months with the maxima can be changed. In the region # 2 the principal maximum was most frequently observed in June, but they also occurred in other months (for example, in January in 2005). In the open Black Sea they were observed in October-December with exception of 2001 in June (# 6) and in May (# 7). Magnitudes of the Chl principal maxima and minima in the open sea, excluding 2001, were rather stable.

It is a surprising result that the bbp variations in the open parts and in the coastal regions are almost synchronous. The enhanced values of bbp covering the whole basin are observed repeatedly in June; this problem is discussed in 4.4.

A variability of the monthly means of SST is shown in Fig.19. As it is demonstrated in [8], changes in winter SST influence an intensity of spring phytoplankton bloom and the June maximum of the particle backscattering coefficient from sea surface temperature.

Inter-annual variability

The seasonal (for chlorophyll concentration and the yellow substance absorption coefficient) and annual (for the particle backscattering coefficient) mean values with their standard deviations in different regions of the Black Sea are given in Table 3.

It should be noted that the mean values of chlorophyll concentration almost in all sub-regions were minimal in 2003, and they increased again in 2004. It is also true for the yellow substance absorption coefficient. As for the particle backscattering coefficient, the minimal mean annual values in the most sub-regions were also observed in 2003 or 2004.