Materials and methods
The study is based on the following CASPCOM data catalogues:
mean annual air
temperature (HMS, Derbent, 1961-2016). mean annual water temperature (HMS,
Makhachkala), mean annual Volga runoff, 1961-2016). Mean values of each of
the time series were distracted from source time series. The series of "residual"
values were compared to one another. According to the results of comparison,
four states of Caspian Sea climate were identified: "cool and dry", "cool and wet",
"warm and dry", "warm and wet".
Results
The climate impact on the marine biota is not limited to the impact of tempera-
ture on biochemical, physiological activity and behaviour of the organisms. The
climate affects the functioning of the marine ecosystem on the whole. The sensi-
tivity of the Caspian ecosystem to the climate and its changes (not
only the cli-
mate, but the sea as a whole) is determined by several factors: isolation, mor-
phology, homogeneity of the water column etc.
Due to its isolation, the sea has no sources of water apart from the river flow and
precipitations. The volume of river flow is by several times higher than that of
precipitations. That is why the sea is particularly sensitive to the increased water
content in the water catchment, not in the sea water area. As the volume of the
Volga runoff exceeds the runoff of all of the remaining rivers, the
fluctuation of
the Caspian Sea level mostly depends on the changes of the Volga water content.
As a result of its isolation, the Caspian Sea is an indicator of moistening of the East
European Plain, which is mainly occupied by the Volga river catchment (it is as
well the indicator of other large-scale climate changes).
North Caspian, being the smallest sea sector by its volume (0.5%) and average
depth (4.4 m), is the least inertial sector
of the Caspian Sea, which immediately
responds to the atmospheric changes above its water area. In addition, the North
Caspian is strongly dependent on the hydrological regime of the Volga and its
runoff fluctuation. The flow of warmth and moist to the atmosphere is the most
intensive in the northern sea part. Therefore, the North Caspian is serves as the
main route of moist and air exchange. The South Caspian is the largest in volume
(65.6%), area (39.5%) and average depth (345 m) part of the Caspian Sea, which is
the opposite of the North Caspian in its responsiveness to external factors. Thus
different sea parts differ from each other not only in their hydrometeorological
parameters, but also in their response to their changes.
The water column of the Caspian Sea, with the exception of its northern freshwa-
ter part, is quite homogeneous in its salinity. This is an important factor, due to
which the depth of the water mixing and, consequently the volume of biogenous
elements entry to the zone of photosynthesis and bioproductivity
of the water
area mainly depend on the climate change. Climatic drivers ensuring deep water
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mixing in the Caspian Sea are of various nature (Tuzhilkin, 2008). These are the
regional atmospheric circulation, which leads to the
formation of eddies, upwell-
ing and downwelling and all the factors leading to the increase of the surface wa-
ter density and its sinking (summer evaporation, winter cooling, ice cover forma-
tion). Deep water mixing is also affected by fluctuations in salinity of the surface
water layer conditioned by the changes in the volumes of river runoff and atmos-
pheric precipitations.
Any of these factors is sufficient for full water mixing (its indicator is the emer-
gence of oxygen in the near-bottom layer of deep water depressions). The most
active water mixing (and, as a result, the enrichment of the bottom layer with
oxygen) was observed in the 70s of the past century, when it was caused by sev-
eral factors (severe winters, reduction of the river flow etc.). Today, as well as 100
years ago, the near-bottom waters of the Middle and the
South Caspian are en-
riched with hydrogen sulphide, which replaces oxygen when it runs out.
The ideas presented above make it possible to suppose that during cool and dry
years bioproductivity rises in the Middle and the South Caspian (due to increased
vertical circulation) and decreases in the North Caspian (as a result of the Volga
flow reduction). Dry and warm years are the least favourable for the marine eco-
system. The data in Table 1 show that in such periods bioproductivity falls in all
the sea parts (in the North Caspian as a result of runoff decrease, and in the Mid-
dle and the South Caspian due to reduced vertical circulation).
Climate
Surface
runoff
Tempera-
ture re-
gime
Sea
level
Vertical
circulation
Bioproductivity
North
Caspian
Middle
and South
Caspian
Dry,
cool
Below
normal
Below
normal
Reduces
Deep
Low
High
Dry,
warm
Below
normal
Above
normal
Reduces
Surface
Low
Low
Wet
cool
Above
normal
Below
normal
Rises
Average
High
Average
Wet
warm
Above
normal
Above
normal
Relatively
stable
Surface
High
Low
Table 1 - Climate status and bioproductivity of the Caspian Sea
The comparison of the time series of the "residues"
of air temperature, Caspian
Sea water temperature and the Volga runoff has shown that in 1964-1977 cool
and dry years prevailed (11 out of 14 years), cool and wet years dominated in
1985 - 1994 (6 out of 10 years), warm and wet years prevailed in 1995-2005 (7
out of 11 years), and warm and dry years - in 2006 - 2015 (6 out of 10 years).
Discussion
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