Caspian Sea

Satellite data


fig19
By convention, the Caspian Sea is divided into the Northern (1), Middle (2) and Southern (3) regions (Fig.25). The Northern Caspian is the shelf area which depth is less than 10 m. This region is under strong influence of river run-off; the largest European river Volga flows into the Northern Caspian, the Ural and Terek rivers too. About 90% of the total river discharge in the Caspian Sea falls within the Northern region.

The Middle Caspian is also influenced by the Volga river run-off. The Volga waters move mainly south along the western coast of the Caspian Sea and turn eastward near Apsheron forming cyclonic circulation in the Middle Caspian. The other part of the Volga waters comes into the Southern Caspian; this region is also influenced by the run-off of the Kura river and the rivers flowing down the Iranian coast.

Fig.26 shows the monthly coverage of the regions in the Caspian Sea by SeaWiFS data and MODIS-Aqua data in 1998-2010. Kara Bogaz Gol, a very specific gulf with salinity of 300‰, is not considered here. The coverage of less than 50% occurred in cold months in the Northern Caspian. Although such data were included in data processing and further analysis, the obtained results for these cases cannot be considered as reliable.


Algorithms

The chlorophyll
Chl and suspended matter TSM concentrations, the particle backscattering coefficient bbp and the absorption coefficient of yellow substance ag were calculated in the Caspian Sea. The used algorithms were not changed. They were derived by using new in situ data obtained in the summer expeditions of 2008-2009 in the northern part of the Caspian Sea (Rybakova et al. 2011).

Chlorophyll concentration

The regression equation for calculation of chlorophyll concentration from SeaWiFS data:

Chl = 0.732·[LNW(555)/LNW(510)]3.40,                                                         (17)


where 
LNW(555) and LNW(510)] are the normalized water-leaving radiances at 555 and 510 nm (n = 55, r2 = 0.708, the error is equal to ±0.33 mg/m3).

The algorithm to derive chlorophyll concentration from MODIS-Aqua data:

Chl = 0.573 (RRS(488)/RRS(551)-2.39.                                                         (18)


where 
RRS(488) and RRS(547) are the above-surface remote-sensing reflectance at 488 and 547nm, respectively (n = 55, r2 = 0.638, the error is equal to ±0.41 mg/m3).

Particle backscattering and TSM

The particle backscattering coefficient 
bbp was derived from the remote sensing reflectance corrected to the bottom reflection (Kopelevich et al. 2007). TSM concentration was calculated by using the regression equation between TSM and the particle backscattering coefficient bbp :

TSM = 67.1 bbp + 0.554,                                                                        (19)


where 
TSM is in mg/l , bbp in m-1 (n=42, R2 = 0.835, the regression error is 0.81 mg/l).

Yellow substance absorption

The values of ag were calculated with the semi-analytic algorithm as described in the section Black Sea, Algorithms. The algorithm worked stably only for the May-September period so the data obtained out of this season are not very reliable.


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 2009.


Fig.27, 28, 29, 30 shows variability of the monthly means of Chl, bbp, ag, and TSM concentration in the Northern, Middle and Southern Caspian from 1998 to 2012. 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 (
River estuaries in the Caspian Sea 2001), 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.27 shows a sharp increase of Chl in the Southern Caspian in July-August 2001 attributed to a consequence of invasion of the ctenophore Mnemiopsis leidyi (Kopelevich et al. 2004). 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/m3) are seen on the color maps in the Southern and Middle Caspian in August-October 2006-2010. It is interesting that the shift of the maximum is observed in 2009-2010 to September-November (see Fig.27).

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.31).


In Table 4 the annual means of Chlbbp, agTSM, and SST with their standard deviations are given for the Northern, Middle and Southern Caspian from 1998 to 2012 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 0.43 mg/m3 in 1998 to 1.71 mg/m3 in 2001 (the second top value was in 2009 – 1.07 mg/m3). In the Northern Caspian the changes were from 1.34 mg/m3 in 2003 to 1.78 mg/m3in 1999 and 2009, and 1.95 mg/m3in 2012.