century against the background of the sharp Volga runoff rise and the
relevantly low sea level
and, consequently, the low volume of the North Caspian water (Fig. 1). A rapid growth of the
sea level throughout 1978-`1995 was accompanied by the decrease of
water renewal rate,
which was typical for the considered period as a whole, but in 1961-1977 and in 1996-2012
was not so pronounced. The main reason of reduction of the water renewal rate in these years
was the reduction of the Volga runoff.
Fig. 1 - Changes of the Caspian Sea level (L, cm) and the rate of North Caspian waters
renewal
(W/V. cm, text) in the period 1961-2012.
The data presented in Table 1 point to the gradual growth of phytoplankton abundance in the
western part of the North Caspian Sea during the flood period.
Phytoplankton abundance
growth in the period of 1996-2012 was accompanied by the growth of its biomass as
compared to the sea transgression period (1978 - 1995). However the highest value of
phytoplankton biomass was recorded during the period of low sea level (1961-1977), when the
large-cell diatomic algae dominated in the biomass formation (Caspian Sea..., 1985).
The abundance and biomass of zooplankton in the north-western part of the Caspian Sea
during the flooding periods of 1996-2012 were significantly higher than in 1961-1977, but the
highest values of zooplankton abundance, as well as its biomass (as per the median value)
were recorded in 1978-1995. There was a slight difference between the second (1977-1995)
and the third (1996-2012) periods as per average zooplankton abundance and biomass.
Moreover, the arithmetic mean of biomass in the third period was higher than that in the
second (due to rarely occurring
high values, i.e. "spotty" distribution).
Periods
Abundance
Biomass
Me
X
mean
Me
X
mean
Phytoplankton, m specimen/m
3
, mg/m
3
1961-1977
107.7
269.1
704
1590
1978-1995
161.2
377.9
274
638
1996-2012
197.6
450.5
573
1406
1961-2012
170.9
397.1
457
1089
Zooplankton, thousand specimen/m
3
, mg/m
3
1961-1977
24.6
45.6
127
292
1978-1995
48.8
99.7
239
636
1996-2012
31.2
85.9
213
689
0
0.2
0.4
0.6
0.8
1
-150
-75
0
75
150
225
W/V
L, cm
L, sm
W/V
53
1961-2012
33.1
73.5
186
518
Zoobenthos, thousand specimen/m
2
, g/m
2
1961-1977
6.94
11.32
36
88
1978-1995
15.62
22.81
63
124
1996-2012
5.44
9.23
19
141
1961-2012
9.7
14.94
41
117
Table 1 - Abundance and biomass of plankton and benthos in the western part of the North
Caspian Sea during the flooding period in 1961-2012 under different hydrological conditions
In contrast to zooplankton, the abundance and biomass of zoobenthos in the western part of
the North Caspian during the flooding period in 1996-2012 were significantly lower than those
in 1961-1977 (if the biomass dynamics is estimated by the median). Similarly to zooplankton,
the highest values of zoobenthos abundance and biomass were recorded in 1978-1995. We
should note that the highest arithmetic mean of the biomass was recorded in the
third period,
whereas it was by an order higher than the median, which points to the extremely uneven
distribution of the benthic organisms - occasional accumulations against the relevantly scarce
background.
Further research was aimed at the study of impact of North Caspian water renewal rate on the
dynamics of abundance and biomass of plankton and benthos. For this purpose, the time
series of the biological parameters were ranged by water renewal rate (in the ascending
order). Judging by the lack of statistically significant trends in the
ranged series of
phytoplankton abundance and biomass, we can conclude that its quantitative development is
irrelevant of the water renewal rate.
The zooplankton abundance and biomass series ranged by water renewal rate display diagram
peaks
at the minimal, average and maximal renewal rate (Fig. 2). This points to the existence of
three ecological zooplankton groups, which quantitative development is accelerated at a
certain water renewal rate.
Fig. 2 - Changes of zooplankton abundance and biomass in the north-western part of the
Caspian Sea in 1961-2012 depending on the water renewal rate (its values are plotted
along the X axis).
The series of zoobenthos abundance and biomass ranged by the water renewal rate clearly
show that its quantitative development grows with the increase of the water renewal rate. As
for zoobenthos biomass, this dependence refers only to the median (Me), and the changes of
X
mean
of the zoobenthos biomass were of the opposite nature. The highest value of X
mean
was
0
50
100
150
0
10
20
30
40
50
60
Xmean
Me
Abundance of zooplankton, thousand
specimen/m3
Me
Xср
0
200
400
600
800
1000
1200
1400
0
50
100
150
200
250
300
350
Xmean
Me
Biomass of zooplankton, mg/m3
Me
Xср
54