© 2000, Annual Reports of the Zoological Institute RAS.

The impact of the Caspian Sea water level fluctuations on the productivity of the Volga River Estuary

Sergey M. Golubkov, Marina I. Orlova, Ludmila P. Umnova

Zoological Institute, Russian Academy of Sciences, Universitetskaya nab., 1, St. Petersburg, 199034, Russia
 

The water level of the Caspian Sea has been fluctuating during the whole history of its existence. The highest level for the 20^th century has been observed at the beginning of the 1930s (a little lower than -26 m absolute), and the lowest in 1977 (-29 m abs.). After 1977 it began to grow and had reached -26.65 m abs. in the middle 1990s. These water level fluctuations change greatly the water depth along shallow northern part of the Caspian Sea. The objective of this study was the evaluation of potential effects of water level fluctuations on productivity of planktonic and benthic communities of the shallow water estuary of the Volga River.

The Volga River Estuary comprises several zones. Upper freshwater zone covers underwater part of the Volga Delta. It is usually divided into kultuk zone, island avandelta and avandelta proper from the North to the South. The kultuk zone is threshold zone between above water and underwater parts of the Delta. There are a lot of shallow bays in this zone. Most parts of the island avandelta are covered by macrophytic vegetation. The avandelta is mostly free of this vegetation. There is a transitional zone between freshwater and saltwater south of the avandelta. Salinity in this zone is as 1-5‰. North of transitional zone there is a shallow water zone of the Northern Caspian Sea with a depth of not more than 5-6 m and water conductivity of about 5-12 S.

First step was to estimate the productivity of planktonic and benthic communities in the above-mentioned zones of the Volga Estuary in the recent years. The investigations were conducted in the western part of the Estuary in September 1994-1997. Primary production and decomposition of organic matter by planktonic communities were measured by closed-bottle methods in oxygen modification on 60 stations along different parts of the Estuary between 45⁰00' - 45⁰40'N, and between 47⁰30' - 48⁰30'E. Temperature, conductivity and transparency of water were measured at all stations. Concentrations of total and organic suspended material were determined by standard methods (Winberg, 1971). Petersen grab was used to evaluate the abundance and biomass of macrobenthos. Production and decomposition of organic matter by bottom animals were estimated by means of growth rate, specific rate of production and by equations describing relationships between respiration and body mass of animals (Alimov, 1981; Zaika, 1983; Golubkov, 1986, 1999; Balushkina, 1987).

Mean primary production at the depth of optimal photosynthesis (Aopt.) was the highest in the transitional zone. Decomposition of organic matter (D) was the highest in the kultuk zone (Table 1). The last-mentioned is the sequence of accumulation and precipitation of suspended matter brought by the Volga River waters to this zone. As a result, the dry weight of total suspended material was very high and water transparency was very low in the kultuk zone of the Estuary (Table 2).

Table 1. Primary production at the depth of optimal photosynthesis (Aopt., cal*l^-1) and in water column (A, kcal*m^-2), decomposition of organic matter (D, cal*l^-1, D', kcal*m^-2) in plankton along different zones of the Volga Estuary

 
The increase of primary production in the transitional zone may be explained by existence of a 'marginal filter' there. 'Marginal filters', peculiar to continent - sea boundary are found in a number of estuaries all round the world. On a global scale the 'marginal filter' is a rather narrow belt where river- and seawaters are mixing (Lisitzin, 1999). The processes occurring in a 'marginal filter' are a combination of a variety of physical, chemical and biological transformations of suspended and dissolved substances of the fluvial water. One of the consequences of these transformations is a 'biological plug', which comprises maximal biomass of phytoplankton. Another consequence is coagulation of dissolved organic matter into fine particles, which leads to a rise of 'silt plug'. At the majority of investigated estuaries 'plugs' are observed at water salinity of 1-5‰ (Lisitzin, 1999). This is also the case for the Volga Estuary, where maximum of organic particulate matter (POM) and chlorophyll 'a' concentration are observed in the transitional zone (Table 2) under salinity of 1-5‰.

Minimum values of A_opt., D, POM and chlorophyll 'a' concentration were observed in the island avandelta (Tables 1, 2). This fact may be explained by the presence of dense stands of emerged and submerged macrophytes, which are spread in this zone. Our previous study has shown that macrophyte beds diminish the content of suspended materials in the water and primary production of plankton (Golubkov et al., 1987). As a whole, our results on plankton primary production in the Volga Estuary coincide with the data of other researchers (Gorbunov, 1976; Maksimova & Metreveli, 1996).

Table 2. Dry weight of total suspended material (SM, mg*l^-1), organic particulate matter (POM), mgÑ*l^-1), chlorophyll "a" concentration (Chl, ìg*l^-1) and Secci disk transparency of water (Tr, m) along different zones of Volga estuary

 
The highest abundance of bottom animals was found in the transitional zone in the 'biological plug', but zoobenthic biomass was the greatest in the island avandelta (Table 3). Strong press of numerous benthivorous fishes on zoobenthos may explain relatively small animal biomass in the transitional zone. As was shown by many researches, fish has a great impact on bottom communities in shallow waters, where there are no macrophytes (Golubkov, 1997). Fish impact leads to a decrease of abundance of relatively large bottom animals, and, as a result, benthic biomass decreases, but abundance may increase. Biomass, production of bottom animal communities and energy flow through zoobenthos, which was estimated by food consumption of non-predatory animals, were the highest in the island avandelta (Table 3). This may be explained by the presence of dense macrophyte beds in this zone. Bottom animals avoid fish predators by using macrophyte refugia. Macrophytes are also a good substrate for epiphyte algae, which are used as food by gastropods and amphipods, abundant in this zone. That was the reason of higher level of food consumption of bottom animals as compared with plankton primary production (Tables 1, 3).

Functional characteristics of plankton and benthos in different zones of the Volga Estuary in modern period may be used to evaluate the influence of water level fluctuations of the Caspian Sea on productivity of shallow water part of the Northern Caspian. Fluctuations of the Caspian Sea level cause spatial changes in position and expansion of the main zones of the Volga Estuary. For instance, the border of microphytic vegetation was situated about of 15-20 km to the south at the end of the 1970s, as compared to the modern period (Fig. 1). It was connected with low sea level at that time. Earlier at the beginning of the 1930s, when sea level was higher than nowadays, most of the modern islands were covered by water and macrophytes beds formed a relatively narrow bar along the coast.

Table 3. Abundance (N, ind.*m^-2) and biomass (B, g*m^-2) of macrozoobenthos, food consumption of non-predatory animals (C, kcal*day^-1m^-2) and production of bottom animal community (P_c)

 
Using our estimation of mean values of primary production and zoobenthos production in the main zones in the western part of the Volga Estuary (Tables 1, 3) and areas of these zones for different periods of the 20th century we have made an approximate evaluation of total primary production of plankton and zoobenthic community production for the western parts of the Estuary covered by macrophytic vegetation (kultuk zone and island avandelta) and zones with brackish shallow waters (transitional zone and Shallow Caspian) for the above-mentioned periods of the 20th century. This evaluation has shown that total productivity of different zones of estuary fluctuates greatly during the century (Table 4).

Regression of the Caspian Sea in the 1970s led to about 4-fold de-crease of productivity of the Caspian shallow water zone (Table 4), which could have influenced the development of many species inhabiting this zone. For example, the Caspian shallow waters are the main foraging areas of sturgeon species. These species have rather long immature stage in their life cycles of about 15-20 years. So, one may expect, that a decrease of productivity of Caspian shallow waters may cause many-fold decrease of sturgeon fish production through 15-20 years. Consequently, the strong regression of the Caspian Sea level at the end of 1970s should be regarded as one of the main reasons of great fall of sturgeon fisheries observed since the middle 1990s.

Fig. 1. Borders of microphytic vegetation in western part of the Volga Estuary in different periods of the 20th century.

Table 4. Total primary production of plankton (P_pl, ton C*day^-1) and community production of zoobenthos (P_zb, ton C*day^-1) in the western part of the Volga Estuary in different periods of the 20th century.

Thus, fluctuations of the Caspian Sea level cause great changes in the productivity of main zones of the Volga Estuary and populations of species living there.

The studies were carried out with financial support of the Russian Foundation for Basic Research (grant 99-04-49614).

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