© 2000, Annual Reports of the Zoological Institute RAS.

Results of hydrobiological and palaeonthological studies at the Northern Caspian Sea

Nikolai V. Aladin, Andrey A. Filippov, Vasily A. Petukhov, Igor S. Plotnikov

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

Studies of zooplankton and zoobenthos were carried out within the framework of combined researches of the Northern Caspian Sea and Volga Delta area. The aim of the present work is to reveal main structural characteristics of aquatic assemblages and modern tendencies in dynamics of the Northern Caspian ecosystems under the influence of changing hydrological regime.

In autumn 1994 in freshwater zone of the Volga-Caspian Channel zooplankton diversity was low while copepods predominated. Near Zhemchuzhnyi Island the number of rotifer taxa was higher. The Caspian fauna was represented by cladocerans of the family Podonidae. The poorest zooplankton was found in lagoon on Zhemchuzhnyi Island: only rotifers, copepods and marine harpactocoids were recorded here.

In July 1995 in the Volga-Caspian Channel zooplankton was represented by rotifers and cladocerans. In the sea area (salinity 1.5-3.5 ppt) rotifers, copepods and cladocerans were found. Only Acartia and Calanipeda, and also bivalve larvae were common for both investigated areas. The number of taxa was higher than in autumn 1994 mainly due to cladocerans. Diversity of rotifers was also higher, especially in fresh water. Abundance of zooplankton was 6800-82400 ind/m³.

In summer 1996 at salinity of 5-10 ppt rotifers predominated, while bivalve larvae were numerous. At higher salinity the number of taxa was lower due to rotifers, which were absent outside freshwater area.

In autumn 1996 diversity of zooplankton in the Volga-Caspian Channel was low: only copepods, freshwater cladocerans and molluscs larvae were met here. In saline water area (about 11 ppt) near Zhemchuzhnyi Island zooplankton biodiversity was higher, and larvae of Balanus sp. were found. In comparison with summer time, species diversity and abundance of zooplankton were noticeably lower.

In autumn 1997 near Zhemchuzhnyi Island zooplankton comprised rotifers, cladocerans, copepods and few molluscs larvae. Most abundant zooplankton was reported from brackish-water island lagoon, though it was represented by only single rotifer species and some copepods.

At the end of August 1998 in the area near Zhemchuzhnyi Island rare for the Caspian Sea cladoceran species, Penilia avirostris, was found. Earlier it was sometimes observed in the Middle and Southern Caspian. However, it is not clear, whether this Black Sea immigrant has formed permanent populations in the Caspian.

The studies have shown that zooplankton overall abundance and species composition are different in areas with different hydrology-hydrochemical regimes. The poorest zooplankton was reported from the Volga-Caspian Channel. Seasonal changes are observed: autumn zooplankton was poorer than summer one. Year-to-year zooplankton variability in the Northern Caspian was uncertain.

Meiobenthos of Volga Delta and adjacent Northern Caspian areas has been poorly investigated (Chesunov, 1979, 1980, 1981, 1983a, 1983b). In 1994-1997 40 taxa of meiobenthos were noted in the research area. The richest group in respect of species number (31) was nematodes. Besides, 2 species of nectobenthic cyclopoids, 2 species of ostracods, 2 species of harpacticoids and 2 species of chironomids were found here. In freshwater zone species list was the largest and comprised 30 taxa. Nematodes had the greatest diversity, 21 species, among which Dorylaimus stagnalis, Tobrilus gracilis, Monhystera sp. were most numerous. These species density was as high as several thousands of individuals per square meter. Contribution of ostracods and oligochaetes to the total meiobenthos biomass, as a rule, was large (up to 75% and 70% correspondingly).

In the island region of the avandelta 9 invertebrate taxa were found in meiobenthos assemblages. Relative number and biomass of meiobenthos separate groups was similar to that observed in freshwater zone.

In permanently salinized zone 12 taxa of meiobenthos were noted, among which 8 were nematodes. Most abundant species were M. nannospiculus, L. longispiculus and S. cuneatus. Density of the last species reached sometimes tens of thousands of individuals per square meter. As well as in the freshwater zone nematodes formed up to 100% of the total meiobenthos numbers while ostracods and oligochaetes produced up to 80% and 50% of total meiobenthos biomass correspondingly.

Analysing meiobenthos dynamics, it is possible to note that in 1997 substantial growth of total meiobenthos biomass took place due to an increase in the numbers of Ostracods.

In 1994-1997 Volga Delta macrobenthos was represented almost exclusively by freshwater and brackish-water species with prevalence of bivalve molluscs, amphipods and oligochaetes by numbers while bivalves and gastropods contributed most to the total macrobenthos biomass. From 1994 through 1996 both numbers and biomass of oligochaetes and polychaetes increased in this area, abundance of amphipods decreased significantly, numbers and biomass of viviparids, cumaceas and isopods rose notably. In the last year of systematic observations (1996) polychaete Hypaniola kowalewskii and bivalve mollusc Hypanis plicata, were registered though they had not been found here before.

Brackish-water and marine species played an important role in the avandelta area benthic assemblages. From 1994 through 1996 there occurred a statistically significant decline in the numbers of bivalve molluscs (zebra mussel in particular), barnacles and chironomid larvae. On the contrary, the numbers of oligochaetes increased. By 1996 some mediterranean species, crab Rhithropanopeus harrissii and bivalve mollusc Abra ovata, appeared in this area and the dominant polychaete species Hypania invalida was replaced by Hypaniola kowalewskii.

Near Zhemchuzhnyi Island macrozoobenthos comprised mainly marine and brackish-water forms with predominance of bivalve molluscs Hypanis spp. and A. ovata, and also, in individual years, polychaetes and oligochaetes. Statistically significant changes during the period of observations included a decline in numbers of oligochaetes (they had disappeared by 1996), and an increase in both numbers and biomass of bivalve molluscs (mainly due to changes in A. ovata abundance). In 1996 barnacles and cockles, not observed here earlier, appeared in macrobenthos.

Zoobenthos sampling conducted in 1995 in the sea region between the Islands of Tyulenii, Zhemchuzhnyi, Chistoi Banki and Morskaya Chapura did not reveal significant changes in macrozoobenthos species composition in this region in comparison with previous observations (Tarasov, 1998). Meanwhile abundance of benthic invertebrates was ten times lower than that in the 1980s. Besides, in 1995 brackish-water molluscs and Nereis were more numerous here while there were fewer oligochaetes than before. These changes could have been connected with the benthic assemblage shift toward Volga Delta area. Such changes are characteristic for the periods of the Caspian Sea level fall and could have been connected with a decrease of river outflow impact on the Northern Caspian benthic assemblages.

As a whole, the revealed changes of macrozoobenthos structure in all investigated areas were opposite to those observed in previous decades in conditions of the Caspian Sea level rise (Osadchikh et al., 1989; Chuikov et al., 1996). Thus, they could have been brought about only by progressing salinization of biotopes and reduction of Volga outflow.

Table. Species diversity of Ostracoda of the Northern Caspian Sea

It is well known that the ancient Caspian extended for more than 1150 km from north to south or completely dried up in former times. These changes were reflected in fauna and flora of the Northern Caspian Sea that could be proved by the analysis of sediment columns collected in the Northern Caspian. These columns contain remains of vegetative and animal organisms, and analysing those it is possible to reconstruct the past biodiversity of any given Caspian water area. In the present work both modern and fossil ostracods were investigated. Composing the species list we analysed not only columns collected by authors, but also materials stored in collections of VNIIGRI, VSEGEI and other institutes. 7 species of ostracods were found in Akchagylian deposits of the Northern Caspian, 12 species in Apsheronian deposits and 21 species in Holocene deposits (Table). 10 ostracod species were found alive on the bottom surface as well as dead shells of 8 more species. Undamaged state and the deposition character of dead shells point to their relation to the recent fauna. In our collections from the Northern Caspian Sea 18 recent species of ostracods were found only. However real Ostracoda diversity in this water area is apparently a little higher because in collections of E.I. Shornikov and KaspNIRH 14 more species were found. Thus, a total of 32 species of Ostracoda from the Northern Caspian Sea are known by now.

The studies were carried out with financial support of the State Scientific Technical Program "Biological Diversity and the Russian Academy of Sciences Commission for the Young Scientists.

References

Osadchikh, V.F., Ardab'eva, A.G., Belova, L.N., Elizarenko, M.M., Kurashova, E.K., Malinovskaya, L.V., Popova, M.K., Stepanova, T.K., Tinenkova, D.H. & T.A. Tatarintzeva. 1989. Characteristic of forage stock development and use by fishes under conditions of the Caspian Sea level increase. In: Kompleksnye rybokhozyaistvennye issledovaniya na Kaspii [Combined fishery studies at the Caspian Sea]. pp. 119-136. Astrakhan. (In Russian).

Tarasov, A.G. 1998. Long-term quantitative changes in the benthic biocenoses of the Northern Caspian Sea. Okeanologiya 38 (1): 105-113. (In Russian.)

Chesunov, A.V. 1979. Free-living nematodes of the Caspian Sea: Doctoral theses. 22 pp. (In Russian).

Chesunov, A.V. 1980. New data on free-living nematodes of the superfamily Monhysteroidea in the Caspian Sea. Zool. Zh. 59 (7): 973-985. (In Russian).

Chesunov, A.V. 1981. Free-living nematodes from species group Therstus flevensis (Monhys-terida) in the Caspian Sea. Byull. mosk. Obshch. Ispyt. Prir. (Biol.) 86 (2): 63-70. (In Russian).

Chesunov, A.V. 1983a. Geographical distribution and ways of formation of free-living nematodes fauna of the Caspian Sea. In: Biologicheskie resursy Kaspijskogo morya [Biological resources of the Caspian Sea]. pp. 83-107. Moskva, MGU. (In Russian).

Chesunov, A.V. 1983b. Spatial distribution of free-living nematodes in the Caspian Sea. In: Biologicheskie resursy Kaspijskogo morya [Biological resources of the Caspian Sea]. pp. 69-82. Moskva, MGU Publ. (In Russian).

Chuykov, Yu.S., Bukharitsin, P.I. & L.A. Kiseleva. 1996. Hydrology-hydrobiological re-gime of Lower Volga. In: Ekologiya Astrakhanskogo regiona [Ecology of Astrakhan region]. Vol. 4. Astrakhan. 253 pp. (In Russian).