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51
Plankton and benthos of the Caspian Sea under different hydrometeorological
conditions
Umerbaeva R.I.*, Kashin D.V.**, Asaeva K.I.***
* "Research Centre of Southern Seas Ecology" Ltd.
** FSBUE "Caspian Research Institute for Fishery"
*** FSBI "Caspian Marine Scientific Research Centre"
Key words: North Caspian, water renewal, phytoplankton, zooplankton, zoobenthos,
abundance, biomass
Introduction
A few years ago several organizations lead by KaspNIRKh Research Institute created a
database
of hydrobiological observations for the northern part of the Caspian Sea. The database covers
the period of 1961-2012 and is aimed at the study of impact of natural and anthropogenic
factors on the status of biological communities, feasibility studies for exploration and
development of oil and gas fields and assessment of vulnerability of this highly productive
ecosystem to oil spills etc. (Kolmykov et al., 2016)
Throughout 1961-2012, the hydrological conditions in the North Caspian changed dramatically
several times: in 1961-1977 the sea level fell by more than 0.5 m; in the following 17 years it
rose fast by almost 2.5 m, and throughout 1996 - 2012 it fell again by about 1.0 m.
In line with
the morphometric dependency relation (Caspian Sea..., 2009), the volume of North Caspian
water throughout this period varied from 315 to 532 km
3
, its area ranged from 741 to 109
thousand km
2
, and the volume of the Volga runoff varied from 166 to 333 km
3
(according to
CASPCOM catalogues of the surface runoff).
To study the impact of hydrological conditions on the North Caspian plankton and benthos, we
suggest using the index "rate of North Caspian water renewal" (Asaeva, Kashin, 2018) which is
the ratio of the Volga flow volume (W, km
3
) to the volume of North Caspian water (V, km
3
).
The objective of the study is to look into the relationship of this index (W/V) to the dynamics of
plankton and benthos of this sea part.
Materials and methods
The study is based on the data on abundance and biomass of phytoplankton, zooplankton and
zoobenthos in the western part of the North Caspian for 24 sectors evenly distributed in the
studied water area, including the shallow zone (less than 5 m), the slope (5-10 m) and the
steep coast (more than 10 m). The study is based on the data received during
the high-water
season (May- June). The median (Me) and the mean arithmetic value (X
mean
) of abundance and
biomass were calculated for every year if the data for at least 12 sectors were available. The
median was required due to asymmetric distribution of hydrobiological data in the time series,
when the mean values of X
mean
and Me diverged. The use of both values facilitates the
characteristics of the compared aggregates. Both values (X
mean
and Me) were then averaged for
the above mentioned time periods (1961-1977, 1978-1995, 1996-2012), characterized by
different hydrological conditions in the Caspian Sea.
Results of the study
Within the considered time period, the Volga flow annually renewed 0.34- 0.88
of the North
Caspian water volume. The fastest water renewal rate was recorded in the 70-80s of the past
52