5.3. Results and discussion

The color maps show that the Northern Caspian is distinguished by enhanced values of all of the bio-optical characteristics. As mentioned above, in shallow waters of the Northern Caspian the bottom reflectance was taken into account. High values of chlorophyll concentration are seen in the Southern Caspian in summer 2001-2002 and then in 2005-2010.

Fig.25, Fig.26, Fig.27, Fig.28 shows variability of the monthly means of Chl, bbp, ag, and TSM concentration in the Northern, Middle and Southern Caspian from 1998 to 2010. The significant correlation was found between changes of the Chl, bbp (TSM) and ag values within each of the sub-regions, especially in the Northern Caspian. These results testify that the formation and variability of the bio-optical characteristics in each sub-region are mainly determined by processes within it, although the interrelationships between the sub-regions certainly exist. In the Northern Caspian seasonal variations of all characteristics are strongly influenced by variability of the river run-off. The influence of the run-off of Volga and other rivers flowing into the Northern Caspian is extended throughout the region due to cyclonic circulation there.

According to [39], waters of low salinity, formed under influence of the Volga river input, reach the Middle Caspian near Makhachkala in September-October. Significant correlation was found between variations of the bbp values in the Middle and Northern regions, but there was no correlation between variations of the Chl values as well as between the ag values.

A significant correlation (with a statistical probability more than 0.999) was found between variations of the Chl values in the Middle and Southern Caspian.

Fig.25 shows a sharp increase of Chl in the Southern Caspian in July-August 2001 attributed to a consequence of the ctenophore Mnemiopsis leidyi [2]. In 2002 the means of hl in the Southern Caspian decreased appreciably as compared with 2001 but they stayed higher than in 1998-2000. In 2003-2004 a level of chlorophyll concentration in the Southern Caspian was lower than in 2002, but it increased again in 2005 and it remained so up to 2010.

In 2005 anomalous algal bloom (AAB) occurred in the Southern Caspian in August-September (see color maps); the algae responsible for the AAB was cyanobacteria Nodularia; the floating alga layer was tens centimeters thick

(http://www.caspianenvironment.org/newsite/Caspian-AAB.htm).

The areas with high chlorophyll concentration (more than 2 mg m-3) are seen on the color maps in the Southern and Middle Caspian in July-October 2006-2010. It is interesting that the shift of the maximum is observed in 2009-2010 to September-November (see Fig.25).

2009 stands out for the highest Chl values in the Southern and Middle Caspian among the last few years. It may be noted that this year also stands lower values of the summer-autumn SST (see Fig.29).

In Table 4 the annual means of Chl, bbp, ag, TSM, and SST with their standard deviations are given for the Northern (N.C.), Middle (M.C.) and Southern (S.C.) Caspian from 1998 to 2010 and for the whole period. It is seen that the highest inter-annual variability is observed for chlorophyll concentration, especially in the Southern Caspian where the annual means were changed from 0.48 mg m-3 in 1998 to 1.58 mg m-3 in 2001 (the second top value was in 2009 – 1.13 mg m-3). In the Northern Caspian the changes were from 1.29 mg m-3 in 2003 to 1.71 mg m-3 in 1999, 2007 and 2009.